Chap. 1.

experiments on lactic acid, from which he drew as a conclusion that it is merely acetic acid contaminated with some saline and animal matter.* Thenard drew the same conclusion from his experiments in 1806. Both of these chemists had obtained the acid which they examined by distillation. But Scheele had expressly stated, that lactic acid when distilled was converted into acetic acid. The existence of lactic acid, therefore, was by no means disproved by their experiments. The subject was taken up by Professor Berzelius, in 1808. In the second volume of his Animal Chemistry, he examined the properties of lactic acid in detail, and endeavoured to establish its peculiar nature. He employed the following process for procuring pure lactic acid :

76

Extract obtained by evaporating whey to dryness, consists of lactic acid, lactate of potash, muriate of potash, phosphate of lime, and some animal matter. It was dissolved in alcohol and mixed with alcohol, holding th of its weight of concentrated sulphuric acid till there was an excess of sulphuric acid in the solution. Some sulphate of potash was precipitated. To get rid of the other acids it was digested over carbonate of lead, till the liquid acquired a sweetish taste. By this the sulphuric acid, the phosphoric acid, and most of the muriatic acid, were separated; but lactic acid forming a soluble compound with lead remained in solution. A current of sulphuretted hydrogen gas being passed through the liquid threw down the lead. The liquid was now digested over quicklime till all the animal matter was separated. It now contained only lactic acid, muriatic acid, and lime. A portion of it was taken and freed from its lime by means of oxalic acid. This portion was then saturated with carbonate of silver. By means of this solution the remainder of the liquid was freed from muriatic acid. Finally, the lime was thrown down by means of oxalic acid, so that nothing remained but lactic acid dissolved in water. To get rid of a small portion of oxalate of lime which it may hold in solution, it is proper to evaporate it to dryness and redissolve it in water.

When oatmeal, rice, and several other vegetable bodies, are mixed with water, and left to spontaneous fermentation, they acquire an acid taste and smell. The acid thus formed was examined by Braconnot, who considered it as a new acid, and

* Ann. de Chim. 1. 288.

+ Ibid. lix. 280. Föreläsningar i D’jurkemien, ii. 430. A translation of his account of lactic acid is published in the Phil. Mag, xli. 241.

Class I.

Div. II.

gave it the name of nanceic acid.* But Vogel afterwards showed it to be identical with the lactic acid of Scheele.+ Lactic acid when obtained by Braconnot's process (which Characters. consists in saturating the acid with oxide of zinc, and purifying the salt by a double crystallization. The salt is now dissolved in hot water, the oxide of zinc thrown down by barytes water, and the barytes by sulphuric acid) is colourless. But when the process of Scheele or Berzelius is adopted it has a brownish yellow colour, and a sharp sour taste, which is much weakened by diluting the acid with water. While cold it has no smell, but when heated it acquires a sharp sour odour, not unlike that of sublimed oxalic acid. It does not crystallize, but when evaporated to dryness forms a smooth varnish which gradually attracts moisture from the air. It dissolves readily in alcohol. When heated it boils, emits a sour smell, and leaves a bulky charcoal not easily burnt. When distilled it gives out empyreumatic oil, water, acetic acid, carbonic acid, and inflammable gas. All the salts which it forms with bases are soluble in water, and hardly any of them can be made to assume a crystallized form.

This acid obviously differs from acetic acid in being less volatile. But whether this difference be not owing to some foreign substance combined with acetic acid has not been determined. The lactates described by Berzelius as existing in all animal fluids, have not been sufficiently examined to make their nature certain.

2. Caseic Acid.

ed.

This is a name given to a substance formed by Proust, by How obtain. subjecting the curd of milk to the putrefactive fermentation, and which he considered as a peculiar acid. Curd was allowed to putrefy under water at a temperature of about 50°. In about 14 days' time the water will be found to hold in solution phosphate, acetate, and caseate of ammonia. The water is renewed from time to time till the putrefying curd gives out no more of these substances. The watery liquids are evaporated to the consistence of a syrup, which is mixed with alcohol, and the mixture left for some time undisturbed, and the whole is then thrown on the filter. A white substance remains on the filter, to which Proust gave the name of caseic oxide. The alcoholic liquid contains the caseate of ammonia. It is subjected to dis

* Ann. de Chim. lxxxvi. 86.

† Annals of Philosophy, xii. 391.

Chap. I.

Characters.

How obtain. ed.

tillation to get rid of the alcohol. The residue is boiled with water and carbonate of lead, which disengages all the ammonia. The liquid being now filtered is exposed to the action of a current of sulphuretted hydrogen gas to throw down the lead. It is again filtered and concentrated to drive off the acetic acid which it contains. The liquid thus treated consists, according to Proust, of a solution of caseic acid in water.

This acid is described as a light yellow syrup, which when heated, passes into a horny, translucent, yellow matter, having a sour, bitter, and cheesy taste, and easily soluble in water. According to Braconnot, who has repeated the experiments of Proust, this substance owes its acid properties to a quantity of acetic acid which it contains. But it consists chiefly of various animal matters. One of the most remarkable of which (the caseic oxide of Proust) he has called apose pedine It is mixed with some resin, some oil, and acetate of potash, muriate of potash, and acetate of ammonia.†

It would appear from this that caseic acid is not a peculiar acid, but merely the acetic disguised by a quantity of foreign matter, which retards its volatility. In this respect it resembles lactic acid.

3. Fibric Acid.

I give this provisional name to an acid substance obtained by M. Peretti by the following process: The muscular flesh of an animal is digested in cold water, and the water evaporated to the consistence of a syrup. This matter is digested in strong alcohol, and the solution being set aside for some days in a close vessel, some small needle-shaped crystals make their appearance. These needles redden litmus paper, and when heated give out oil and carbonate of ammonia, with a smell of roast meat. They dissolve easily in water and alcohol, and may be mixed with sulphates and nitrates without undergoing decomposition.‡

These facts are scarcely sufficient to determine the real nature of these crystals. They consist probably of an alkaline super

lactate.

History.

SECTION II.—OF FORMIC ACID.

This acid is first mentioned in the Philosophical Transactions for 1671, in a paper by Mr. Ray, giving an account of the

* Ann. de Chim. et de Phys. x. 29.

Jour, de Pharm. xii. 274.

Ibid. xxxvi. 165.

observations of Mr. Halse, and the experiments of Mr. Fisher, on the acid juice which is spontaneously given out by ants, and which they yield when distilled. Mr. Fisher compares this liquor with vinegar, but points out some differences between them. Scarcely any addition was made to these facts till Margraff published a dissertation on the subject in the Berlin Memoirs for 1749, in which he describes the method of obtaining the formic acid from the formica rufa, or red ant, and points out its properties with his usual precision and method.† A new dissertation was published on the same subject by Messrs. Arvidson and Oehrn in 1782, in which the discoveries of Margraff were confirmed, and many new particulars added. Hermbstadt's paper on the same subject appeared in Crell's Annals for 1784. His researches were directed chiefly to the purification of the formic acid. He demonstrated that the juice of ants contained several foreign bodies, and among others, that a portion of malic acid might be detected in it. Richter published experiments on formic acid about the year 1793, pointing out a method of procuring it in a very concentrated state.‡ Deyeux soon after examined it, and found it analogous to the acetic acid.§ This opinion of the French chemist was confirmed in 1802, by Fourcroy and Vauquelin, who published a dissertation on ants, and concluded from their experiments, that the formic acid is nothing else than a mixture of the acetic and malic acids. The opinion maintained by these celebrated philosophers induced Suersen to examine the subject by experiment. This chemist, in an elaborate dissertation on formic acid, published in 1805,¶ shows that most of the facts pointed out by Fourcroy and Vauquelin had been already ascertained by preceding chemists; that the experiments which they detail were not sufficient to warrant their conclusions; that formic acid, when properly prepared, contains no malic acid; and that it possesses properties different from the acetic. Gehlen was induced to resume the examination of this acid, by some observations which I made upon it in a former edition of this work. He published an elaborate dissertation on it in 1812, in which he compares the properties of formic acid with those of acetic acid, and points out the peculiar characters of formic acid with

* Phil. Trans. v. 2063. † Margraff's Opusc. i. 291. Gehlen, iv. 7. Fourcroy, x. 491; Eng. Trans. I have not seen either the dissertation of Richter or Deyeux.

Phil. Mag. xv. 118.

Gehlen's Jour. iv. 1.

Class I.

Div. II.

59

Chap. I.

How obtain. ed.

Properties.

great clearness and precision.* Since that time it has been analyzed by Berzelius, and an artificial method of preparing it has been discovered by Dobereiner.

The simplest method of procuring formic acid in a state of purity is that of Margraff as corrected by Richter. Suersen accordingly had recourse to it. This method is as follows: Infuse any quantity of ants in about thrice their weight of water, put the mixture into a silver or tinned copper still, and draw off the water by distillation as long as it continues to come over without any burnt smell; for the distillation must be stopped as soon as that smell begins to be perceived. Saturate the water in the receiver with carbonate of potash, and evaporate to dryMix the white mass thus obtained with as much sulphuric acid, previously diluted with its weight of water, as is sufficient to saturate the potash. Introduce the mixture into a retort, and distil slowly to dryness. The liquid which comes over into the receiver is to be again rectified by a very moderate heat, to get rid of any portion of sulphuric acid that may be present.

ness.

Gehlen, to be still more certain of the purity of his formic acid, saturated the acid prepared by the above process with carbonate of copper, and by the requisite evaporation, separated the formate of copper in crystals. He mixed in a retort 13 ounces of these crystals with 8 ounces 310 grains of sulphuric acid of the specific gravity 1.864 and distilled. He obtained 6 ounces 410 grains of formic acid in a state of purity.

Dobereiner discovered that if we mix together in a large retort one part of crystals of tartaric acid, 2 parts of deutoxide of manganese, and 2 of sulphuric acid of the specific gravity 1.85, previously diluted with twice its weight of water, and apply heat, much carbonic acid gas is disengaged, and the matter in the retort swells up and has a great tendency to run over. After the disengagement of gas is at an end, if we distil over the liquid we obtain dilute formic acid, which may be concentrated by saturating it with potash or soda, and mixing the formate with the requisite quantity of sulphuric acid and distilling.†

Wöhler has found that the process succeeds equally if we substitute starch for tartaric acid. And Liebeg even affirms that many other organic bodies may be substituted for tartaric acid without injuring the process.

It has a sharp sour taste and cannot be crystallized, even by

* Schweigger's Journal, iv. 1. + Gilbert's Annalen, Ixxi. 107.