PEIRCE AND WILLSON. — THERMAL CONDUCTIVITIES. 47 



Some of our thermopiles have been cahbruted for us at temperatures 

 between 0" C. aud 100° C. by Mr. C. G. Tersous of the staff of the Jef- 

 fersou Laboratory, aud he has assisted us in much of our other work. 



Thk Thermal Conductivity of Marble. 



With the apparatus described in this paper we have made a large 

 uumber of experiments. As has beeu already intimated, we are not 

 entirely satisfied with the source of heat that we have used for temjjera- 

 tures higher than 300° C. because of the ditliculty of keeping these 

 temperatures constant for long intervals of time, while for tempera- 

 tures between 0° C. and 100° C, it has been easy to get closely agree- 

 ing results many times over. We have, nevertheless, made a good 

 many determinations at the higher temperatures, aud, while we are not 

 yet ready to state definitely the law of variation with the tempera- 

 ture of the thermal conductivities of materials in which we have found 

 such variations, we may say that, of the substances which we have ex- 

 amined, two, a special brand of glass of which we have a number of 

 large plates, and dry white marble,* show no appreciable change in 

 thermal conductivity within the limits of our measurements. We shall 

 therefore content ourselves in this preliminary paper with giving the 

 results of a number of determinations, made at different low tempera- 

 tures, of the conductivities of about twenty specimens of marble of 

 different kinds. Incidentally we shall need to describe very briefly 

 some experiments upon the glass plates just mentioned. 



It will appear that the conductivity of a specimen of marble at 

 ordinary mean temperatures may de^jend to the amount of several per 

 cent, as Messrs. Herschell and Lebour have shown, upon the amount 

 of moisture which the specimen holds. For this reason we have aimed 

 at an accuracy of only 1 % in the determinations here recorded. A change 

 in conductivity much less than this was of course easily observable. The 

 difference of temperature between two thermopiles, one of which is only 

 a few degrees hotter than the other, can be measured with considerable 

 accuracy, but it will be sufficient here to state the results correct to tenths 

 of degrees. 



* The conductivity of the specimen of marble upon wliich R. Weber has made 

 a set of extremely accurate measurements appears to change by only one two- 

 thousandth part of its own value between 0° C. and 100° C. 



