wise, and, when completed, can be folded or rolled up in the same manner as a length of common linen.

After a brief delay, for the purpose of drying, the next and last process of cutting is effected. By the introduction of modern machinery, this part of the preparation of isinglass is performed with surprising celerity, and the material is cut into very fine shreds.

The cutting machine is a cylinder with some five or six keenedged blades fixed in a tangential direction to the cylinder. The same engine which serves to roll out the isinglass, as already described, suffices to turn this little machine at the rate of some 800 or 1,000 revolutions per minute; taking a low estimate, we will suppose it turns 800 times. On examining the cylinder we find five or six blades set in it, and as each of these knives severs a shred from the width of the “ribbon,' while the cutting process is going on, it follows that four or five thousand shreds are cut in the short space of one minute.

Such is the plain and simple method of preparing cut isinglass.

There are, however, many consumers who still prefer the oldfashioned style of hand-cut isinglass. In this case, the thin leaf is pulled to pieces with the fingers or divided into strips with scissors, a work mostly performed by women.

The shreds of isinglass, softened in cold water and examined under the microscope, are seen to possess a fibrous structure, a few vessels, granular cells, and nuclei being scattered here and there; it is, in fact, an organised substance (fig. 137).

THE ADULTERATIONS OF ISINGLASS.

The principal adulteration of isinglass is with gelatin, an article in every respect inferior to isinglass.

Usually shreds of gelatin are mixed with those of isinglass. Occasionally the gelatin is incorporated with the isinglass while it is in sheets.

Most frequently, however, gelatin is substituted for isinglass. The best isinglass, of course, is Russian ; this is often deteriorated by admixture with a very inferior article, termed Brazilian isinglass ; in other cases, this is substituted for the better and more valuable description of isinglass.

Results of the Eramination of Samples. Of twenty-eight samples of isinglass subjected to examination, ten, or more than one-third, of the samples consisted entirely of GELATIN.

THE DETECTION OF THE ADULTERATIONS OF ISINGLASS. Between isinglass and gelatin several well-marked distinctions exist; some of these are sufficiently simple to enable the ordinary observer himself to distinguish the one article from the other.

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All that is necessary to effect the discrimination is to spread a few of the filaments out on a slip of glass, to moisten them with water, and after the lapse of a few minutes to note well the appearances presented by them.

Isinglass and gelatin differ, especially in the following characters :

The shreds of isinglass, when immersed in cold water, become white, opaque, soft, and swollen.

The swelling is equal in all directions, so that, when viewed with

a low power of the microscope, the shreds appear more or less quadrangular.

In boiling water they dissolve nearly without residue.

The smell of the dissolved isinglass, when hot, is somewhat fishy, but not unpleasant.

The moistened shreds, or the solution, exhibit to test paper a neutral, or faintly alkaline, and rarely a slightly acid reaction.

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Under the microscope the filaments exhibit a well-marked fibrous structure.

In acetic acid they swell up, and become soft and jelly-like, the greater part of the structure being lost.

Lastly, 'The ash which results from the incineration of good Russian ising lass is of a deep red colour; it contains but a small portion of carbonate of lime, and never amounts to more than nine per cent. of the isinglass used.'1

The shreds of gelatin, on the contrary, when placed in cold water, swell up, acquire increased transparency, and become translucent and glass-like.

The form which the shreds take in swelling is peculiar: they do not, like those of isinglass, swell equally and remain quadrilateral, but become expanded, flat, and ribbon-like, the broad surfaces corresponding to the incised margins.

The dry shreds on the uncut surfaces frequently present a peculiar, shining lustre, not unlike that of tinsel.

In boiling water they do not entirely dissolve, but in most cases a copious deposit falls to the bottom of the glass.

The smell of the hot infusion is like that of glue, and therefore disagreeable.

The moistened filaments, or the solution of gelatin, usually exhibit a strong acid reaction: this in some cases is due to the substances used in bleaching it.

They show no structure under the microscope, but only the marks of the instrument employed in cutting them.

Immersion in dilute acetic acid hardens gelatin.

Lastly, the ash is different from that of isinglass in amount, colour, and composition. '100 grains of gelatin give from 2.3 to 2:6 grains of ash, which is white, contains much carbonate of lime, with some chlorides and sulphates.'-Letheby.

It is therefore very easy to distinguish between isinglass and gelatin, even when the shreds of the two articles are mixed together in the same parcel.

The discrimination is, however, much more difficult when they are both incorporated in the same shreds or strips ; nevertheless, by means of the microscope, this adulteration, first described by Dr. Redwood, may frequently be discovered.

If, on examination with that instrument, the shreds, after immersion in cold water for a few minutes, exhibit a thick border of a clear and structureless substance, there is no doubt but that the reds are coated with gelatin.

Some of the better kinds of Brazilian isinglass are manufactured in the same way as Russian, and sold at a cheaper rate. No doubt, in some instances, this is mixed with or sold as the best, and it has been ascertained that acids and other chemicals have been used to improve its colour ; but the test of good isinglass is in the jelly made therefrom.

The jelly made from Russian isinglass dissolves readily, furnishes scarcely any sediment, and is remarkably firm, pure, and translucent.

On the other hand, Brazilian isinglass makes a far inferior jelly, with these remarkable differences: that whilst Russian isinglass is firm and free from deposit, Brazilian isinglass leaves a deposit of insoluble matter amounting to 20 or 30 per cent., is less readily dissolved, and the jelly is opalescent and milky.

On making blanc-mange with the purest Russian isinglass, milk is needed to impart the snow-white colour of that jelly; but in the case of Brazilian isinglass hot water alone will render it nearly of that colour. It is almost needless to add that the blanc-mange is much inferior in quality, and the large percentage of insoluble matter renders the jelly proportionately weak.

The quality of any isinglass may easily be tested by dissolving a small portion in a glass vessel, with about a tablespoonful of boiling water. The best Russian isinglass will instantly dissolve, and scarcely a particle of sediment remain; the soluble matter in this article being, according to the best authorities, ninety-eight grains in every hundred.

The same test applied to Brazilian isinglass will extract the gelatin, but the shreds, from their fibrous character, do not entirely dissolve; they turn white and retain their form, unless disturbed, in which case they break up, and form a deposit at the bottom of the vessel.

'If Russian isinglass be adulterated with Brazilian, the admixture may easily be detected by the insoluble shreds, or white deposit, which is sure to appear in proportion to the amount of Brazilian isinglass that may be introduced. The smell of the latter also is strong, far from pleasant, and forms a great contrast with the faint, inoffensive, seaweed-like odour of Russian isinglass.

CHAPTER XXIII.

GELATIN AND ITS ADULTERATIONS.

DEFINITION OF ADULTERATION,

Any addition of salt or sugar.

PREPARATION OF GELATIN. We have been at some pains to procure the following information respecting the manufacture of gelatin.

Ordinary gelatins are made from those pieces of skins which are cut off by the tanner as unfit for making leather, in consequence of thickness. The best description is prepared from the skins of calves' heads; these are separated from the whole skins after they have passed through the process of liming, to remove the hair from them.

The skins are next well washed to get rid of the lime, and all the pieces of flesh and fat are carefully cut out; some manufacturers soak them for a short time in a dilute solution of muriatic acid, to remove any remaining portion of lime ; but this practice is both injurious and unprofitable. The acid forms with the lime chloride of calcium, which, if it is not carefully removed by washing, is boiled up with the skins, and, being soluble, remains in the gelatin; a portion of the skins is also dissolved by the acid, and is thrown away in the water employed in washing them, which thus occasions a loss in weight.

In some cases the skins are boiled whole, in others they are cut into small pieces, or even reduced to a pulp by a machine especially constructed for the purpose.

If the skins are cut into fine pieces instead of being put into the boiler whole, the gelatin will be better, that is, it will be of a lighter colour ; and the process is more economical, as one-half the time will be saved in the boiling, and much less heat and fuel required. As the gelatin is darkened by prolonged boiling, the reduction of the skins to a pulp is a point of very great importance in the manufacture of gelatin-so much so, that Mr. Swinburne has obtained a patent for this method of preparation.

The skins are boiled with water, in the proportion of about one gallon of water to seven pounds of skin; a small quantity of common salt is added to preserve the gelatin. After it has boiled for about twelve hours, it is strained and clarified with white of eggs, and then run upon glass plates; as soon as it is solid, it is cut into slices and

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