26 Prof. B. Osgood Peirce on the 



drip. Before the expiration of this interval the extra water 

 introduced into Q with the ice has drained off, and the indi- 

 cations have become steady. After this the apparatus is 

 allowed to run for about two hours until 300 grams of ice, or 

 less, has been melted, and then Q is refilled. The drip-tube 

 always contains a few drops of water, but this amount remains 

 sensibly constant during the progress of one experiment. 

 The drip is collected in a graduated vessel, and the approxi- 

 mate amount is noted from time to time to see whether the 

 flow is steady. The whole is then more accurately determined 

 by weighing, at longer intervals. 



The regularity of drip is a far more sensitive test of the 

 approximate attainment of the final state of the body experi- 

 mented on and its surroundings than is a sensibly constant 

 temperature gradient on the axis. In most experiments with 

 the apparatus just described, a sufficiently steady state has 

 been attained in about seven hours from the beginning of the 

 heating. Sheets of blotting-paper were generally inserted 

 between the prism to be tested and the hot and cold boxes, to 

 serve as elastic pads, and to prevent the possible wetting of 

 the edge of the prism by moisture condensed on the ice-box. 

 The presence of this paper prolonged the time of waiting for 

 the final state to be attained, but did not influence the results 

 of the measurement of the conductivity of the prism. When 

 filled with ice, Z and K weigh about 300 kilograms, and the 

 additional pressure due to the bolts is considerable ; so that, 

 when the prism is made up of brittle material like glass, the 

 blotting-paper or an equivalent must be used to prevent the 

 prism from injury. Since hard rubber becomes very soft at 

 temperatures above 100° C, it did not seem worth while to 

 test its thermal conductivity at high temperatures ; and it is 

 not necessary to give any account here of the apparatus 

 intended for use at temperatures as high as the boiling-point 

 of sulphur. 



After the absolute conductivity of a particular specimen 

 has been carefully determined, between various pairs of tem- 

 perature-limits, in the apparatus just described, the conduc- 

 tivity of any other specimens can be easily obtained in the 

 same apparatus merely by determining the temperature- 

 gradient in the final state on the axis of a prism built up of 

 the slab already tested, and the slab to be examined, with 

 their attendant thermopiles and such other thin slabs as may 

 be conveniently used. By varying the order of the slabs on 

 different occasions, the temperatures at the faces of the slab to 

 be examined can be altered, it being always understood that 

 the thermal elements must be placed between slabs of approxi- 



