Nitric acid may be detected by the ordinary tests for that acid. The juice may be boiled with metallic copper, when red fumes will appear, should nitric acid be present. Or, it may be much diluted and filtered, and one portion be made neutral with burnt magnesia, and boiled to expel all free ammonia; afterwards, by acting upon the liquid with a copper zinc couple, any nitrates may be turned into ammonia, distilled over, and titrated in the usual way.

F. Scribani* adds to the suspected lemon juice an aqueous solution of ferrous chloride, strongly acidified with hydrochloric acid and free from ferric salt. The liquid is boiled for a few minutes, and then a little sulphocyanide is added. If nitric acid has been present, it will have oxidised the ferrous salt into a ferric salt, and a deep blood-red colour will be produced by the

test.

§ 292. The Analysis of Lime Juice, with a view to ascertain its strength, is confined to the determination of the amount of citric acid and citrates.

The amount of citric acid is determined frequently for technical purposes by the aid of a special hydrometer, called "a citrometer," but this method is not exact enough for the purposes of the food-analyst. Nevertheless, it will always be well to take the specific gravity of the juice by a specific gravity bottle, or by the methods detailed at page 71, first boiling off any alcohol which the juice may contain, and making it up to the same bulk as before the dealcoholisation.

The amount of free acid may be estimated by means of decinormal soda. A known quantity of the juice is taken and coloured with phenol-phthalein, and then d. n. soda is run in until the colour changes. About 10 cc. of the ordinary raw juice may be taken and diluted to 50 cc.; but the concentrated juice must be much diluted before titration.

For

The free acid known, the next step is to determine the amount of citrates and other organic acids combined with bases. this purpose, the measured quantity of the juice which has already been neutralised by soda, is evaporated down, charred, and the charred mass treated with a known volume of decinormal sulphuric acid, which must be sufficient to more than neutralise the carbonates.

The acid solution is filtered and neutralised by d. n. soda; this will give the necessary data from which to calculate the amount of sulphuric acid used by the carbonates produced by the action of heat on the organic acids. This amount is equi

*Gaz. Chim. Ital., viii. 294.

valent to the total amount of organic acid; if expressed as citric acid, forty-nine parts of sulphuric are equal to seventy of H2CHO,H2O, or sixty-seven of 2H,CH ̧O2H2O.

5

The amount of free acid already obtained is now subtracted from the total acid, the difference being that which is combined with bases.

To ascertain the amount of real citric acid present in the juice, it must be determined as citrate of lime, for it need hardly be said that the process given above does not distinguish between malates, meconates, or any other organic acids converted by heat into carbonates. To determine the real citric acid, Mr. Warrington recommends the following processes from 15 to 20 cc. of raw lime juice are exactly neutralised by d. n. soda, the whole made up to about 50 cc., and heated to boiling.

While boiling, so much of a solution of chloride of lime is added as is known to be rather more than equivalent to the total amount of organic acids present. The boiling must be continued for half an hour, and the precipitate collected and washed with hot water. The filtrate and washings are evaporated to a small bulk (not more than 15 cc.), and a little ammonia added to exact neutralisation, if the liquid gives an acid reaction; a further precipitation takes place, and this second precipitate must be collected on a filter. The filters are dried and burnt up at a low heat, and their neutralising power with regard to acid is determined. For this purpose, the ash may be dissolved in standard hydrochloric acid, and titrated back; each cc. of normal HCl neutralised is equivalent to 070 grm. of crystallised citric acid. If either oxalic or tartaric acids should be present, the results are, of course, inaccurate.

*Journ. Chem. Soc., 1875, 934.

PART VIII.-CONDIMENTS: MUSTARD, PEPPER, &c.

MUSTAR D.

§ 293. Mustard is made from the seeds, finely ground, of the Sinapis nigra, or black mustard, or from those of the Sinapis alba, or white mustard, or again, from a mixture of both varieties. The manufacturer reduces the seeds to powder, and passes the product through a series of sieves. The portion in the first sieve is called the dressings, that which passes through is an impure mustard flour. The impure flour, on being passed through a second sieve, yields the pure flour of mustard and a second quantity of dressings. The dressings are utilised, by being submitted to pressure, for the sake of the fixed oil they contain.

Microscopical Structure of the Seed. The white mustard seed is made up of the husk and the seed proper. The seed proper is simple in structure, consisting entirely of minute oil-bearing cells; their size averages 00041 inch in the finely powdered seed; and they look extremely like starch corpuscles, but neither polarise light nor strike a blue colour with iodine.

The husk is more complex, and consists of three membranes:

1. An outer membrane, composed of two kinds of large transparent cells, which are described by Dr. Hassall thus:-Those of the first kind are of an hexagonal figure, and united by their edges so as to form a distinct membrane, the centre of each cell being perforated; the cells of the second kind occupy the apertures which exist in the previously-described cells, and they are themselves traversed by a somewhat funnel-shaped tube, which appears to terminate on the surface of the seed. Immersed in water, these cells swell up to several times their original volume, occasion the rupture of the hexagonal cells, and become themselves much wrinkled or corrugated, the extremity of the tubes in some cases being seen protruding from the proximate termination of the cells. It is possible, however, that what are here described as two different kinds of cells really form distinct parts of the same cells. It is from these cells that the thick mucilage obtained by digesting mustard seeds in water is derived.

2. The second layer, or middle tissue, consists of very minute, angular, coloured cells.

3. The inner or third layer of the husk consists of a single layer of angular cells.

The black mustard, in its structural composition, differs from the white only in not containing the large perforated cells of the husk, the outer membranes consisting of two or three layers of large, transparent, hexagonal cells, the other structures being similar to those already described.

§ 294. The accompanying tables (L., LI.) give some careful analyses by C. H. Piesse and Lionel Stansell* of black and white mustard :

TABLE LI.-ANALYSES OF ASH OF MUSTARD SEED.

§ 295. The Chemistry of Mustard is extremely interesting; both seeds, white and black, contain a fixed oil (from about 36 per cent.), and a sulphocyanate of sinapin and myrosin. Black mustard seeds contain, in addition to the foregoing, myronate of potash (about 5 to 6 per cent.) When the powdered black mustard seeds (or the mixed black and white) are moistened with water, the myronate of potash acts upon the myrosin, and produces the volatile oil of mustard. White mustard seeds, on the other hand, contain also a sulphur principle, sinalbin, not found in black.

Sinapin, CH2NO.-Sinapin exists as a sulphocyanate, both

Analyst, 1880, p. 161.