J. Dewar — Boiling Point of Liquid Hydrogen. 299 



In each of the last four results if P = — X760 mm the formula- 



n 



become respectively 



dT = ?iX0-3563 dP, and dT = ^x 0-3516 dP, 

 dT = 7ZX0-35V5 dP, and dT = nXO'3548 dP; 



in other words, any error in reading P is magnified in its 

 effect on T 1 directly in proportion as P is diminished. This 

 affords some explanation of the weakness of the results in 

 Experiment (No. 3). 



In like manner, from an error in P , we get 



Here if x = 1/50, t = 13°, P = 760 mm., T, = - 180° ; 

 dT == — 0*1188 dP^ or an error of 1 mm. in P would only 

 alter the boiling point of oxygen by a ninth of a degree; but 

 with the same data at - 250°, dT - - 0*0264<£P , so that the boil- 

 ing point of hydrogen would only be altered by a tenth of a 

 degree for a change of 4 mm. on an initial pressure of about 

 one atmosphere. 



In this case also if P =— X 760 mm we get similar results to 



those in the case of P, namely, 



For x— 1/50, dT=— wxO-1188 dP and dT=— /ix 0-0264 dP . 

 For. x— 1/115, dT—— ^XO-1192 dP and dT=—nX 0-0266 dP n . 



The general results of an error in either P or P is, that the 

 more reliable experiments are those in which the initial pres- 

 sure is as high 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 — -J X 760 nearly, we have two readings — 

 one at the boiling point, the other in solid hydrogen, namely, 

 19*7 mm. and 14*4 mm., whose difference is 5*3 mm. This 

 corresponds to dT = 3 X 0*3516 (- 5*3) degrees, or 5°*59. 

 The calculated temperatures for these pressures are — 253°*37 

 and — 258°-66, whose difference is 5°-29, a satisfactory agree- 

 ment. 



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 



