[Read before the Wellington Philosophical Society, 29th August, 1874.]
Having announced, in November, 1871, * that clay-slate hydrates when in contact with water, as shown by the coagulation test devised by me, I have been desirous to obtain corroborative evidence in support of this statement, and so prosecuted the matter further; and being under the impression that the hydration of this substance would evolve heat, I ground some clay-slate to a very fine powder and put it in a small quantity of distilled water, when an elevation of temperature occurred in the mixture equal to about 2° F. above the temperature of the materials used.
To make sure that none of this rise was due to heat generated during the act of crushing, and retained in such a manner that the exact temperature of the powdered substance could not be ascertained, I repeated the experiment, but with this variation—the crushed slate was bottled off and not used till twenty-four hours afterwards. The thermometric results were the same.
The water after the mixing was slightly alkaline, owing no doubt to the presence of a minute quantity of alkalies derived from the slate, to the hydration of which or their compounds a part of the elevation of temperature noticed might have been due. To obtain, therefore, some indication of the quantity of this, I crushed some glass to quite as fine a powder as I had the slate instanced, allowed it twenty-four hours to cool (as I may state here I have for all succeeding experiments upon substances dried by heat), and then mixed it with water in same proportions as observed for the slate, when an elevation of temperature barely equal to 1° F. occurred. The water was of course strongly alkaline directly after the mixing; clearly, therefore, the alkalies of the slate had no important share in producing the elevation of temperature observed.
At this stage I struck aside a little to experiment upon coal, clay, and other substances, in the hope of obtaining facts extending over a wider field, and so capable of being handled more correctly and with greater ease, and I found carbonate of lime, as also quartz and brown coal or clay, did not give any indication of an evolution of heat when mixed with water; steatite, however, and anhydrous coal, also hydrous coal, clay, and lignin, when dried gently or wholly, did so. Steatite had as much heating power as slate, while all the remaining substances cited were superior to it in this respect. In the case of naturally anhydrous coal, or hydrous coal and clay dried at from 90° to 212°, or over desiccating substances (at common temperatures), I frequently obtained a rise of temperature from 3° to 6° F.
[Footnote] * Proceedings of New Zealand Institute, vol. iv., p. 381.
These results tend to show that, as a general rule, any so-termed hygroscopic substances, when deprived of even the smallest portion of their water, and then allowed to regain this, generate heat in doing so. It will at once appear that, if in place of submitting water to these dried substances we submit aqueous vapour, the evolution of heat would be much greater, and in fact I find that these substances desiccated and exposed to common air rise in temperature very notably.
It only remains now to consider how much of this elevation of temperature is due to mechanical, and how much to chemical agency.
It is obvious a portion of this is due to friction, occasioned by the rapid inrush of water to the pores of these substances. I believe the calorific effect of such inrushes has not yet been measured, if indeed noticed, before, and in all likelihood they will be deemed so small as to be barely perceptible, or at most not at all necessary to take into account in determining the origin of the increase of temperature instanced; but as I felt anxious to get the approximate calorific result due to chemical agency, I have made an attempt to effect this by comparative tests.
In this attempt I substituted other liquids for water, liquids which, experimentally, I found had no action upon the solids used, or only to a very minute extent.
My results were as follows:—
Clay-slate mixed with water raised a thermometer placed in the mixture 2° F. above the temperature of these substances, at the time of mixing them. A portion of the same sample of clay-slate, and in quantity as before, mixed with pure kerosene in same volume as that of the water used, only raised the thermometer 1 1/4° F.
Dried brown coal, similarly treated, gave an elevation of temperature equal to 4°; F. with water, and only 2° F. with oil. The same conditions were observed as to quantities and volumes as in the first experiment.
As the kerosene used would have a less specific heat than water, the amount of heat due to friction in the first experiment would not be so much as 1 1/4°, and in the second experiment not so much as 2°, leaving a balance of heat equal to something more than 3/4° and 2° F. for the slate and coal respectively, which balance is, I conclude, due to chemical action; and as I believe kerosene is more diffusive than water, and so would rush these substances with greater rapidity than water would, thus producing more friction, the balance of heat found is, perhaps on this account, again less than the actual amount due to chemical action.
Returning now to the subject I started with, the supposed direct hydration of clay-slate by water, and bringing the results just stated to bear upon this question, it certainly appears that this substance evolves heat when mixed
with water, which heat is the result of chemical action, and the only agency to which I can attribute this chemical action is hydration.
Thus the statement hazarded as to the direct hydration of clay-slate by water receives support upon the ground I have just traversed.
Applying now the facts just elicited in a general manner, it appears—
That in the disintegration of rocks or soils heat is evolved.
That the differences in temperature sometimes observed between contiguous strata may be due, wholly or partly, to this cause.
That our native anhydrous coals hydrate upon their surfaces when exposed to water or aqueous vapour.
That hygroscopic water is chemically combined water.
That the quantity of water present in certain rocks or minerals may, when known, frequently indicate the highest temperature to which they have been subjected.
That vegetable matter (leaves, twigs, etc.) generally developes heat by hydration; also by friction, when the temperature of the air surrounding it is lowered.
In regard to these statements it requires, in the case of (3), gravimetrical experiments to support it, which I shall presently endeavour to obtain, and if they should prove it a correct one, that is, that anhydrous coal can hydrate, and to any notable extent, it will certainly appear that this substance has been formed at a somewhat elevated temperature, perhaps approaching to nearly 100° C.
While upon the subject of the formation of coal, I would just like to observe here, that I cannot avoid thinking the effects of pressure in consolidating this, and indeed other minerals, also rocks, are considered much greater than they really have been, and this because it seems that these substances will generally, if not always, be charged with water, oil, or gas, and, if this is so, I conceive the consolidating action of pressure would be very greatly mitigated, and would be in some proportion to its actual volumetrical effect upon the liquid receiving it. I cannot see how particles suspended in, or throughly soaked with a liquid, can be made to approach each other by pressure, except by allowing the liquid to escape, and it does not appear, in the case of rocks, etc., at some depth, that there can be any such way of escape, at least a sufficiently ready one, for the liquids or gases lodged in their pores.
In reference to the statement (4) that the hygroscopic water of substances retaining it is combined water, this appears certain from what has been described, and also from other considerations. Thus, to take an analogous case, the salt chloride of silver forms a definite crystallizable chemical compound with ammonia, but, though acknowledged as such, it can only be preserved in an atmosphere of ammonia, and if this gas is taken away as
evolved, the mineral loses the whole of it, even at common temperatures. Now, I conceive water has relations to the so-termed hygroscopic substances, similar to those ammonia sustains to argentic chloride; it chemically combines with these substances, and the compounds thus formed are not permanent, except, at a temperature not higher, and a tension of aqueous vapour not less, than that at which they were produced.
Lastly, as to a production of heat by the chemical combination of water with the substances of plants, a combination brought about by changes in the temperature of the air, or in the tension of its vapour, this is no doubt a very useful provision for preventing atmospheric changes of this nature affecting the vegetable world abruptly. *
[Footnote] * As I have since ascertained that heat is also evolved during the hydration of anhydrous wool, it is probable that we have in this a provision of the same nature for use in the animal world too.—W. S., 23rd April, 1875.