44 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In most of our experiments with the large apparatus just described, a 

 sufficiently steady state has been attained in about five hours from tlie 

 beginning of the heating. Sheets of blotting paper were generally in- 

 serted 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. We 

 have tried several different materials, and of these the blotting paper 

 is the most satisfactory. We may note in passing, however, tbat the 

 indications of thermal couples placed between soft jiads and the hard 

 prisms are often very anomalous, two thermal junctions placed side by 

 side sometimes differing very widely. In all the experiments that we 

 regard as trustworthy the slab to be tested with its attendant thermopiles 

 was placed between two other slabs of the same material^ in forming the 

 prism. 



Most of our mercury thermometers were made by Alvergniat, or by 

 Richards & Co., but our final standard was Tonnelot No. 11,142, upon 

 which a very complete set of tests bas been made at the International 

 Bureau of Weights and Measures. 



For temperatures higher than 100° C. we had two platinum ther- 

 mometers of the general form described by Messrs. Griffiths and Cal- 

 lendar. These served an excellent purpose, though the wire, about 

 0.2 mm. in diameter, seemed from the form of the calibration curve 

 not to be very pure. The resistance of one of them, as measured by 

 a Carey Foster Bridge was about 29.25, 3G.78, 42.85, 45.31, or 55.43 

 ohms, according as it was immersed in melting ice or the vapor, at 7G0° c.c. 

 pressure, of water, anilin, naphthalin, or mercury. We have another ther- 

 mometer made of pure platinum wire furnished by Messrs. Johnson and 

 Matthey, 0.005 inch in diameter. This we intend to make our standard. 



All our thermal elements were made either of platinoid and copper, or 

 of German silver and copper; some were of wire, and some of narrow 

 ribbon carefully rolled for our use. Each specimen of platinoid or 

 German silver was " butt-jointed," generally by silver solder, to a piece 



