Liquid Hydrogen for the Production of High Vacua. 231 



this result ? Has the platinum resistance thermometer arrived at a- 

 limiting resistance about 35 aba., so that at a lower temperature it 

 refuses to change in resistance, the curve having become practically 

 asymptotic to the axis of temperature *\ On the other hand, has the 

 influx of heat by the leads, and the correction on account of this 

 change of resistance, become so great as to vitiate the results at these 

 excessively low temperatures 1 Again, it may be suggested that the 

 thermometer was not properly cooled, or that the liquid hydrogen does 

 not lower in temperature to any marked extent under exhaustion like 

 other liquids. All these conjectures can only be set at rest by a repeti- 

 tion of the experiments with a new thermometer of much higher initial 

 resistance, and under conditions of better heat isolation. No blunder 

 having been detected in the observations, for the present we must 

 assume that the platinum resistance thermometer No. 7 acts in the 

 manner described. It would be premature to discuss the inferences to 

 be drawn from these results until they are confirmed qn another 

 variety of platinum wire made into a resistance thermometer. But as 

 this will involve the use of considerable quantities of liquid hydrogen, 

 it will take some time to complete the investigation. 



The same kind of anomaly appears in the case of the use of a thermo- 

 j unction at these low temperatures, but this is a separate matter, and 

 must be dealt with in a further communication. 



I am indebted to Mr. J. E. Petavel for assistance in the electrical 

 measurements, and also to Mr. Robert Lennox and Mr. Heath for 

 their general help in the conduct of the experiments. 



" Application of Liquid Hydrogen to the Production of High 

 Vacua, together with their Spectroscopic Examination." By 

 JAMES DEWAR, M.A., LL.D., F.R.S. Received and Read 

 December 15, 1898. 



As an illustration of the extraordinary power of the new cooling 

 agent liquid hydrogen, the extreme rapidity with which high vacua 

 can be produced by its use is, perhaps, one of the most striking. The 

 absolute boiling points of hydrogen, oxygen, and chlorine are respec- 

 tivety 35, 90 and 240, in other words oxygen boils at a temperature 

 two and a half times higher than liquid hydrogen, and liquid chlorine 

 similarly at two and a half times that of liquid oxygen. From 

 this we infer that liquid hydrogen as a cooling agent ought to be rela- 

 tive to liquid air as effective as the latter is compared to that of liquid 

 chlorine. Now chlorine at the temperature of boiling oxygen is a hard 

 solid, some 80 below its melting point, and in this condition has an 

 excessively feeble vapour pressure. When liquid hydrogen freezes 

 air out of a sealed tube by immersing the end in the liquid, it is to be 



