charcoal in its composition. The chief difference between fixed or fat oils drawn from vegetables by expression, and volatile or essential oils, is, that the former contains an excess of charcoal, which is separated when the oils are heated above the degree of boiling water; whereas the volatile oils, containing just proportion of these two constituent ingredients, are not liable to be decomposed by that heat, but uniting with caloric into the gaseous state, pass over in distillation unchanged.
In the Memories of the Academy for 1784, p. 593. I gave an account of my experiments upon the composition of oil and alcohol, by the union of hydrogen with charcoal, and of their combination with oxygen. By these experiments, it appears that fixed oils combine with oxygen during combustion, and are thereby converted into water and carbonic acid. By means of calculation applied to the products of these experiments, we find that fixed oil is composed of 21 pairs, by weight, of hydrogen combined with 79 parts of charcoal. Perhaps the solid substances of an oily nature, such as wax, contain a proportion of oxygen, to which they owe their series of experiments, which I hope will throw light upon this subject.
It is worthy of being examined, whether hydrogen in its concrete state, uncombined with calorie,
