Page:Experimental researches in chemistry and.djvu/222

1880.] &c., most of the metals, and also the earths, which were absolutely 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 temperatures 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 processes of distillation effectual, which otherwise would fail. Thus with the essential oils, many of them which would require 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 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, and pass it forward; only the facility with which the carrying agent is condensed when it consists of steam, allows of the condensation 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 peculiar 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 volatilised, might be considered as #wed at common temperatures, could,