﻿Phenomena connected with the Boiling of Liquids. 169 



elasticity of the vapour of water that has just left off boiling we 

 may suppose to be one-hundredth less than the pressure of the 

 air; hence 1 volume of hydrogen and 99 volumes of vapour 

 would produce a mixture equal in elasticity to the atmospheric 

 pressure *. 



17. Some of the German text-books f on physics assign to 

 Rudberg the honour of an important observation made in 1837, 

 namely, that although water boils at a higher temperature in a 

 glass than in a metal vessel, yet the temperature of the steam is 

 in both cases the same, the pressure being the same. This fact 

 was known to Cavendish and the other eminent men who pre- 

 sented the Report on Thermometers to the Royal Society already 

 referred to (2). 



18. In 1842 Marcet published a long memoir " On certain 

 circumstances that influence the Boiling-point of Water" J. The 

 conditions of the case are thus stated : — Boiling takes place in 

 any liquid at the moment when the repulsive force of heat is 

 sufficient to overcome the effect of cohesion among the particles 

 of the liquid plus the atmospheric pressure. But if the liquid is 

 in a vessel of which the adhesion of the sides to the particles of 

 the liquid is more than the cohesion of the latter among them- 

 selves, this adhesion must be overcome, and more heat will be re- 

 quired than if the simple cohesion of the particles were concerned. 



19. Marcet considers that iron, zinc, and other substances 

 tend to lower the temperature of boiling water, because they 

 have a less molecular adhesion for water than glass has. If the 

 vessel be coated with a thin layer of sulphur, gum-lac, or any 

 similar substance that has no sensible adhesion for water, the 

 boiling-point is lowered, and the temperature of the water and 

 of the steam are identical. In such case the boiling-point may 

 be lower by some tenths of a degree than in metal vessels. The 

 boiling-point varies in flasks made of the same glass, and even in 

 the same flask at different times, irrespective of pressure. Differ- 

 ences were also noticed between flasks fresh from the maker and 

 those that had been used for experiments. In such as had held 



* Schonbein repeats Bostock's experiments, of course without being 

 aware of it. He says, " bits of wood are remarkably active so long as 

 their pores are full of air, but not at all when this is expelled." And 

 again: — " All bodies that contain air or liberate it, set air or gas free from 

 their solutions." This is precisely the theory of M. Gernez, " On the Dis- 

 engagement of Gases from their Supersaturated Solutions, "as given in the 

 Comptes Uendus for November 19, 1866, pp. 883-88/, and examined by 

 me in the Philosophical Magazine for August 1867. 



f See Eisenlohr, Physik, 1860, p. 356. Rudberg's paper is on the 

 Construction of Thermometers. It is inserted in PoggendorrFs Annalen, 

 vol. xl. p. 49, and is more modest in its claims for originality than the 

 books referred to. 



X Annates de Chimie et de Physique, ser. 3. vol. v. p. 449. 



