the Boiling Temperature of Hydrogen. 207 



measured with the hydrogen thermometer, I successively- 

 plunged the apparatus into melting ice, into a mixture of 

 solid carbonic acid and ether, and into liquid oxygen boiling 

 under atmospheric pressure and at a diminished pressure of 

 15 millim. ; and I thus determined the resistances of the 

 platinum thermometer at these low temperatures which cor- 

 respond to 0°, -78°'2, -182°-5, and -208°'5 respectively, 

 according to the hydrogen thermometer. It is to be remarked 

 that the temperature of the carbonic acid mixture ( — 78°*2) 

 is constant only when the carbon dioxide is in excess, and 

 when it has the consistence of butter : as soon as the ether 

 begins to appear separately at the surface, the temperature 

 gradually rises. 



The variations of all \h.Q three platinum thermometers I used 

 were quite proportional, and, calculated for 1000 ohms of 

 resistance at 0°, were as follows : — 



Temperature according- to the Resistance of the Platinum 



Hydrogen Thermometer. Thermometer. 



0° 1000 ohms. 



- 78°-2 800 „ 



-182 a 5 523 „ 



208 a 5 453 



>? 



Thus for each 1° of the hydrogen thermometer between the 

 limits 



from 0° to - 78°-2 we have a fall of 2'557 ohms. 



„ - 78°-2 „ -182°-5 „ „ 2-655 „ 



„ -182°-5 „ -208°-5 „ „ 2-692 „ 



This last number, viz. 2'692 ohms =1°, I used for the 

 extrapolation in order to measure temperatures lower than 

 — 208 o, 5, at which the platinum temperature could not be com- 

 pared with that of hydrogen. Assuming that the variation of 

 the resistances is directly proportional to the variation of the 

 temperatures below — 208 o, 5, I committed an apparent error, 

 which, nevertheless, cannot be great on account of the feeble 

 curvature of the line of resistances ; for the temperatures 

 extrapolated were not far distant from — 208°* 5. 



To determine the resistance of the platinum thermometer 

 at the moment of the expansion of hydrogen from a high 

 pressure, I brought into the steel vessel a, cooled by means 

 of liquid oxygen (at 15 millim. pressure), hydrogen under a 

 pressure of 120 to 160 atm. ; in the resistance-box I diminished 

 the resistances, by which means the equilibrium was destroyed, 

 and the galvanometer marked a considerable deflexion. 



