The crude butyric acid thus obtained and freed from water by shaking with calcium chloride, is a mixture of acetic, butyric, and capric acids, but does not contain propionic and valerianic acids. To obtain from it pure butyric acid, fractional distillation is required. For manufacturing on a large scale, a copper distilling apparatus with silver head and silver cooling pipe is used, the bulb of a thermometer being placed in the head. In the first rectification, the receiver is changed after the thermometer has risen to 311° F.; the portion passing over between 311° and 329° F. is caught up by itself, and the receiver being again changed, distillation is continued until finished. The first distillate contains mostly acetic acid, besides a small quantity of butyric acid, the second the greater portion of the butyric acid besides a little acetic and capric acids, while the third consists chiefly of capric acid. For preparing butyric ether for technical purposes, the fraction passing over between 311° and 329° F. is sufficiently pure. To obtain chemically pure butyric acid, the rectification of the portion passing over between 311° and 329° F. is in the same manner repeated, until finally a product with a constant boiling point at 324.2° F. is obtained. Butyric acid fermentation proceeds more rapidly by using, instead of rotten cheese, putrefying meat, and in place of sugar, starch paste or mashed boiled potatoes, 1 part of meat to 4 parts of starch or a corresponding quantity of potatoes being employed. The same products are formed as in the preceding process, but much more rapidly, fermentation being finished, according to Schubert, in 5 to 6 days. Butyric acid, C_{4}H_{7}O.OH, or C_{3}H_{7}COOH, is a liquid of a very sour taste and odor, and at an intense cold congeals to a crystalline mass which melts at 32° F. In a pure state it boils at 324.2° F. It is soluble in water, but separates again if soluble salts are added to this solution. Its specific gravity, after being completely freed from water, is 0.974. Besides the normal butyric acid, there is known another one called _isobutyric' acid_ or _dimethyl acetic acid_. It is distinguished from the normal acid by being less soluble in water and by its boiling point, which lies at 309.2° F. It occurs in St. John's bread or carob, in the volatile oil from _Arnica montana_ and in croton oil.