Mr. Faraday on the Limits of Vaporisation. 71 



which are perfectly fixed. The arguments adduced, were 

 drawn first from the power of gravity, as applied by Dr. 

 Wollaston, to shew that the atmosphere around our globe had 

 an external limit, and then from the power of cohesion ; either 

 of these seemed to me alone sufficient to put a limit to vapori- 

 sation, and experiments upon the sufficiency of the latter force 

 were detailed in the paper. 



The conclusion was, that although such substances as ether, 

 alcohol, water, iodine, &c., could not as such be entirely de- 

 prived of their vaporising force, by any means we could apply 

 to them, but still, if in free space or in air, would send off a 

 little vapour, yet there were other bodies, as iron, silver, cop- 

 per, &c., most of the metals, and also the earths, which were 

 absolutely fixed under common circumstances, the limit of 

 their vaporisation being passed; and further, that there were a 

 few bodies, the limits of whose vaporisation occurred at such 

 temperatures as to be within our command, and therefore 

 passable in either direction. Thus mercury is volatile at tem- 

 peratures above 30, but fixed at temperatures below 20, and 

 concentrated sulphuric acid, which boils at temperatures about 

 600, is fixed at the ordinary temperature of the atmosphere. 



It is well known in the practical laboratory that vaporisation 

 may be very importantly assisted so as to make certain pro- 

 cesses of distillation effectual, which otherwise would fail. 

 Thus with the essential oils, many of them which would re- 

 quire a high temperature for their distillation if alone, and be 

 seriously injured in consequence, will, when distilled with water, 

 pass over in vapour with the vapour of the water at a much 

 lower temperature, and, being condensed, may be obtained in 

 their unaltered state. 



It has been supposed that the vapour of the water, either by 

 affinity for the vapour of the essential oil or in some other 

 way, has increased the vaporising force of the latter at the 

 temperature applied, and so enabled it to distil over; but 

 there is no doubt that if air or any other similar elastic medium 

 were made to come in contact with the mass of essential oil at 

 212 in equal quantity, and in a manner to represent the 

 vapour of water, it would, according to well known laws, carry 

 up the vapour of the essential oil perhaps to an equal extent, 



