On the Atomic Weight of Glucinnm. 



15 



Appendix B. 



Note on Siemens' Pyrometer. 



The following are the details of construction and graduation of the 

 pyrometer employed in the vapour-density determinations. About 

 100 cm. of thin platinum wire (0*13 mm. diameter) was coiled round 

 a thick piece of ordinary clay tobacco-pipe, and three thicker 

 platinum wires were soldered with gold to the two ends. To these 

 thicker wires were soldered three copper wires, which went to the 

 bridge where the resistance was measured. The whole was enclosed 

 in a porcelain tube, one end of which had been closed by a plug of 

 clay, and the wires were so arranged that the junctions between the 

 copper and platinum came at the same height, just within the tube. 

 By suitably arranging the three wires and a known resistance in the 

 arms of the bridge, measurements could be made of the resistance of 

 the thin coil without that of the leads, so that any change in resist- 

 ance was due to the altered resistance of the coil alone. The wire of 

 which the coil was made had previously been heated several times to 

 redness. 



In order to graduate the instrument it was immersed in melting ice, 

 boiling water, aniline, diphenylamine, sulphur, selenium, and zinc. 

 For boiling these substances (with the exception of water) iron tubes 

 welded together at one end, of 5 cm. diameter and 46 cm. length, 

 were used. The upper portion of the tube for about 8 cm. was sur- 

 rounded with a coil of thin lead tubing, through which a stream of 

 cold water was allowed to flow. This acted as a very efficient con- 

 denser, and kept the upper part of the tube cool enough to handle 

 easily. The results obtained are expressed in the following table, in 

 which the first column gives the substance, the second its melting 

 or boiling point, and the third the actual resistance found. In the 

 fourth, column the resistances are reduced to that at 0° = 1, and in the 

 fifth are given the corresponding resistances, calculated from the 

 empirical formula — 



r t = 1 + 0-0027* - 0-00000019^. 



Substance. t. r f . r = 1. Calculated. 



Ice 0° 13-42 1-000 1-000 



Water 100 17'02 1*268 1-268 



Aniline 184 20*08 1*496 1*493 



Diphenylamine.. 310 24*43 1*819 1*819 



Sulphur 448 29 14 2*171 2*172 



Selenium 665 36*08 2*688 2*712 



Zinc 940 45*38 3*381 3*370 



