J. Dewar — Boiling Point of Liquid Hydrogen. 297 



Of the remaining experiments in Table I, (No. 3) was made 

 in order to show the effect of a very small initial pressure, one- 

 sixth of an atmosphere. The results were unsatisfactory, 

 owing to the sticking of the mercury giving uncertain readings. 

 In this case an error in the reading of a low pressure has six 

 times as great an effect as if the initial pressure had been about 

 an atmosphere. If the temperature deduced for the boiling 

 point of oxygen is corrected, and the same factor of correction 

 applied to the observed liquid hydrogen boiling point, then it 

 becomes— 25 1°*4:. 



It is of particular moment to have some estimate of how far 

 errors in the observed quantities employed in Chappuis' formula 

 affect the final value of T. 



In the case of an error in t, on differentiating equation (2) 

 we get 



If x = 1/50, t = 13°, T, = - 180°; then dT = 0-00339 dt, or 

 it would need an alteration of 2£° in tto alter T by l/100thof a 

 degree at the boiling point of oxygen. In the same circum- 

 stances when T x = - 250, dT = ; 00136 dt, so that an altera- 

 tion of between 7° and 8° in the value of t would only affect 

 the boiling point of hydrogen by l/100th of a degree. 



From equation (4) the error in T varies with x very nearly. 

 This for the second thermometer where x = 1/115, a variation 

 of t to the extent of 6°, would only affect the boiling point of 

 oxygen by l/100th of a degree ; and it would require an altera- 

 tion of 17° in t to affect the boiling point of hydrogen to the 

 same extent. 



In Table I the values of t enclosed in brackets are assumed 

 values ; this investigation shows that no serious error is involved 

 in these assumptions. 



In the case of an error in P l5 a similar process gives 



_ . (a-8)P 273 + * • 



dT ~ Vp -SP) 2 " 273 + *-*^ dP (5) 



If #=1/50,2=13*. P = 760mm.; T x = -180°; JT =0'3563 

 dP, so that an error of 1 mm. in P would only alter the boil- 

 ing point of oxygen by a third of a degree. In the same cir- 

 cumstances at - 250°, dT = 0*3516 </P, which is practically 

 the same result at the boiling point of hydrogen as at that of 

 oxygen. 



For the second thermometer these two equations become 



At - 180°, dT = 0-3575 dP. 

 At — 250°, dT = 0-3548 dP. 



