50 Prof. J. Dewar. The Boiling Point of Liquid Hydrogen, 



occurs in the gas derived from the Bath wells. A sample of helium 

 prepared as above described, which had been passed over red-hot 

 oxide of copper to remove any hydrogen, was found by Lord Bayleigh 

 to have a refractivity of 0*132. The refractivity of Ramsay's pure 

 helium being 0-1238, and that of neon 0*2345, it results that my 

 helium contained some 7-4 per cent, of neon, according to the refrac- 

 tivity measurements. This would make the partial tension of the 

 neon in the helium thermometer cooled in the liquid hydrogen to be 

 about 4 mm., and this being taken as the saturation pressure the boil- 

 ing point of neon is about 34° absolute. The initial pressure (No. 9) 

 was taken rather less than an atmosphere, and the temperature of the 

 boiling point of hydrogen was given by this thermometer as - 252° - 68. 

 A further observation (No. 10) was taken on another occasion with the 

 same thermometer, and the value found was - 252° '84. The fact that 

 the boiling point of hydrogen, as determined by the helium thermo- 

 meter, is in substantial agreement with the results obtained by the use 

 of hydrogen itself is a conclusive proof that no partial condensation 

 of the neon had occurred. 



Of the remaining experiments in Table I, (No. 3) was made in order 

 to show the effect of a very small initial pressure, one-sixth of an 

 atmosphere. The results were unsatisfactory, owing to the sticking of 

 the long column of mercury giving uncertain pressure readings. In 

 this case an error in the reading of a low pressure has six times as 

 great an effect as if the initial pressure had been about an atmosphere. 

 If the temperature deduced for the boiling point of oxygen is corrected, 

 and the same factor of correction applied to the observed liquid hydro- 

 gen boiling point, then it becomes - 251° '4. 



It is of particular moment to have some estimate of how far errors 

 in the observed quantities employed in Chappuis' formula affect the 

 final value of T. 



In the case of an error in t, on differentiating equation (2) we get 



di - 11 (273 + f- sT!)*** W ' 



If x = 1/50, t = 13°, Ti = - 180° : then dT = 0*00339^, or it would 

 need an alteration of 2J° in t to alter T by 1/1 00th of a degree at the 

 boiling point of oxygen. In the same circumstances when T x = - 250, 

 dT = 0-00136 dt, so that an alteration of between 7° and 8° in the 

 value of t would only affect the boiling point of hydrogen by 1/lOOth of 

 a degree. 



From equation (4) the error in T varies with x very nearly. Thus 

 for the second thermometer where x = 1/115, a variation of t to the 

 extent of 6°, would only affect the boiling point of oxygen b}^ 1/1 00th 

 of a degree; and it would require an alteration of 17° in t to affect 

 the boiling point of hydrogen to the same extent. 



