In experiments in the small way, the lamp will answer every purpose of the sand bath, as the sliding rest containing the retort admits of its being placed at any given distance from the flame. In _ addition to this, the Argand lamp can be adjusted by the rack to almost any degree of intensity below its maximum. Ano ther method of defending the retort from ' the too rapid effect of the heat consists in coating the outside with a mixture of horse dung and clay, or loam. When a limited degree of heat is required in tillation, recourse is had to a bath of some liquid, whose boiling point will give the degree of heat required, such as water, oil, or mercury. If; for instance, it were 'required to separate any substance from - Water more volatile than that liquid, it ' ' necessary to employ a water n which to place the retort; keep fig the water in the bath below its boil. iag point.
The laws of distillation, however, are .o modified by other circumstances, as to ' render some of the preceding rules in tome measure exceptionable. If the 'different bodies subjected to distillation had no chemical affinity for each othe,r, it I probable that each substance would put on its elastic form only at that tem perature, at which it would, in a separate 40fate, be converted into vapour. But we Lfrequently find, that one volatile substance 'will carry along with it other bodies of
. _considerable fixity. From the affinity which water has to air, we observe the evaporation of the farmer to take place at all temperatures below its boiling point ; and though it has been thought that water might be freed from saline matter by dis ' tillation, it is found, by experiment, that several salts are carried over along with the vapour of the water, which, in their • dry state, would undergo decomposition before they would be induced to assume the elastic form. Hence water, by the common mode of distillation,- cannot be rendered pure. From the circumstance, that the air is capable of raising water and other liquids at a low temperature, we are enabled to perform the distillation of such liquids, by making the slightest degree of difference of •temperature be tween the retort and the receiver. ter and alcohol may be obtained perfect ly pure, by placing the retort in the tem perature of 100°, and the receiver in that of 50° of Farenheit's scale.
The salts most liable to rise with wa ter in distillation are, carbonate ammo nia, muriates of lime and magnesia, and nitrate of Indeed, this tendency appears to be directly as the solubility of the salt, or rather as its deliquescent pro perty, which is as its affinity tor water.