1830.] On the Limils of Vaporization. 207 



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

 lutely fixed under common circumstances, the limit of their 

 vaporization being passed ; and further, that there were a few 

 bodies, the limits of whose vaporization occurred at such tem- 

 peratures as to be within our command, and therefore passable 

 in either direction. Thus mercury is volatile at temperatures 

 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 vaporization 

 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 vaporizing force of the latter at the tem- 

 perature 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, and pass 

 it forward ; only the facility with which the carrying agent 

 is condensed when it consists of steam, allows of the condensa- 

 tion of every particle of the essential oil vapour, whereas the 

 permanency of the elastic state of the air would cause it to 

 retain a large proportion of the vapour of the oil when cold, 

 and consequently a diminished result would be obtained. 



There are, nevertheless, some appearances which seem to 

 favour the idea that water occasionally favours vaporization, 

 not merely in the manner referred to above, but by some pecu- 

 liar process ; and it was to ascertain whether substances which, 

 from a consideration of the general reasoning already referred 

 to, and the high temperature at which they sensibly volatilized, 

 might be considered as fixed at common temperatures, could, 



