Elements of the Theory and Practice of Chymistry, 5th ed.

by Pierre Joseph Macquer

Into a glass retort put the quantity of fresh Butter you intend to distil. Set the retort in a reverberatory; apply a receiver, and let your fire be very gentle at first. The Butter will melt, and there will come over some drops of clear water, which will have the peculiar smell of fresh Butter, and shew some tokens of Acidity. If the fire be increased a little, the Butter will seem to boil: a froth will gather on its surface, and the phlegm, still continuing to run, will gradually come to smell just like Butter clarefied in order to be preserved. Its Acidity will be stronger and more manifest than that of the first drops that came over.

Soon after this, by increasing the fire a little more, there will rise an Oil, having nearly the same degree of fluidity as fat Oils; but it will grow thicker as the distillation advances, and at last will fix in the receiver when it cools. It will be accompanied with some drops of liquor, the Acidity whereof will always increase, while its quantity decreases, as the distillation advances.

While this thick Oil is distilling, the Butter contained in the retort, which at first seemed to boil, will be calm and smooth, without the least appearance of ebullition; though the heat be then much greater than when it boiled. Continue the distillation, constantly increasing the fire by degrees as you find it necessary for the elevation of the thick Oil. This Oil, or rather this kind of Butter, will be at last of a russet-colour. There will rise along with it some white vapours exceeding sharp and pungent.

When you observe that nothing more comes over, though the retort be quite red-hot, let the vessels cool, and unlute them. You will find in the receiver an aqueous acid liquor, a fluid Oil, and a kind of fixed brown Butter. Break the retort, and you will find therein a charred matter; the surface of which, where it touched the glass, will be of a shining black, and have a fine polish.

OBSERVATIONS.

The analysis of Butter proves that this substance, which is an oily matter in a concrete form, owes its consistence to the Acid only, with which the oily part is combined: that is, it follows the general rule frequently mentioned above in treating of other oily compounds; the consistence whereof we shewed to be so much the firmer, the more Acid they contain. The first portions of Oil that come over in the distillation of Butter are fluid, because a pretty considerable quantity of Acid rose before them, which being mixed with the phlegm gives it the Acidity we took notice of.

This Oil, being freed from its Acid, and by that means rendered fluid, rises first; because it is by the same means rendered lighter. The kind of Butter that comes over afterwards, though it be fixed, is nevertheless far from having the same consistence as it had before distillation; because it loses much of its Acid in the operation. This Acid is what rises in the form of white vapours. These vapours are, at least, as pungent and irritating as the Sulphureous Acid or Volatile Alkalis: but their smell is different: it hath a resemblance, or rather is the same, with that which rises from Butter, when it is burnt and browned in an open vessel. But, when concentrated and collected in close vessels, as in the distillation of Butter, they are vastly stronger: they irritate the throat so as to inflame it; they are exceeding sharp and pungent to the smell, and are so hurtful to the eyes that they quickly inflame them, as in an ophthalmy, and make them shed abundance of tears. The great volatility of this Acid is entirely owing to a portion of the phlogiston of the Butter with which it is still combined.

It may be asked why Butter, or the oily part of Milk which hath the consistence of a fixed Oil, is more replete with an Acid than the Oils of the vegetables whereof the Milk was formed; as these Oils are almost all fluid, which indicates their containing less Acid before than after they were digested in the body of an animal. This must appear the more extraordinary, because the Acid contained in the liquors of animals is sheathed and imperceptible, and consequently incapable of combining with the Oils of vegetables so as to give them this consistence.

I think it will be easy to give a satisfactory answer to this question, if it be considered, that the Oils, which exist in the vegetable juices whereof the Milk is formed, are far from being combined with the whole Acid of those vegetables; because there is hardly a plant that doth not yield a great deal of Acid, even without the help of fire. Now, there is reason to think, that one of the principal effects of digestion is, to combine and unite this Acid, with the oily parts of vegetables, more intimately than it was before.