300 J. Dewar — Boiling Point of Liquid Hydrogen. 



dT= — 0-0266 dP + 0-3548 dP, 



in which dP = — 11 mm. and dP = — 0"5 mm. whence dT = 

 0°-152 ; the observed result is - 252°;683 + 252°-806 or 0°-123, 

 which is also satisfactory and explains how so great a drop as 

 11 mm. in P has, nevertheless, so slight an affect on the result. 

 An alteration in the value of x has but little relative affect 

 on the results. As before we have 



(273 + *)(273+T x ) 

 Cil - l ' (273 + t-aTJ 1 ClX (?) 



If x = 1/50," 2 = 13°, then 



AtT^-180 , dT=-57'085 dx, 



At ^ = — 250°, dT= — 19-4205 dx, 



and for the second thermometer (x — 1/115) in like circum- 

 stances, 



dT= — 57*895 dx. 

 and dT= — 19-802 dx. 



For instance, if x were altered from 1/50 to 1/80 the result 

 would be to raise the boiling point of oxygen by o, 43 and that 

 of hydrogen by o, 15. 



Finally, the alteration of a for any particular gas being in 

 any case small affects the value of T practically only in its 

 main factor T r To hundredths of a degree, therefore, the 

 change in T is inversely proportional to the change in a, or, in 

 other words, is directly proportional to the corresponding 

 absolute zero. 



For instance, in Experiment (No. 11) had we used the same 

 value of a as for hydrogen the boiling point of dry CO a would 

 have been — 79°-35. 



The following table shows what alterations would be required 

 for each of the thermometers, in the values of £, P, P , and x 

 to alter the boiling point of oxygen or that of hydrogen by 

 1/10 or 1/100 of a degree. The table is calculated for t=lS b ; 

 and in the cases of P and P the initial pressure is taken to be 

 about 1/nth of an atmosphere. 



Thus, for example, if the initial pressure in either thermom- 

 eter were about half an atmosphere an error of 1/7 mm. in 

 reading P would alter T by a tenth of a degree. 



If we take the average values given by these experiments as 

 being the most probable, then the boiling point of oxygen is 

 — 182 0, 5 and that of hydrogen is — 252 u *5. The temperature 

 found for the boiling point of oxygen agrees with the mean 

 results of Wroblewski, Olszewski, and others. If the boiling 

 point of oxygen is made — 182°, which is the highest value it 



