CHANGE FROM THE LIQUID TO THE SOLTD STATE ON VAPOUR-PRESSURE. 475 
vapour-pressure of ice at (t— 1) = P,i — (P^— PV-i) 
where P = vapour-pressure of ice. 
I y = „ ,, water. 
H v = heat of vaporization of water. 
H F = „ fusion of ice. 
t = temperature of ice. 
hY.+iiYA 
HY, 
The formula devised by Regnault for vapour-pressures of water between 0° and 
100°, was held to apply to temperatures as low as —15°. Even if this is not strictly 
correct, the alteration in the differences between the vapour-pressures of water and 
ice would be inappreciable. 
35. In calculating the results the following data were employed :—■ 
(I.) Heat of vaporization of water at 0° = 606"5 calories (Regnault). (H v .) 
„ „ „ -0°-5 = 606-85 
„ „ „ — L°'5 = 607'55 
and so on, increasing by 07 calorie for each degree. 
(II.) Heat of fusion of ice at 0° =79-15 calories (Regnault). (H f .) 
„ „ „ — 0 o '5 = 78'90 „ 
„ „ „ -l°-5 = 78-40 „ 
and so on, decreasing by 0"5 calorie for each degree. 
(III.) Specific beat of ice, 0'5 calorie (mean of various experiments). 
(IV.) Ratio of difference of pressures for 1° of vapours of ice and water. 
The ratio between 0° and —1° was calculated thus : 
Between --1° and — 2 C 
606- 85+ 78-90 
606-85 
607- 55 + 78-40 
= 1-130. 
= 1-129, 
607"55 
and so on. 
(V.) Vapour-pressures of water from Regnault’s empirical formula 
log e = ci d - b.ct! —c./T, between 0° and —16°. 
(It is noteworthy that the curve representing results calculated by Magnus from 
his own observations for temperatures below 0° runs parallel to that here given, with 
a difference varying between 0'075 and 0'08 L millims.) 
3 p 2 
