Thermal Conductivity of Vulcanite. 25 



in diameter. When set in motion by a slight push, it con- 

 tinues to rotate for about a minute before coining to rest. 

 K is so truly hung that the outside can be used as a pulley, 

 and the whole can be rotated by the use of the belt shown in 

 fig. 1. The vanes reach to about 2 millim. of the floor of 

 the box, and when the whole is filled with cracked ice and 

 then rotated, the ice at the bottom which rubs on Z soon gets 

 nnd holds a very smooth surface. A hole in the bottom of Z 

 carries away the drip and prevents any accumulation of water 

 on the floor of the ice-box. We were at first troubled by 

 irregularities arising from honeycombing of the ice in the 

 ice-box; and to remedy this a suitably loaded brass tripod is 

 used to pack the ice by light blows delivered at intervals of 

 21 seconds by the aid of the lever L. A train of four wheels 

 is necessary to reduce the speed of K to one revolution in 

 20 seconds, though only two wheels are shown in the 

 drawings. The tripod slides in guides which revolve with K, 

 and a swivel at the top prevents the cord from twisting. 



The rotation of K and of the inside ice-holder, Q, which is 

 connected with K by a thin yoke, are matters of much im- 

 portance. The continual rubbing of the ice over the flat 

 surface of the casting seems to be necessary, if the latter is to 

 be kept at a uniform constant temperature for hours. The 

 heat produced in rotating Q is so little as to be quite negligible. 

 The ice in K is piled up so as to cover P completely: and we 

 have been unable to detect any difference between the tempe- 

 ratures within and without P by fine, properly protected 

 thermal junctions introduced for the purpose. If, while K 

 revolves, Q is kept still, the amount of ice melted in Q becomes 

 irregular, though the whole amount of drip in two or three 

 hours is not very different from the amount of steady drip in 

 an equal time when Q is rotating. Only selected lumps of 

 ice are put into Q. The ice to be used is first broken up into 

 pieces weighing something like 15 grams each, by means of 

 an ice-cracking machine, and these pieces are then put into 

 ice-water so that their sharp edges may become slightly 

 rounded. They are then drained and dropped into Q. In 

 this way a slight amount of water attached to the ice is intro- 

 duced into Q ; but the error due to this cause appears to be of 

 slight importance. In some experiments the ice to be used 

 was carefully dried in cold blotting-paper ; but this precaution 

 does not seem to be necessary, though the use of small bits of 

 ice with sharp edges is to be avoided. Q's capacity is about 

 2000 cub. centim. After Q has been freshly filled in the 

 course of any experiment while K is rotating, no record is 

 kept for some time, perhaps fifteen minutes, of the amount of 



