EFFECTS OF HEAT ON FLUIDS. 427 



this improbable, and the deposit formed on the interior of the 

 glass tube in which the experiment is made has all the pro- 

 perties of platinum-black; so if the spark from a Ruhmkorffcoil 

 be taken in the vapour of water for several days, a portion of 

 gas is freed which is pure hydrogen, the oxygen freed being 

 probably changed into ozone and combined with the mercury 

 or dissolved by the water. 



I have alluded to the eudiometer by which I analysed the 

 gases obtained in these experiments ; it was formed simply of 

 a tube of glass frequently not above 2\ millemetres in dia- 

 meter, with a loop of wire hermetically sealed into one end, 

 the other having an open bell-mouth. By a platinum wire a 

 small bubble of the gas to be examined could be got up 

 through water or mercury into the closed end of the tube, and 

 by the addition of a bubble of oxygen or hydrogen gas a very 

 accurate analysis of very minute quantities of gas could be 

 made ; I have analysed by this means quantities no larger 

 than a partridge-shot. 



I need hardly allude to results on the compound liquids, 

 such as oils and hydrocarbons, as the fact that permanent gas 

 is given off in boiling such liquids would not be unexpected ; 

 but the above experiments seem to show that boiling is by no 

 means necessarily the phenomenon that has generally been 

 supposed, viz. a separation of cohesion in the molecules of a 

 liquid from distension by heat. I believe, from the close 

 investigation I made into the subject, that (except with the 

 metals, on which there is no evidence) no one has seen the 

 phenomenon of pure boiling without permanent gas being 

 freed, and that what is ordinarily termed boiling arises from 

 the extrication of a bubble of permanent gas, either by 

 chemical decomposition of the liquid, or by the separation of 

 some gas associated in minute quantity with the liquid, and 

 from which human means have hitherto failed to purge it ; 

 this bubble once extricated, the vapour of the liquid expands 

 it, or, to use the appropriate phrase of M. Donny, the liquid 

 evaporates against the surface of the gas. 



My experiments are, in a certain sense, the complement of 

 his. He showed that the temperature of the boiling-point 

 was raised in some proportion as water was deprived of air. 



