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:—