PETRCE. — THERMAL CONDUCTIVITIES OP CERTAIN ROCKS. 657 



The rotation of A" and of the inside ice-holder, Q, which is connected 

 with K at the top by means of a yoke, were matters of much importance. 

 The continual rubbing of the ice over the flat surface of the casting seems 

 to be necessary if the upper face of the prism is to be kept at a uniform 

 constant temperature for hours. The heat produced in rotating Q slowly 

 was so little as to be quite negligible. The ice in K was piled up so as 

 to cover P completely, and I was unable to detect any difference between 

 the temperatures within and without P by fine, properly protected ther- 

 mal junctions introduced for the purpose. If while K revolved, Q was 

 kept still, the amount of ice melted in Q became irregular, though the 

 whole amount of drip in two or three hours was not very different from 

 the amount of steady drip in an equal time when Q was rotating. Only 

 selected lumps of ice were put into Q. The ice to be used was first 

 broken up into pieces weighing something like fifteen grams each, by 

 means of an ice-cracking machine, and these pieces were then put into 

 ice-water so that their sharp edges might become slightly rounded. They 

 were then drained and dropped into Q. In this way a slight amount of 

 water attached to the ice was introduced, but with the method employed, 

 the error due to this cause appeared to be of slight importance. In some 

 experiments the ice to be used was carefully dried in cold blotting-paper, 

 but this precaution did not seem to be necessary if the use of small bits 

 of ice with sharp edges was avoided. Q's capacity was about two thou- 

 sand cubic centimeters. After Q had been freshly filled in the course 

 of any experiment while K was rotating, no record was kept for some 

 time, perhaps fifteen minutes, of the amount of drip. Before the expi- 

 ration of this interval the extra water introduced into Q with the ice 

 had drained off, and the indications had become steady. The drip 

 tube always contained a few drops of water, but this amount remained 

 sensibly constant during the progress of an experiment. The drip was 

 collected in a graduated vessel and the approximate amount was noted 

 from time to time to see whether the flow was steady. The whole was 

 then more accurately determined by weighing, at longer intervals. The 

 regularity of drip seemed to be a far more sensitive test of the approxi- 

 mate attainment of the final state of the body experimented on and its 

 surroundings than was a sensibly constant temperature gradient on the 

 axis. In most experiments with the apparatus a sufficiently steady state 

 was attained in about seven hours from the beginning of the heating. Z 

 and K together weighed when filled with ice about three hundred kilo- 

 grams, and many hundreds of pounds of cracked ice were needed for a 

 single day's experiment. The area a of the bottom of the small ice-pot 

 vol. xxxvm. — 42 



