out the experiments. The cylinder A was further furnished with a metal stirrer, driven at a constant speed by a small electromotor. The mechanical arrangement of the stirrer is shown in the drawing.

When making a series of observations the insulating substance was first packed in CC, care being taken that the cylinder A was exactly at the centre of BB. A measured quantity (300 cc.) of ice-cold water was then introduced into A, and steam was blown into BB through an orifice D, escaping at the bottom by the tube E. BB was thus kept throughout the experiment at a temperature of 100° C. The stirrer having been set in motion, the thermometer in A was observed at intervals of five minutes. The rise in the temperature of the water in A is evidently a measure of the amount of heat passing through the insulating substance from the steam-jacket in the given interval of time. At first the temperature of A rose slowly, owing to the flow of heat from the steam-jacket being largely employed in heating the insulating substance; but the rate of rise in temperature gradually increased, until after a period varying from forty to sixty minutes it attained a maximum, and then began slowly to diminish. The numbers given in the subsequent tables were, with one or two exceptions, all observed during this latter period, when it was assumed that the temperature gradient was uniform throughout the insulating material. It is, of course, important in making experiments of this kind to allow sufficient time to elapse for this uniform gradient to establish itself. That during the latter part of the experiment there should be a gradual diminution of the flow of heat into A is to be expected, inasmuch as the difference of temperature between the outside steam and the inside water becomes less as the experiment progresses.

The first series of observations were made with pumice, such as is used for insulation on steamers, which was kindly provided by the New Zealand Shipping Company. As it was probable that the insulating-power of any material would be influenced by the size of the grains employed, the pumice was first sifted so as to divide it into the following three grades: Coarse, passed through seven meshes to the linear inch, but not through twenty meshes; medium, passed through twenty meshes, but not through fifty-six meshes; fine, passed through fifty-six meshes. The pumice was not dried, but used as it came from the warehouse.

The following Table I. gives the readings of the thermometer at intervals of five minutes, starting from the moment when the water in the internal cylinder was at 20° C. precisely. The three columns of the table refer to three experiments, one made with each of the three grades of pumice:—

An exactly similar set of observations was then made with charcoal, also supplied by the New Zealand Shipping Company, and of the kind used for insulation. The charcoal was in its ordinary state, and undried; it was roughly crushed in a mortar, and divided into the same three grades, with the sieves used for the pumice. Table II. gives the results:—

From the above tables of figures we learn that when pumice and charcoal are in the form of powder they are not such good insulators as when in larger grains. Thus, in the case of the pumice the temperature of the water in the internal cylinder arose 10.88 deg. in half an hour when surrounded by the coarse material, and 12.32 deg. when surrounded by the powder. In the same way the temperature rose 14.22 deg. in half an hour with the coarsely-divided charcoal and 15.64 deg. with the powder. The difference between the coarse and medium materials is much less marked, but it would appear that the coarse is slightly the best insulator.

The figures in the tables also show that of the two substances used, pumice and charcoal, the pumice was much the better insulator. Thus, while on the coarse charcoal the temperature rose 10 deg. in twenty minutes, it only rose 7½ deg. with the coarse pumice.

It has already been mentioned that the rate at which the temperature increases gradually diminishes throughout each series. This is noticeable in every case. Thus, in the case of the coarse pumice the rise in temperature during the first five minutes was 2.02 deg., and during the last 1.50 deg. It has been explained that this is due to the difference in temperature between the outside steam and the inside water being less at the end than at the beginning, or, in other words, the temperature gradient across the insulator becomes less as the experiment goes on. For example, with the coarse pumice the difference between outside and inside was 80 deg. at the beginning and 64.5 deg. at the end. The heat-flows should then be in this proportion, which is very nearly 4:3, which is seen to be the case. The fact that the diminution is slightly greater than that which should take place theoretically is probably due to the increased evaporation of the water at the higher temperatures, by which, of course, its temperature is rendered slightly lower than it would be if no evaporation took place. This evaporation it is difficult to avoid.

After each of the experiments with charcoal had been completed, the internal cylinder, on being removed, was found to be wet on the outside, and the charcoal in the neighbourhood of the surface was found to be in a like condition. This was obviously due to the moisture in the charcoal having been distilled, as it were, from the hotter portions to the cooler. This distillation was necessarily accompanied by a transfer of heat; so that it was clear that the flow of heat across the charcoal was partly due to the moisture, and the same may have been the case with the pumice, although, with the latter substance, there was no appreciable moisture on the cylinder.

In order to avoid the errors introduced by the presence of

moisture, the pumice and charcoal were both carefully dried by heating on a metal plate for several hours, and then allowed to cool in closed vessels, so that they could not absorb water from the air. Series of observations were then made with the dry substances, exactly as before. The coarse material only was employed in each case, as it had been found that when finely divided it was less efficient. The results are given in Table III. Experiments were also made with dried slag-wool, a material largely used for covering boilers and steam-pipes, and which was furnished to me by the Railway Department through the kind intervention of Mr. Macdonald. The results with slag-wool are included in Table III.:—

The numbers in this table show that, when carefully dried, charcoal is a better insulator than pumice, and that slag-wool is better than either. Thus, at the end of half an hour, or six periods of five minutes, the rise in the temperature of the water was—With charcoal, 9.04 deg.; with pumice, 10.59 deg.;