The yield of butter per cow is subject to great variation. Some breeds of the animal are remarkable as milkers; such, for instance, as the Alderneys and Kerrys--indeed, I may say all the small varieties of the bovine race. There are instances of cows yielding upwards of twenty pounds of butter per week, but these are extraordinary cases. In Holland a good cow will produce, during the summer months, more than 180 lbs. of butter. In these countries I think the average annual yield of a cow is not more than 170 lbs. It sometimes happens that cows yield a large quantity of milk and a small amount of butter, but it far more frequently occurs that the cow which gives most milk also yields most butter. An estimate of the amount of butter contained in milk may be made by determining the amount of cream. This may be effected by means of an instrument termed a _lactometer_, which is simply a glass tube about five inches long, and graduated into a hundred parts. The specimen to be examined is poured into this tube up to zero or 0, and allowed to stand for twelve hours in summer and sixteen or eighteen in winter. At the end of that time the cream will have risen to the top, and its per-centage may be easily seen. In good milk the cream will generally extend 11 to 15 degrees down from 0. This instrument, although very useful, is not reliable in every case, especially in detecting the adulteration of milk. I have already stated that the complete separation of the butter from the other constituents of the milk is never accomplished in the dairy. Now although the proportion of curd in the butter is very small--rarely more than two per cent. and often not a fourth of one per cent.--yet it is more than sufficient, under a certain condition, to cause the butter to become speedily rancid. That condition is simply contact with the air. If the curd, before it becomes dry and firm, is subjected to the influence of the air, it rapidly passes into a state of fermentation, which is very soon communicated to the fatty and saccharine constituents of the butter (substances not spontaneously liable to sudden changes in composition) and those peculiar compounds--such, for example, as butyric and capric acids, are generated, which confer upon rancid butter its characteristic and very disagreeable odor and flavor. The fermentation of the curd is prevented by incorporating common salt with the butter, and by preventing, so far as possible, the access of air to the vessels in which the article is placed. If fresh butter be placed in water--which apparently protects it from the influence of the air--it will soon become rancid. The reason of this is, that water always contains air, which differs in composition, though derived, from the atmosphere, by being very rich in oxygen. Now, it is precisely this oxygen which effects those undesirable changes in the casein, or curd, to which I have so repeatedly referred; hence its presence in a