Sweet milk becomes sour, evolves a considerable quantity of gas during churning, and its temperature ascends four or five degrees. Oxygen is unquestionably absorbed, and it is probable that a portion of the sugar of milk is converted into acid products. I have already stated that even the most carefully prepared butter contains a small proportion of casein and sugar of milk. This casein is the good genius of the cheese-maker, but the evil genius of the butter manufacturer. How? In this way:--When butter containing a notable proportion of casein and sugar of milk is exposed to the air, the following changes take place: the casein passes into a state of fermentation, and acting upon the sugar of milk, converts it, firstly into the bad-flavored lactic acid, and secondly into the bad odorous butyric, capric, and caproic acids. The first of these compounds in a state of purity emits an odor resembling a mixture of vinegar and rancid butter; the second possesses an odor resembling that of a goat--hence the name _capric_; the third has an odor like that of perspiration. In addition to these acids, there is another simultaneously generated--the caprylic, but it does not unpleasantly affect the olfactory nerve. The casein also injuriously affects the fatty constituents of the butter; under its influence they absorb oxygen from the air, and become converted into strong-smelling compounds. The washing of butter is intended to free it from the casein and unaltered cream, and the more perfectly it is freed from those impurities the better will be its flavor, and the longer it will remain without becoming rancid. Some people believe that too much water injures the quality and lessens the quantity of butter. It cannot do the former, because the essential constituents of butter are totally insoluble in water; it may do the latter, but, if it do, so much the better, because the loss of weight represents the amount of impurities--milk, sugar of milk, &c.--removed. I have already remarked that butter is so susceptible of taint that even a perspiring hand is sufficient to spoil it; naturally cool hands should alone be allowed to come in contact with this delicate commodity, and the hands should be made thoroughly clean by repeated washings with warm water and oatmeal--the use of soap in the lavatory of the dairymaid being highly objectionable. Wooden spades are now being commonly made use of in manipulating the butter, and there is no good reason why they should not come into universal use.