by rendering the oil black. Hence the only way of preserving these oils colorless and transparent, is by keeping them in bottle perfectly full and accurately corked, to hinder the contact of air, which always discolours them.
Successive rectifications of this oil furnish another phenomenon confirming our theory. On each distillation a small quantity of charcoal remains in the retort, and a little water is formed by the union of the oxygen contained in the air by distilling vessels with the hydrogen of the oil. As this takes place in each successive distillation, if we make use of large vessels and a considerable degree of heat, we at last decompose the whole of the oil, and it entirely into water and charcoal. When we employ a slow fire, and especially when we employ a slow fire, or degree of heat little above that of boiling water, the total decomposition of these oils, by repeated distillation, is greatly more tedious, and more difficulty accomplished. I shall give particular detail to the Academy, in a separate memoir, of all my experiments upon the decomposition of oil; but what I have related above may suffice to give just ideas of the composition of animal and vegetable substances, and of their decomposition by the action of fire.
