Div. II.

Vauquelin informs us that the process is facilitated by sub- Class I. stituting bicarbonate of potash, or crystallized carbonate of soda, in the proportion of 5 parts for every 100 parts of the expressed pulp. The rest of his process is the same as that of Braconnot just described.

Thus obtained pectic acid has the form of a transparent and Properties. colourless jelly. Its taste is perceptibly acid, and it reddens paper stained blue by litmus. It is scarcely soluble in cold water. Boiling water dissolves it in small quantity. The filtered solution is colourless, and does not let any sediment fall on cooling, and scarcely reddens litmus paper. It is coagulated into a jelly colourless like glass, by alcohol and all metallic solutions without exception; by lime and barytes water, by acids, by muriate and sulphate of soda, nitrate of potash, &c. Even sugar when added to the water, causes the pectic acid to assume the form of a jelly.

When dried upon a capsule it assumes the form of transparent plates, which do not adhere to the glass vessel. In this state it scarcely swells when put into cold water; but it dissolves a little in boiling water, and the solution coagulates by all the different reagents as at first. When distilled in a retort, it yields a good deal of empyreumatic oil; but no ammonia nor muriatic acid, and leaves behind it a bulky coal. When mixed with water it has the property of disengaging carbonic acid from the alkaline carbonates.

When mixed with an excess of caustic potash in a platinum crucible, and gently heated, agitating the mixture carefully, it soon melts and assumes a brown colour. If we evaporate the liquid without increasing the heat much, we will observe that when the water is driven off what remains in the crucible again assumes a white colour, and if we examine it we shall find it to consist of oxalate of potash. By this process the pectic acid has been converted into oxalic acid.*

With potash, pectic acid forms a salt which has little taste, salts. and is very soluble in water. Weak alcohol throws it down in the form of a jelly. When this jelly is dried it assumes a form resembling gum arabic, and scarcely adheres to the glass in which the evaporation was carried on. When heated it swells, and leaves a brown matter, which is ulmine united to potash. Pectate of potash, when in solution in water, is coagulated by all the substances which produce the same effect on pectic acid.

Vauquelin; Ann. de Chim. et de Phys. xli. 55.

Chap. I.

History.

The earthy and metallic salts decompose it by double affinity.
According to the analysis of Braconnot it is a compound of

Hence it would appear that its atomic weight is about 33. Most of the other pectates are insoluble, and may be obtained by double decomposition.

Cold sulphuric acid has little action on it; when heat is applied sulphurous acid is given off and ulmine formed. When nitric acid is distilled off it, the residue furnishes oxalic acid.

Braconnot, in consequence of the property which this acid has of converting a vast quantity of water into a jelly, has proposed the introduction of it under that form as a very palatable food. The method of preparing these jellies is as follows: Boil a little pectic acid in the quantity of water which is to be converted into a jelly, then dissolve in the water a sufficient quantity of sugar previously seasoned by being rubbed over the skin of an orange, or by any other wished for seasoning, or add to the water a little alcohol previously properly seasoned. In either case the whole assumes the form of a jelly, the flavour of which will of course depend upon the nature of the seasoning employed.

These jellies liquefy with extreme facility when a little ammonia is poured upon them.

Braconnot is of opinion that the soluble pectates constitute a complete antidote against all metallic poisons, such as those of lead, copper, zine, antimony, and mercury. His reason is, that the salts of all these metals are decomposed by pectate of potash, and the metalline pectate formed is quite insoluble, and therefore not likely to prove injurious to the animal economy. Though this inference is not quite unobjectionable, yet the insolubility of such pectates would form a reason for administering pectate of potash in all cases when a poisonous metallic salt had been taken into the stomach. Braconnot excepts corrosive sublimate, nitrate of silver, and tartar emetic, because the insoluble pectates obtained from them are soluble in an excess of pectate of potash.

Upon the whole, the properties of pectic acid are interesting, and its nature and properties deserve a much more complete investigation than they have hitherto met with.

SECTION XXVI.-OF STEARIC ACID.

The examination of the constitution of soaps was begun by

Chevreul about the year 1813. He at that time gave to the Institute an account of an acid substance which enters into the composition of soaps, and to which he gave the name of margarine. In 1816 he distinguished it by the name of margaric acid. About the same time he found that margaric acid extracted from different bodies existed in two different states, and as the one contained more oxygen than the other, he distinguished them at first by the name of margarous and margaric acids. But he thought it afterwards better to give to margarous acid the name of stearic acid, and to retain the term margaric acid, which had been applied to the other at first.

The method of procuring stearic acid is as follows: Make a soap by boiling mutton suet and caustic potash together, with a sufficient quantity of water, till the whole is converted into soap. Dissolve one part of this soap in 6 parts of warm water, and mix the solution with about 40 parts of cold water, and leave it for some time in a temperature about 60° or between 60° and 70°. A substance precipitates having a pearly lustre, which is a mixture of bistearate of potash and margarate of potash. Collect this substance on the filter and wash it. The liquid that has passed through the filter being mixed with a little acid to saturate the potash which it now contains in excess, will yield an additional quantity of this twofold soapy salt. By repeating this process several times all the bistearate of potash and margarate of potash is obtained, and the water retains only the oleate of potash. The bistearate and margarate of potash is to be dried and dissolved in about 20 times its weight of hot alcohol of 0.82. When the alcohol cools a quantity of bistearate of potash mixed with margarate of potash precipitates, and oleate of potash and margarate of potash remains in solution in the alcohol.

The precipitated bistearate of potash and margarate of potash is to be again dissolved in boiling alcohol, which is to be set by to cool. The bistearate which now precipitates is much freer from margarate, and by repeating the solution in boiling alcohol two or three times, it is at last got perfectly pure. To determine whether the bistearate is quite pure, take a little of it and decompose it by muriatic acid. Put the acid which separates (after washing it) in water, the temperature of which is to be gradually raised. If the acid does not melt till the temperature of the water reaches 158° it is pure stearic acid. If

* From 7g, tallow.

Class I

Div. II.

Chap. I.

Properties.

it melts at a lower temperature it contains a mixture of margaric acid. The stearate of potash being thus obtained in a state of purity, it is decomposed by boiling it with muriatic acid and water. The stearic acid is to be separated when the mixture gets cold, and it may be freed from all admixture of muriatic acid by fusing it repeatedly in pure water.

Stearic acid has a fine white colour, and when the fused acid is allowed to cool below 158° it crystallizes in fine brilliant needles. It is tasteless and destitute of smell. It is insoluble in water. While liquid it may be mixed with hot alcohol in all proportions. If we heat 1 part of alcohol of the specific gravity 0-794 with one part of stearic acid to the temperature of 167°, we obtain a solution which remains liquid till cooled to 122°. It then crystallizes in brilliant scales, and it becomes quite solid at 113°. When stearic acid isheated with its own weight of ether of the specific gravity 0-727 in a tube shut by the finger, a solution takes place, which is limpid at 160°, but at 134° it becomes solid, and consists of a congeries of brilliant plates.

This acid combines with concentrated sulphuric acid without undergoing decomposition. It has the property of reddening vegetable blues. It combines with the bases and forms a class of salts called stearates. At the temperature of 212° it disengages carbonic acid from the carbonates of potash and soda.

When this acid is distilled it gives out a little carbonic acid and a little carburetted hydrogen, and an oily matter passes over which is stearic acid a little altered in its properties. When stearic acid is heated sufficiently in an open vessel it burns like wax. When digested with nitric acid it is converted into an acid which possesses peculiar properties, but which hitherto has been very little examined.*

From the analyses of different species of stearates by Cheyreul, it appears that the atomic weight is very nearly 33-33. The acid in its usual state is a hydrate, being composed of

The small surplus of water is doubtless mechanically lodged between the particles of the acid.

*See Braconnot; Ann. de Chim, xciii. 254; and Chevreul sur les corps gros, p. 28.

This acid was analyzed by Chevreul by means of oxide of Class 1. copper, and he found the constituents to be

Div. II,

Now the number of atoms corresponding to these weights and to the atomic weight of the acid is the following:

Composition.

The analysis of Chevreul gives 27 atoms oxygen instead of 2 atoms. But this would make the atomic weight too high. We see that stearic acid must be of a very complicated nature indeed, since it contains no fewer than 72 atoms.

SECTION XXVII.-OF MARGARIC ACID.

This acid was one of the first which Chevreul discovered, History. but it was only by degrees that he was able to distinguish between stearic and margaric acid. It may be obtained by the following process:

Soap made of olive oil and potash is to be macerated for 24 hours in twice its weight of cold alcohol. The oleate of potash is dissolved and the margarate of potash remains undissolved. It must be well washed with cold alcohol and then dissolved in 200 times its weight of boiling alcohol. When the liquid cools the margarate of potash shoots into crystals, but still contaminated with a little oleate of potash. Dissolve it again in hot alcohol and allow it to crystallize a second time. To see whether it be now pure margarate of potash, decompose a portion of it by means of muriatic acid, and observe whether the acid obtained fuses at 140°; if it does, it is pure margaric acid, but if it does not, it is contaminated with oleic acid, and the salt must therefore be again dissolved in hot alcohol and crystallized. We must proceed in this way till we obtain pure margarate of potash. This salt is to be decomposed by muriatic acid, and washed and purified precisely in the way directed in the last section for purifying stearic acid.

* Chevreul sur les corps gros, p. 21.