434 Mr. C. Tomlinson on some Phenomena 



who in 1837 attributed the action of bits of wood, wire, &c. in 

 liberating vapour from boiling liquids to the air contained in 

 their pores or adhering to their surfaces, their action ceasing 

 when the air is driven off. M. Gernez quotes this con- 

 clusion with applause; but he does not quote Schonbein's 

 doubts, forgetting perhaps that the doubts of a man of genius 

 are often of more value than the conclusions of an ordinary man. 

 I pointed to the fact some years ago* that Schonbein was by no 

 means satisfied with the theory which attributed the action of 

 solids in liberating gases or vapours from liquids to their carry- 

 ing down air, a film of which was supposed to adhere to all 

 bodies exposed to it ; and he expressed his opinion that any one 

 would perform an important service both to physics and to che- 

 mistry who could satisfactorily account for the varied phenomena 

 connected with the subject of nuclei. 



In 1842 Marcet attributed to differences of adhesion of the 

 liquid to the sides of vessels of different material those differ- 

 ences in the boiling-point which had been so often noticed. In 

 1844 Magnus recurs to the same idea, and also to that of cohe- 

 sion among the particles of the liquid ; but they make no refer- 

 ence to the presence of gas in the liquid ; and it was not until 

 1846, or three quarters of a century after the promulgation of 

 De Luc's theory, that we have a De Luc redivivus in the person 

 of M. Donny. According to him, boiling is not an inherent 

 property of liquids ; they only boil when they contain air — that 

 is, when they are not pure. Heat liberates bubbles of air near- 

 est to the source of heat; each air-bubble presents to the liquid 

 molecules surrounding it a surface which promotes the vaporiza- 

 tion of these molecules ; and when the tension of the vapour is 

 sufficient to counterbalance the pressure to which these bubbles 

 are submitted, nothing further opposes the development of this 

 vapour, which then forms currents that traverse the liquid and 

 give rise to ebullition. Hence, according to this view, ebullition 

 is a kind of evaporation, extremely rapid, which operates upon 

 those interior surfaces of the liquid which limit a bubble of some 

 aeriform fluid. If the quantity of air in the liquid be small, the 

 boiling-point may rise; the boiling-point is constant only when 

 the liquid contains air. It is difficult, if not impossible, to get 

 rid of the dissolved air. The boiling-point is also singularly 

 influenced by the forces of cohesion and adhesion, the cohesion 

 of the molecules of water being superior to a pressure of three 

 atmospheres, or a column of water of 30 metres. This conclu- 

 sion was arrived at by heating water in a chemically clean tube 

 in a bath of chloride of calcium as high as 138° C. (280°*4 F.). 



Donny's experiments excited considerable attention among 

 * Phil. Mag. for September 1869. 



