During the fermentation of milk a large proportion of a peculiar acid is generated, which from this circumstance has received the name of lactic acid. This is separated by treating sour whey with lime, filtering the liquid now holding lactate of lime in solution, and precipitating with oxalic acid, which throws down the lime, leaving the lactic acid still dissolved. The solution, having been evaporated to the consistence of syrup, is treated with alcohol, which dissolves the acid, leaving the impurities; and the lactic acid is then obtained by distilling off the alcohol.

Lactic acid is in the form of a colourless liquid, of a syrupy consistence, a very sour taste, and the specific gravity 1.215.

It has been found in the stomach during digestion, and is probably one of the acids with the aid of which the fermentative principle of the gastric juice is enabled to effect a solution of nitrogenous articles of food. it readily enters the circulation, and, being offensive there in its uncombined state, is probably thrown off by the emunctories, and among others by the kidneys. its remedial applications are dependent on its solvent properties, first in the stomach as regards the food, and secondly in the urine as regards the phosphate of lime. The affections in which it may be expected to be useful are dyspepsia, with insufficient acid for the purposes of digestion, and the phosphatic lithiasis, when there is an abundant deposition of phosphates in the urine. From half a fluidrachm to a fluidrachm may be taken three times a day. Allusion has been made, in the preceding article, to the advantages to be occasionally derived from administering it with pepsine. The same effects may probably be obtained from the use, to a moderate extent, of sour buttermilk as an article of diet.