228 Prof. J. Dewar. On the Boiling Point of 



to Normal Air Temperature of the Platinum Thermometers," used in 

 the low temperature researches of Professor Fleming and the author, has 

 been good enough to calculate a special formula for this thermometer 

 No. 7. He finds the formula 



(R + 43-95S933) 2 = 2-03596488 (t+ 1193-1460) 



expresses the relation between the resistance and temperature in 

 centigrade degrees. This expression gives a probable error of only 

 0-16 C. over a range of more than 300 C. When this thermometer 

 was placed in boiling hydrogen, the resistance became 0-129 ohm, and 

 remained constant at this value. Calculated into the Dickson formula, 

 this value of the resistance corresponds to a temperature of - 238'4 C. 

 If we assume the resistance reduced to zero, then the temperature 

 registered by the thermometer ought to be - 244 C. At the boiling 

 point of hydrogen, therefore, if the law correlating resistance and 

 temperature can be pressed to its limits, a lowering of the boiling point 

 of hydrogen by 5 or 6 C. would produce a condition of affairs where 

 the platinum would have no resistance, or become a perfect conductor. 

 Now we have every reason to believe that hydrogen, like other liquids, 

 will boil at a lower temperature the lower the pressure under which it 

 is volatilised. The question arises, how much lowering of temperature 

 can we practically anticipate. For this purpose we have the boiling 

 point and critical data available from which we can calculate an ap- 

 proximate vapour pressure formula, accepting 35 abs. as the boiling 

 point; 52 abs. as the critical temperature, and 19'4 at. as the critical 

 pressure ; then as a first approximation 



1 Q*7'Q 



log^ = 6-8218 - L ^ mm (1). 



If instead of using the critical pressure in the calculation we assume 

 the molecular latent heat of hydrogen is proportional to the absolute 

 boiling point, then from a comparison with an expression of the same 

 kind, which gives accurate results for oxygen tensions below one 

 atmosphere, we can derive another expression for hydrogen vapour 

 pressures, which ought to be applicable to boiling points under reduced 

 pressure. 



The resulting formula is 



1 P9'7 



log^> - 7-2428 - ^L mm (2). 



Now formula (1) gives a boiling point of 25 '4 abs. under a pres- 

 sure of 25 mm., whereas the second equation (2) gives for the same 

 pressure 26'1 abs. As the absolute boiling point under atmospheric 

 pressure is 35, both expressions lead to the conclusion that ebullition 



