1904] Physical Constants at Low Temperatures. 



251 



If parabolas with vertical axes (Callendar) be described through the 

 points (1) B, C, D; (2) C, D, E; (3) C, D, F, they will be different, 

 since the curve of observation is not itself parabolic. In the case of 

 these parabolas, where the resistances at C and D are common to all, 

 the product of 8 and the parameter (P) of the corresponding parabola 

 is constant ; hence the smaller 8 becomes, the more open — or less 

 curved — will be the parabola along the arc we have to deal with. This 

 result is in accordance with the curve in fig. 2, and explains why, as I 

 have already pointed out, the two parabolas for high and low 

 temperatures are not only different, but also may differ from each 

 other by any amount, within certain limits depending on the nature of 

 the unknown curve of temperature and resistance. 



I am greatly indebted to Mr. J. D. H. Dickson, M.A., Fellow of 

 Peterhouse, for help in the calculation and reduction of the observa- 

 tions. 



" Physical Constants at Low Temperatures. (1) — The Densities 

 of Solid Oxygen. Nitrogen, Hydrogen, etc." By Professor 

 James Dewar, M. A., LL.D., D.Sc, F.E.S. Eeceived March 9, 

 — Eeacl March 17, 1904 



1. The following experiments on the solid densities of oxygen, 

 nitrogen and hydrogen were carried out as part of a former investiga- 

 tion dealing with gaseous densities at low temperatures.* 



The method adopted was to measure the volumes of the gases 

 sucked into a cooled space of known capacity, when the temperature 

 was such as in the first place to induce liquefaction and finally 

 solidification. For such experiments to be successful the rate of 

 liquefaction and the cooling must be under thorough control, other- 

 wise the cooled space may not get completely filled with solid. 

 Further, the volume of gas condensed ought to be as large as possible, 

 in any case about 20 litres, in order to diminish errors inseparable 

 fr6m the mode of manipulation. The inertia of the bell-jars of the 

 large gas-holders causes some variation in the pressure ; and errors of 

 their calibration, and want of uniformity of temperature in the mass 

 of gas, are all important factors. As my object was to ascertain 

 experimentally the limiting density in the solid state, the elimination 

 of these variations was not so important as it would have been for the 

 study of fluid density. 



The dry purified gas was contained in a gas-holder connected by a 

 pipe with a glass bulb of 20 or 30 c.c. capacity, sealed to a narrow 

 tube some 10 cm. long, with a glass stop-cock at the end. The 



* " The Specific Volumes of Oxygen and Nitrogen Yapour at the Boiling Point 

 of Oxygen," ' Roy. Soc. Proc.,' vol. 69. 



