I now come to examine the second essential Part of Milk, _viz._ that which is cheesy, earthy, and somewhat saline. I am not of Opinion that the Acid of the Stomach is increased by this Part, for there is no Acid naturally in the Stomach; if there were, it would be mischievous. Although it be certain that Cheese is acid, and turns sharper by Age, yet those Particles which are precipitated into Chese, are vastly different in the Chyle and the Milk, from what they are in a State of Separation, and after being exposed to the Air. The Salts, which before were nitrous, and of a middle Nature, somewhat volatile, and mixed with oily, sulphureous, or earthy Particles, being agitated by an inward Motion, become more stiff and complicated. These Salts, while in a State of Union with the Milk and Chyle in the Body, by Means of the progressive Motion, are more disunited and smaller, the serous and oily Particles keeping them asunder; and there is neither Time nor Rest allowed them in their natural State to produce fresh Combinations, as they have when deprived of their progressive Motion, in a State of Separation from the Body. That Milk in warm Weather turns sowre, is to be imputed to its intestine Motion, where the Salts, before small and somewhat nitrous, mixed with the oily Particles by the Influx of the Air, change their natural Texture and Figure, and become more rigid and heavy, and so precipitate the light, viscid, and earthy Particles. That the Air contributes much to this Change, appears from hence, because that alone produces a remarkable Quantity of acid Salts in some Bodies. If a Piece of Alum be calcined in the open Fire, upon exposing it again to the Air, it shall double its Weight; so that a large Quantity of aluminous acid Salt may be drawn from thence: And although Milk be coagulated in the Breasts, it happens either from an acid Acrimony in the Blood, or its Motion being stopped, and some Obstructions of the milky Vessels. It doth not appear from any Experiment yet known, that healthy Milk fresh drawn contains any Acid; the Manner in which this Part of the Milk acquires this Tendency, I conceive to be this: We have already asserted, that Milk, in its natural State, contains no Acid, although after being exposed to Warm Air, by Means of some Fermentation and inward Motion, it becomes acid, which is to be look’d upon as a new Production, no way relating to Milk in its natural State. The cheesy Particles of Milk, if I may so call them, when in the Body differ extremely from those which out of the Body form the Cheese; for while in the Body, they are in the Shape of earthy, subtile, viscid Particles, mixed with the Milk, Chyle, and Blood; they give a due Consistence to the Milk, by duly mixing the oily, fat and serous Particles with them, and while in their due progressive Motion, keep the Milk in a proper Temperature, and occasion a slower Motion of the Milk through the milky Vessels. SECT. 9.