Platinum Thermo metry at Kc,w Obseri:ati>r;/. 37 



in escaping with no damage except to his clothes. In consequence of 

 this defect, we have had to employ about a score of tubes, and these 

 have not been strictly uniform in length, thickness, or diameter. We 

 have also had to employ a considerable number of small asbestos cones, 

 of the pattern recommended by Mr. Heycock and Mr. Neville, and the 

 tightness with which these have fitted the tubes has varied sensibly. 

 Thus the sulphur point observations have been taken under more vari- 

 able conditions than those at the steam point. 



We have not indeed observed any certain consequences to follow any 

 of the minor changes described above, but more careful experiments 

 would be necessary to justify the conclusion that the variations are 

 immaterial. I am, in fact, inclined to think that variations in the 

 length of the asbestos cones, or in the tightness with which they fit the 

 tube, are sufficiently probable sources of uncertainty to deserve investi- 

 gation. 



Error in the Formula assumed far the Variation of the Boiling Point of 

 Sulphur with Pressure. 



35. In another direction there is, I think, little doubt that appre- 

 ciable error exists, viz., in the reduction of sulphur point observations 

 to the standard barometric pressure. Professor Callendar and Mr. 

 Griffiths accept a simple linear relation 



Af = (p - 760) x 0-082 (10), 



where A is the increment in the temperature of sulphur vapour, on the 

 air scale, when the barometer is p mm. instead of 760. This is based 

 on some experiments by Regnault, who examined a wide range of pres- 

 sure, but had so few experimental points that interpolation is very 

 uncertain. 



A priori, having regard to the corresponding phenomenon in steam, 

 one would hardly anticipate great accuracy from a linear relation, 

 unless restricted to a very limited range ; while in ordinary every-day 

 use we must be prepared to cover at least a range of 30 mm. The 

 Kew experiments, having been taken without any special regard to the 

 atmospheric pressure, naturally cover a fairly wide range, and thus 

 afford a favourable opportunity of testing the accuracy of (10). To 

 this end I have carefully reduced all the sulphur point experiments in a 

 uniform way. 



Assuming Callendar's formula 



t-pt = 8[(</100) 2 -(*/100)], 

 we may, near the sulphur point, conveniently give it the form 



pt-pt, = (t-ts) [1-100^3(24 -100)] -I00-*8(t-t t y (11), 



