stant pressure apparatus to the aspirator. The levels of the liquid in the manometer were kept the same during an experiment. In cases of difficulty in getting this setting, the constant level apparatus was set in operation, water entering until atmospheric pressure was restored; the volume of water which entered was deducted from the volume of water run out of the aspirator. The rate of outflow was checked by a stop-watch in all coses. A filter-pump was arranged so that it could be used for suction from the aspirator, and when this was not desirable the air to be pumped into the reservoir entered through an inlet on the pipe-line. The different absorption vessels were of such sizes that by always using the same volume of solution, “depths” of liquid of 7.5, 5.5, 3.7 cm. respectively were obtained.

To carry out an experiment the three-way taps are opened, so that air only passes through the jets. It is very important to have air passing through while the solution is being poured into the absorption vessel, else liquid enters the jet. After the liquid has all been poured in, the taps are turned off until a bubble of air remains on the tip of the jet. The acid gas is then allowed to pass in at a rate regulated by a tap on the aspirator. The thermometer in the inlet tube gives the temperature of the gas mixture (practically air temperature) and a thermometer in the solution gives the temperatures at the beginning and end of an experiment.

When the “erect” jets were first used difficulties were encountered, owing to the working-back of the liquid into the barrel of the jet and thence to the inlet tap, but by coating the inside surfaces of the jets with paraffin wax this was obviated. The tips of the jets were cleaned, so that a glass surface was exposed for the formation of the bubbles.

The alkali solutions, whether of sodium, potassium or barium hydroxides, were free from carbonate.

In all cases the partial pressure of the hydrogen chloride is taken as 130 mm. Although no difference in the amounts of fog formed under similar conditions from moist or dry gas were found, in all cases the gas was dried by calcium chloride.

The fog was analysed by the method previously given. The “partial pressure” of the hydrogen chloride given in the tables has been calculated by taking one mg. of hydrogen chloride as giving a pressure of 0.5014 mm. Hg. at 20° C., or 0.5098 mm. at 25° C. From the increases in weight of the calcium chloride tubes, assuming that water vapour obeys the gas laws, the partial pressures given in the tables were calculated.

In all cases “per cent. HCl” refers to the percentage of acid by weight in the fog particles. All weights of acid and of fog are given in mg. per litre of air used. The amount of free acid in the particles has been determined in each case as a check on the silver titration, but is not given in the tables.

Owing to the variation of the amount of fog formed with change in concentration of the alkali, it was necessary to use the smallest possible volume of air in the experiments in which the “rate” was altered. Two litres has been adopted as standard, as with this volume the concentration has not been too greatly reduced and a weight of fog sufficient for analysis has been obtained.

(a) The necessary presence of alkali: It has been repeatedly observed that in neutral or acid solutions of salts, dyes and colloids, and in distilled water no fogs are formed under the conditions of the present experiments. This is clearly shown also in the data obtained when the effect of alkali concentration on fog formation was being studied—immediately the solution became acid, fog formation ceased.

A number of salts of weak acids were examined also. Solutions of the following (in normal solution generally), sodium acetate, bicarbonate, disodium phosphate, potassium dihydrogen phosphate, and sodium biborate gave no fogs. Thin fogs were obtained with sodium carbonate (IN), and good ones with trisodium phosphate (IN). These results show that a certain hydroxyl-ion concentration is required before fog formation begins.

(b)Alteration in radius of the jet and in the depth of the liquid: In practically every case an increase in the radius of the jet causes a greater production of fog. It would appear that at about 5.5 cm. depth the production of fog is at its maximum, for here the two widest jets give considerably more fog, especially at the higher rates, than at either 3.7 cm. or 7.5 cm. With the narrowest jet, there is comparatively little difference in the amount of fog produced at each depth of liquid. With the other jets there is a tendency for less fog to be produced at 3.7 cm. than at 5.5 cm., or 7.5 cm., especially at the higher rates, although the variation is only a few milligrammes per litre of air.

As is noted later, the concentration of acid in the particles is greatest at 3.7 cm., so that the weight of hydrogen chloride escaping absorption decreases as the depth of liquid increases, except for the two widest jets which show maxima at 5.5 cm. On plotting the data obtained, it is found that as the depth of liquid increases and the radius of the jet decreases, the amount of fog formed, and especially the weight of acid vapour escaping absorption tends to vary linearly with the rate of passage of the gas.

(c)Alteration in rate of passage of the gas: In all cases any increase in the rate of passage of the gas causes an increase in the amount of fog formed. With the simple aqueous solutions, in general, this increase is not a linear function of the rate. There is evidence that below a certain critical rate the fog production falls off very rapidly, but owing to experimental difficulties, when only small quantities of fog are obtained, it is not certain whether this critical rate exists for solutions containing no colloid or dyestuff.

It may be stated here that the fog formation is not due merely to the comparatively rapid rates at which the gas is bubbled through the solutions, for with gelatine, etc., solutions, a thick fog is formed when only one bubble every one or two seconds passes up through the liquid.

As an example of the results obtained when the radius of the jet and the rate were altered, in Table 2 are given data obtained when a 1.26 N sodium hydroxide solution was used.

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(d) Alteration in concentration of the alkali: A large number of experiments have been made in which the same alkali solution has been used until completely neutralized. As shown by the representative data in Table 3, the weight of fog formed has been determined after the passage of each litre of air through the solution. In all cases the fog production falls off as the alkali concentration decreases. As soon as the solution becomes acid, no more fog is formed.

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Solutions of potassium hydroxide and barium hydroxide give similar results, although at corresponding concentrations less fog is obtained than with sodium hydroxide.

If the logarithms of the mean concentrations of alkali during the passage of each litre of air and of the corresponding weight of fog be taken, a straight line is obtained when they are plotted. Moreover under fixed conditions of depth and original concentration of alkali the lines corresponding to the different jets are parallel.

From the data obtained in these experiments Table 4 is compiled, showing the amount of fog formed in the first litre of air passed through sodium hydroxide solutions of the mean concentrations given at 20° C.

If these figures are plotted in the ordinary manner it will be seen that in dilute solutions small additions of alkali exert very large effects; in the more concentrated, the amount of fog obtained is almost proportional to the alkali concentration.

The experiments made with the sodium carbonate and trisodium phosphate solutions may now be considered. The data obtained are given in Table 5, the rate being 225 c.c./min., and the depth of liquid 7.5 cm.; at 22° C

There is a rapid fall in the amount of fog formed as the solutions are neutralized. Buffer action may possibly account for the constancy of some of the carbonate figures; the phosphate solution reaches the disodium stage too quickly for the above to be observed, but it doubtless exists in this case also.

Sodium carbonate is not so greatly hydrolyzed as sodium phosphate, and consequently the hydroxylion concentration will be somewhat smaller in the carbonate than in the phosphate solution. This difference is plainly shown by the differences in the amount of fog obtained from the first litre of gas. Now, from the data given in Table 4, the concentrations of free sodium hydroxide which would give the same weight of fog under the same conditions are 0.06 and 0.45 normal respectively; these are of the same order of magnitude as would be expected from the degrees of hydrolysis.

It has not been possible to fix a definite value for the hydroxylion concentration below which no fog appears, but it must be greater than 10^-5 normal, since the other phosphates and sodium bicarbonate give no fog.

(e) Effect of the above factors on the concentration of acid in the fog particles: The acid concentration remains remarkably constant, varying only from 8 to 11 per cent., although the radius of the jet, the rate of passage of the gas, and the depth of the liquid have all been varied. At the two depths, 3.7 cm. and 5.5 cm., for a given rate the concentration of acid is the same; at 7.5 cm. for the same conditions the acid content is about one per cent. lower. At 3.7 cm. the acid content is almost independent of the radius of the jet; in the other cases there is a tendency for the acid content to fall as the radius of the jet is decreased. In all cases there is a slight tendency for the concentration to fall as the rate is reduced.

(f) Water vapour partial pressures: In all cases, after making due allowances for differences in temperature, the pressure of the water vapour in the filtered air remains fairly constant at about 95 per cent. of the saturation value for the corresponding air temperature, or 80-90 per cent. of that for the mean temperature of the solution during an experiment. When large quantities of fog are obtained the relations are not so simple, as in some cases values of more than 100 per cent. saturation are obtained. This point is considered later, when dealing with the effect of adding colloids, dyes, etc., to the alkali solutions. With small quantities of fog the vapour pressure is independent of the nature of the solution, the radius of the jet, the depth of the liquid, and the rate of passage of the gas. This presumably shows that the particles attain some sort of equilibrium with the vapour with which they are in contact. This point will be considered more fully when dealing with the sizes of the particles in the fogs.

2. During some of the experiments it was noticed that traces of certain organic compounds exerted a marked positive effect in fog production. For example, in an experiment with a 0.313N sodium hydroxide solution 10 drops of a 0.02 per cent. aqueous solution of methyl red were added, in order to see at what stage all the alkali was neutralized, but much more fog than usual was obtained, namely, 36.7 mg., instead of 22.2 mg. Hence it appeared to be of interest

to obtain strictly quantitative data on the effect of small quantities of colloids or capillary active substances on fog formation.

The following have been used as “capillary active substances”: iso-amyl alcohol, sucrose, gelatine, gum arabic, safranine, methylene blue (hydrochloride), congo red, orange G, glycerine, saponin, kaolin, aquadag, and clay.

Solutions or suspensions of these substances in sodium hydroxide were made up, and the amount and the analysis of the fog formed were determined for the different experimental conditions. The analyses given in experiments on the progressive neutralization of the alkali, except where otherwise stated were carried out on the total amount of fog formed, and are therefore average values.

Finally, most of the substances were examined with the conditions of experiment fixed, except for variations in the concentration of the capillary active substance.

Gelatine and Gum Arabic: The solutions were made up by allowing a weighed quantity of the solid to disperse in the alkali solution. Both the gelatine and gum arabic gave no fogs in absence of alkali.

In the case of gelatine solutions of different concentrations were made up, one of which was 0.5N with respect to sodium chloride. It was found that the amount of fog was not very dependent on the concentration of gelatine in the stronger solutions, but even traces of gelatine have a very great effect relative to the amount of fog obtained with a simple aqueous solution. Some of the data are given in Table 6.

Addition of sodium chloride to the solution caused a slight increase in the amount of fog formed.

Small alterations in the concentration of the alkali did not have a great effect on the formation of the fog.

When the rate of passage of the gas was varied, at about 110 c.c./min., the fog production decreased rapidly. This is shown by the data in Table 7. This peculiarity is shown by other solutions also.

A number of experiments were carried out at different rates and with different jets and solutions to find out how the fog production varied during the neutralization of the alkali. An example is given in Table 8. The solution was 0.591N sodium hydroxide containing 0.25 per cent. of gum arabic.

Iso-amyl alcohol, saponin, glycerine and sucrose: The amyl alcohol had a very definite action in increasing the amount of fog. In a 0.594N sodium hydroxide solution containing 1.0 per cent. of alcohol, at a depth of 7.5 cm., the gas being passed at 225 c.c./min., using the jets of radius 1.32 and 0.82 mm., from the first litre of air 79.3 and 83.7 mg. of fog respectively were obtained. Even in 0.01 per cent. solution its effect was quite definite.

Saponin was used in 0.01 per cent. solution. Some difficulty was experienced due to frothing. This solution gave three times as much fog as a simple aqueous solution under similar conditions. The acid content of the particles was somewhat less than in other cases, namely, 6.5-7.0 per cent.

Glycerine and sucrose did not have any effect on the amount of fog formed.

Safranine and methylene blue: Safranine was used in 0.61N sodium hydroxide. It has been found to be of the same order of activity as gelatine. For experimental conditions such that a pure aqueous alkaline solution gave 31.9 mg. of fog, the 0.02, 0.008, 0.0013 per cent. solutions gave 117.9, 83.2, 37.3 mg. respectively. Analysis of the fogs gave a percentage of 8.89 of hydrogen chloride.

Methylene blue was extremely active, so that a dilute solution containing 0.0013 per cent. was used as a stock solution for dilution in some of the experiments. The dye was absorbed very strongly

by all glassware, etc. This property seemed to run parallel with activity in fog formation since safranine and methylene blue were much more strongly absorbed than congo red or orange G.

The same phenomenon was noticed here as with the gelatine solutions, namely, that in the higher concentrations, the chiloride in the fog particles was somewhat greater than in the more dilute; for example, the fogs from the strongest solutions gave on analysis 10.2 per cent., and the fogs from the more dilute 8.9 per cent. For quantitative results see Table 21, when it will be seen down to what extreme dilutions the effect of methylene blue can be detected.

Congo red and orange G: Neither of these substances was very active, although the effect of their presence could be detected easily in a 0.0067 per cent. solution. For conditions such that 31.9 mg. of fog were formed from the simple solution, congo red and orange G each in 0.1 per cent. solution gave 71.2 and 90.2 mg. respectively. The percentages of acid were 8.11 and 8.34 per cent. respectively.

Suspensions: Colloidal solutions may be considered as very fine suspensions, so that it is of interest to see what effect a coarse suspension would have on the absorption of the acid vapour. With suspensions of kaolin, graphite (aquadag), and clay, dense fogs were obtained in each case.

Two different concentrations of acid vapour have been used, the partial pressures being 36.7 and 139.2 mm. Experiment has shown that not only is the amount of fog formed, but also is the concentration of acid in the particles very much dependent on the original partial pressure. For example, in a solution containing 0.039 per cent. of aquadag the weights of fog were 88.7 and 129.6 mg. respectively, the corresponding concentrations of acid in the particles being 7.22 and 8.55 per cent. In Table 9 are given some of the results obtained with kaolin. For aquadag, see Table 21.

A sample of carefully washed and fractionated Buxton clay was used. The particles were very small, taking some days to settle a few centimetres. This suspension was very active, as is shown by the following figures quoted from the full experimental results.

With a 0.124 per cent. suspension 208.0 mg. of fog obtained, and even with a 0.006 per cent. suspension (which appeared very slightly cloudy to the naked eye) 106.5 mg. of fog were obtained. The mean concentration of acid in the particles was 9.56 per cent.

All the experiments with the suspensions have been carried out under the same conditions namely, with a jet of radius 1.32 mm., and the gas passing at 208 c.c./min., except for the first set of experiments with kaolin, when the rate was 204 c.c./min.

The activity of aquadag approaches, and of Buxton clay exceeds, that of gelatine or safranine, while the kaolin has the same order of activity as congo red.

3. Hydrobromic acid has been found to be much more active than hydrochloric acid, this being parallel to its greater “fuming” properties. Neither hydriodic nor nitric acid has yet been tested, but each would doubtless give fogs.

As only a small quantity of hydrobromic acid was available, it was not possible to do many experiments, and, in addition, the partial pressure of the bromide vapour was smaller in each successive experiment; but the partial pressure was estimated in each case.

In all the following experiments the jet of radius 1.32 mm. was used at a depth of 7.5 cm., the gas being passed at 208 c.c./min.

Distilled water gave no fog with a gas in which the partial pressure of the bromide was 79.7 mm. The water vapour partial pressure was 19.4 mm., the air temperature being 21.7° C. This vapour pressure is the same as that found later when fog formation was taking place.

Two different sets of experiments have been carried out using the same concentration of soda (1.26N), but with the fresh solution and the same after the bromide partial pressure had fallen to about 70 per cent. of its original value. It will be noticed that the fog obtained at corresponding intervals in the second set is about two-thirds of that in the first set, so that the production of fog has fallen off almost in proportion to the reduction in the partial pressure of the bromide. The data of these experiments are given in Table 10, the figures in the column headed “mean” being obtained by analysis of the whole of the fog collected, and of the alkali solution at the end of the set of experiments.

When the same gelatine and gum arabic solutions were used with hydrogen bromide as had been employed with hydrogen chloride, very dense fogs were formed, the bubbles as they passed through the liquid being filled with fog. The concentration of acid in the particles remains practically constant, whether the solution contains any colloid or not.

In Table 11 the data obtained in experiments on the production of fog with progressive neutralization of the alkali are given. In each solution the sodium hydroxide was 0.59N. The “mean” values were obtained as in Table 10.

The percentages of bromide in the particles are very large; but when these are converted to normalities the concentration is nearly the same as in the case of the chloride, being, in fact, slightly higher. This would appear to show that the particles in each case are of the same size. How far this is true will be seen later when the sizes of the particles are considered.

In the gas, after removal of the particles, the water vapour pressure is constant, and has the same value under corresponding conditions as with hydrogen chloride, although with the gelatine and gum arabic solutions there is a tendency for it to be somewhat greater than in the previous experiments.

If the quantity of fog obtained be plotted as has been done for hydrogen chloride, the same type of curve is obtained. Corresponding to the greater production of fog in these experiments and to the slower falling off as the alkali is neutralized, the lines are not so steep as for hydrogen chloride. It is interesting to note that the lines for gum arabic and gelatine are again practically parallel.

The vapour pressures of the bromide in all these experiments have been considerably lower than with the chloride, yet the amount of fog formed under the same conditions is greater. This is probably due to the lower rate of diffusion of the bromide in the gaseous phase.

4. Although alcohol might have been included among the “capillary active” substances, it does not appear to be particularly effective. For example, a 7.6 per cent. solution gives an increase of only about 25 per cent. over the plain aqueous solution of the same alkali con-

centration. The strong alcoholic solutions give dense fogs, which begin to form as the bubbles pass up through the liquid. In this case the presence of a volatile substance lends interest to the experiments.

The solutions used contained 7.6 and 33.0 per cent. by weight of ethyl alcohol, and were 1.223N and 1.238N respectively, with respect to sodium hydroxide.

Alcohol was estimated by the method of Benedict and Norris (J. Am. Chem. Soc. 1898, 20, 293), in which a solution of potassium dichromate in sulphuric acid is used. The procedure finally adopted was as follows. A known volume of the very dilute alcohol solution (0.1 per cent. or less) is run from a burette into a small flask; three times this volume of concentrated sulphuric acid are then added slowly with cooling under a good stream of water. An excess of dichromate solution is slowly added from a burette, after which the flask is heated over a small flame, taking fifteen minutes to reach 98° C., the temperature then being held at this point for five minutes. The flask is then cooled and added to about 300 c.c. of cold water in a large conical flask. After again cooling, the excess of dichromate is destroyed by adding an excess of standard ferrous ammonium sulphate, this excess being in turn estimated with standard permanganate.

The dichromate was standardized by means of a 0.100 per cent. solution of alcohol. This solution was of approximately the same strength as the solutions obtained after extracting the asbestos filters with water.

Since the presence of chloride causes high results, the following procedure was adopted. The asbestos was removed and extracted with distilled water, the volume being kept as small as possible (not more than 100 c.c. with the smaller quantities of fog), the washings being run into a graduated flask. An excess of 0.03185N silver sulphate solution was then added, and, after being well shaken the liquid was made up to the mark. After standing for a time, the solution was filtered, and an aliquot taken for analysis. Another portion of the solution was then titrated with ammonium thiocyanate to determine the excess of silver. From the gross amount of alcohol found on analysis a deduction was made for alcohol absorbed by the asbestos, this having been determined in blank experiments.

An attempt has been made to estimate separately the alcohol and the water vapour partial pressures in the gas after the fog particles had been filtered out. The most satisfactory method for the conditions under which it was required to work, appeared to be that of Thomas (J. Soc. Chem. Ind., 1922, 41, 33T). In this method freshly broken calcium carbide, which must be as free as possible from calcium oxide, is used to remove the water vapour, the alcohol passing on to be obsorbed in concentrated sulphuric acid. The factor for converting the change in weight of the carbide to water was found by blank experiments to be 2.91. Blank experiments showed that no acetylene was absorbed by the acid.

In all the experiments where fogs were formed, the partial pressures were estimated, whence the ratio water/alcohol was calculated, but the results in some cases were somewhat irregular. The ratios

water/alcohol found in the vapour correspond satisfactorily with those given in the standard tables for mixtures similar to those employed in these experiments.

The details of a typical experiment are given:—

The fog was washed out into a 250 c.c. flask containing 15 c.c. of the silver sulphate solution. After filtering, 100 c.c. of solution required 2.15 c.c. of ammonium thiocyanate solution (equal to the sulphate in concentration) for precipitation of the excess silver; whence the chloride present used 9.63 c.c. of silver solution. This represents 11.18 mg. of hydrogen chloride so that the particles contained 4.94 per cent. by weight of chloride.

For the alcohol estimation two portions of 25 c.c. each were taken, to which were added as described above, 25 c.c. of the chromic solution. To destroy the excess chromic, 10 c.c. of a N/10 ferrous solution were used, for the titration of the excess iron 3.27 c.c. of N/10 permanganate were required; hence the alcohol was oxidised by the equivalent of 10.95 c.c. of the ferrous solution, so that in 250 c.c. of solution there were 109.1 mg. of alcohol (since 1 c.c. of the ferrous solution was equivalent to 0.996 mg. alcohol). The correction for adsorption was 10.3 mg., so that finally the fog contained 98.8 mg. alcohol giving a percentage by weight of 43.6.

Series of experiments similar to those already reported for the other alkaline solutions have been carried out. Some of the results are given in Table 12. It will be noticed that with the weaker solution there is not a great deal of difference in the amounts of fog formed at corresponding rates with the two jets employed; but with the stronger solution the narrower jet gives the greater amount of fog. This has been noticed for the still stronger solution used in Part 2 of these experiments. Also, in the weaker solution, the concentration of acid in the particles approaches much closer to that found with the ordinary aqueous solutions than does that with the stronger solution. In the latter case, the figures are in agreement with those found in Part 2. Owing to the difficulty of preventing liquid from creeping back into the jet, no results could be obtained with the widest jets.

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A peculiarity that was shown by varying the rate of passage of the gas was that although at higher rates the amount of fog was almost proportional to the rate, at the lower rates the amount of fog produced fell off much more rapidly than this straight line law required. The data of the above tables show this.

The fogs produced at each stage of the neutralization of the stronger solution have been analyzed. As found with other solutions, the composition of the particles and of the vapour remains practically constant throughout. The data in Table 13 illustrate this. The composition of the particles and of the vapour remains constant when the rate of passage of the gas is varied as shown by Table 12.

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In connection with these experiments it will be noticed that the percentages of alcohol found in the particles are somewhat greater than those in the original solutions. In view of the work of Kablukow (Zeit. phys. Chem. 1903, 46, 399) it is probable that the composition of the vapour from the alkaline solutions is richer in alcohol than that given by a simple aqueous solution of the same alcohol content. If a correction is applied, assuming that sodium hydroxide will have the same effect on the vapour composition as has potassium iodide, then it is found that the percentage of alcohol to be expected in the liquid in equilibrium with the vapour given off is close to that actually found.

It is well-known that pyridine fumes strongly in the presence of hydrogen chloride, so that it is not surprising that dense fogs are formed when air carrying the acid gas is bubbled through a pyridine solution.

A M/2 solution of pyridinc was used. Experiments on the progressive neutralization of the solution showed that fog formation ceased only after the solution became distinctly acid. This is probably due to the hydrolysis of the pyridine salt allowing vapour to be given off to form a fog until a sufficient acid concentration is built up to repress the hydrolysis. The bubbles rising through the liquid were filled with fog, so that in this case, as with some of the other solutions already mentioned, at least part of the non-absorption of the acid would be due to the slow diffusion of the hydrogen chloride condensates and in accord with the known difficulty of removing suspended matter from a gas by bubbling. The composition of the particles remained constant during the neutralisation of the base. No determinations of the pyridine content of the particles have been made. Some of the data obtained are given in Table 14.

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This pyridine solution was also used in experiments in which the rate of passage of the gas was varied. With both the jets employed it was found that at the higher rates, say, above 110 c.c./min., the amount of fog obtained varied linearly with the rate, but below this rate the formation of fog rapidly decreased. (See Table 14 a). In this case also the narrower jet was the better fog producer. From Table 14(a) it will be seen that the acid concentration in the particles falls as the rate decreases, although the water vapour partial pressures remain constant. In some cases the pressure is greater than corresponds to saturation at the given temperatures, but this is probably due to the temperature of the gas being higher than that of the solution, owing to the latent heat of condensation of the water in the large quantities of fog which are obtained.

5. (a) Sizes of the Particles: The chamber in which the fog was allowed to settle was constructed as follows: An inner cylinder of thin, hard glass was surrounded by a thick outer one, the space between the two being sealed at each end. Wide bore taps were used on the inlet and outlet tubes, in order to eliminate as much as possible contact of the fog with any surfaces with consequent coagulation of the particles. Two narrow slits the length of the cylinder were made in the blackened outside wall; these slits were at right angles, one for the entry of light filtered through a water cell, and the other for observation of the level of the fog column. The space above the fog appeared black, while the fog appeared white, the intensity of the dispersed light being dependent on the density of the fog. Along the last-mentioned slit was placed a calibrated glass scale by means of which the height of the surface of the fog could be read at any time. At each 2.5 cm. the times taken for the surface to fall from the zero mark were read on a stopwatch. The surface of the fog remained level during an experiment, except when the densest fogs were being observed.

The following data have been employed in the Stokes-Cunning-ham equation used for calculating the radii of the particles from their rates of fall: for aqueous, 7.6 and 33.0 per cent. alcoholic, pyridine, and hydrobromic acid solutions, the radius is calculated from r = X √v where X has the values 8.92, 9.00, 9.23, 8.92, 8.40 × 10^-4 respectively, the densities of the respective particles having been taken as 1.04, 1.02, 0.97, 1.04, 1.17.

Experiment showed that the radius was the same at all stages of the neutralization of the solution. But the particles increased somewhat rapidly in size during the time of settling, as the rates of fall were greater near the bottom of the scale than at the top. With the dense fogs the particles appear to be smaller than in the other cases. The radii given in the tables which follow have in such cases been determined from the rate of fall of a separate layer which sometimes appeared, or of the original surface at a later period of the settling process. Table 15 gives the radii of the particles in a number of dense fogs. In the first part of the table the solutions were 0.62N with respect to sodium hydroxide.

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An examination has also been made of the variations of the radii when the colloid, etc., the radius of the jet, and the rate were altered. In general the particles were larger as the concentration of colloid decreased, as the radius of the jet decreased (rate constant), and as the rate decreased (radius of jet constant). When the colloidal solutions or suspensions were used, the radii were distinctly less than with the ordinary aqueous alkali solutions. This may be seen from Table 16 where the data given are selected as being representative of the many obtained; in all the solutions the alkali was 0.62N sodium hydroxide.

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With the alcoholic solutions the particles given by the stronger solution were somewhat the smaller. In both cases the particles grow rapidly with very considerable thinning-out of the fog. This rapid growth has been observed previously by Barus (Carnegie Inst., No. 62, 1907, p. 113). In Table 17, “original” and “final” refer to the radii calculated for the beginning and the end of the readings.

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Hydrobromic acid gave similar results to hydrochloric acid as is shown by the data of Table 18. In all cases the particles are smaller than those obtained under similar conditions with the chloride.

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From the various data obtained, it appeared that the original size of the particles is possibly of the same order as for fogs from other reactions, namely, 5 × 10^-5 cm.: for example, pyridine has given particles of radius 5.0-7.2 × 10^-5 cm., methylene blue of 7 × 10^-5 cm., and Buxton clay of 10 × 10^-5 cm., These have been found to be very unstable, often rapidly increasing to a mean radius of 2 × 10^-4 cm. Whether the comparatively large particles found for the ordinary aqueous solutions have grown from such smaller particles it is not possible to say.

It is to be noted that apparently an equilibrium state is reached by the particles, since the vapour pressures and concentration of acid in the droplets remain constant and independent of the condi-