determined by Hydrogen and Helium Gas Thermometers. 53 



as possible. Hence Nos. 4, 9, 10 are in this respect the most reliable 

 for hydrogen. Also, it is of much more importance that P should be 

 accurate than that P should be so ; in fact, for hydrogen an error in P 

 has 14 times as much effect as the same error in P . 



We can verify these results from Table I. In Experiment (No. 2), 

 where P = I x 760 nearly, we have two readings — one at the boiling 

 point, the other in solid hydrogen, — namely, 1 9*7 mm. and 14*4 mm., 

 whose difference is 5 "3 mm. This corresponds to dT = 3 x 0*35 16( - 5*3) 

 degrees, or 5° -5 9. The calculated temperatures for these pressures 

 are - 253 0, 37 and - 258 0, 66, whose difference is 5°-29, a satisfactory 

 agreement. 



If we compare Experiments Nos. 4 and 9, in both of which the same 

 value of a is used, we can pass from the former to the latter by the 

 formula 



dT = - 0-0266 dP () -|- 0-3548 dP, 



in which dP = - 11 mm. and dP = - 0'5 mm., whence dT = o, 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 effect on the result. 



An alteration in the value of x has but little relative effect on the 

 results. As before we have 



/T T (273 + 0(273 + ^) 7 

 (il ~ 11 (273 + t - xT,f dj " (7) ' 



If x = 1/50, t = 13°, then 



at Ti = - 180°, dT - - 57*085 da, 

 atTj = - 250°, , dT = - 19*4205^, 



and for the second thermometer (x = 1/115) in like circumstances, 



and dT = - 57*895 dx. 



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 0°'43 and that of hydrogen by 

 0°-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 Ti. To 

 hundredths of a degree therefore the change in T is inversely pro- 

 portional 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 C0 2 would have been 

 - 79°-35. 



