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part of the three plates; the three plates, taken conjointly, have only 5 of a line of thickness, and how is it possible to believe that, during an experiment, which is continued always for some moments at least, the corresponding points of the three plates can maintain different temperatures? This would be contrary to all the phenomena of the conductibility for caloric, and the constant tendency of this agent to attain an equilibrium. Besides, it is of little importance; the electric current developes heat in traversing the helix of Brequet's thermometer, and I have found, in a memoir anterior to that now in question,* that the intensity of the current, concluded from the elevation of temperature which it determined in this helix, does not sensibly differ from that deduced from the indication of other galvanometric processes. M. Lenz will not probably have been aware of the memoir to which I have just referred.

But yet, I do not wish to conclude, from the preceding remarks, that the intensity of the current is exactly proportional to the indications of Brequet's thermometer; I have never made this pretension, and that the more, because I believe it would be very difficult to find an instrument whose indications would be in exact relation with the variations of the current. If I had made use of a metallic thermometer, it is on one hand, that I believe it to be useful, when studying electric currents, to examine the variations of their intensity, under the relation of all their effects ; on the other hand, it was impossible to avail myself, in the study of that species of currents which I wished to analyse, of the chemical galvanometer, or of the magnetic galvanometer, because of the alternative non-interruption in the direction of the successive currents. In fact, with the chemical galvanometer, the two gases, which are the product of the decomposition of the water, arrive, almost simultaneously, on each of the platinum wires which serve as poles, and they are combined immediately, in great proportion, to recompose water; and with the magnetic galvanometer, the two alternative currents succeeding each other with great rapidity, the needle obeys, indifferently obeys, one or the other, following its position by relation to the wire of the instrument, or remains perfectly in equilibrio.

M. Lenz, in repeating and varying my experiments, made use of the magnetic needle ; but then he has only employed a single instantaneous current, such as we obtain by detaching from a magnet a morsel of iron surrounded by a metallic wire in which a current is developed by induction. I do not hesitate to attribute the differences which exist between the results which he has obtained and those to which I was led, to the diversity of the currents which have separately been made use of by us. That M. Lenz finds it more interesting to determine the effects of a single current, nothing can be more legitimate. But, at the same time, I may be permitted to oc

Mem. de la Soc. de Phys. et d'Hist. Nat., t. vii, p. 457, and Ann. de Ch. et de Phys., t. Ixii, p. 147.

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cupy myself with phenomena which produce a succession of instantaneous currents alternately directed in contrary directions. I believe that this study may have important consequences, and I have commenced it; it is not by caprice, but for motives dated far back, and which have made me adopt with eagerness a simple and easy method of procuring a series of currents placed in the condition which I have above referred to.

( To be continued).

On the Extraction and Decolorization of Gelatine. By J. C. Booth

and M. H. Bove.* In a patent granted to S. G. Dordoy, of Surrey County, “ for certain improvements in the manufacture of gelatine size and glue," (see London Journal for January, 1842), the patentee proposes the employment of chlorine, for bleaching the materials employed, previously to the extraction of the gelatine, in which respect the patent mainly differs from former attempts at bleaching by operating on the gelatinous fluid after its extraction.

“For every 100 lbs. of the animal substances, eight ounces of chlorate or chloride of lime, potash, soda, baryta, or other similar compounds are dissolved, or thoroughly mixed in, or with, two or more gallons of hot or cold water; four pounds of hydrochloric or other acids being added and stirred thoroughly. This mixture is to be poured into the vessel containing the water and animal substances, the materials being stirred continually while the mixture is added. The animal substances should be kept entirely covered with the water for twenty-four hours.”

The above proportions are considered sufficient for thin skins, as of sheep, but for heavier pieces, such as those derived from oxen, calves, &c., two or three steepings will be requisite, each continued the same time, until the substances appear of an uniform, transparent whiteness. The substances are thoroughly washed in pure water at ordinary temperatures, and then water at 160° Fahr. is poured on, and the temperature maintained at 100° Fahr. from twelve to twentyfour hours, when the gelatinous solution is strained off. Fresh supplies of water are successively added, each portion at a temperature of 20° higher than the preceding, until at last the water is boiled.

This patent, like many others, makes a sweeping claim “ to cover all the ground," but it might be restricted to the use of chlorine or its compounds with oxygen, for bleaching animal matters previous to the extraction of gelatine, and not subsequent to this process, which had been repeatedly tried. In the latter case it has been found that although the colour of the gelatine may be improved, its binding quality is materially impaired; but there is every reason to believe that the colouring matters may be obviated by the application

Journal of the Franklin Institute, May, 1842.

of bleaching previous to extracting gelatine, since it is the opinion of chemists that this substance is not ready formed in the animal matters employed. Since, then, the patent contains good principles, it may be well to inquire into some of its details.

As is too often the case with chemical patents, its descriptions are vague and incompatible with the definite laws of chemistry. Thus it is not immaterial whether we employ the same amount of acid (of what strength ?) with the same quantity of chlorate of potassa, soda, lime, or baryta, or with the chlorides of the same bases. Probably the chloride of lime is the only compound that can be employed economically ; and in this case, if, as the patentee directs, the acid, which is twice as much as necessary, be poured first into the solution of the chloride, a considerable amount of chlorous or bleaching material will be lost. It would be advisable either to dissolve the chloride in two or more gallons of cold water, and then add the acid previously diluted with three or more gallons of water, or to add the clearly drawn off solution of chloride to the water over the animal substances, and then to add the acid diluted largely with water. In either case, a bleaching liquor will be obtained which will act more uniformly and for a greater length of time without being exhausted.

The successive extraction of gelatine by water at low and increasing temperatures is worthy of notice, although not novel; but in this case the latter solutions should be employed for gelatine of decreasing qualities.

On the Preparation of Aluminous Mordants. By James C. Booth

and M. H. Boye.*

WHATEVER relates to improvements in dyeing and cloth printing being of high importance, we have thought it worth calling attention to the above subject from the appearance of two patents, the one taken out in England by R. Hervey (Rep. Pat. Inv., December, 1841), and the other in this country (Jour. Frank. Inst., March, 1842). They are worthy of consideration from the comparative cheapness with which the aluminous mordants may be prepared.

The most important point is the formation of sulphate of alumina by the direct action of sulphuric acid upon clay which has been calcined in a reverberatory furnace. Supposing the process to be successful, as described by the patentee, it offers great advantages, since an abundance of clay comparatively free from iron may be obtained in many districts of country, and even a portion of that iron must be rendered insoluble by calcination. In one respect the sulphate of alumina may be more advantageously employed as a mordant or colour-base, than the sulphate of alumina and potassa (common alum), since a given weight of it will contain a larger proportion of colour

Journal of the Franklin Institute, May, 1842.

base than the same weight of alum ; but, on the other hand, crystallized alum is so uniform in composition, that in its employment we operate more definitely and certainly, according to given weights, while the sulphate of alumina is difficult to be brought to a crystalline state. It is, moreover, liable to form basic salts, so that we cannot know with certainty and readiness the exact amount of alumina present in a solution of the salts. For these reasons the patentee is in error in supposing that the sulphate of alumina is superior to alum for printing, dyeing, &c.

The sulphate of alumina may be more successfully employed in the manufacture of the acetate, by means of acetate of lime or lead, for by decomposing alum perfectly, it is requisite to add a sufficient quantity of acetate to precipitate the whole of the sulphuric acid, not only that combined with alumina but also with the potassa, so that a considerable amount of the acetate is employed to no useful purpose ; whereas for every proportion of the sulphate of alumina alone, that is decomposed by an acetate, an equivalent quantity of acetate of alumina is formed. The most economical method is to precipitate with acetate of lime as long as a precipitate is formed, and then to throw down the small balance of sulphuric acid by acetate of lead. The second method described in the English patent, of precipitating alumina by alkali, or its soluble carbonate, and then redissolving the alumina in acetic acid, may often be practised economically, but there is then some difficulty in obtaining a liquid of uniform strength.

The remarks which have been made apply to both the American and English patents, and particularly to the latter, which is more copious. Since each of the points claimed in the patents have been the subject of previous experiments and manufacture, it appears to us that it would be a difficult matter to sustain the combination, since other simple methods of removing the iron may be resorted to.

On the Application of Voltaic Electricity to Calico Printing.


Before entering into the details of the method of applying voltaic electricity to the printing of calico, and other textile fabrics, such as silk, paper, &c., it may be useful to give a brief sketch of the priuciples upon which calico printing, dying, &c., is at present carried on.

If we dissolve a small quantity of corrosive sublimate in a wineglass of water, and add to this a little hydriodate of potassa, an immediate change is produced; and the two solutions, which in a separate state were perfectly clear and colourless, are instantly rendered opaque by the formation of a splendid scarlet pigment, the periodide of mercury, which gradually subsides to the bottom of the glass.

If, in this experiment, we substitute acetate of lead for corrosive

sublimate, the precipitate is a rich yellow; and so, by varying the metallic solution and the reagent employed in its precipitation, any desired colour may be obtained. In fact, it is by this means that colours are ordinarily formed for the use of the artist; and by an elaborate extension of the same principle, the calico printer is enabled to adorn the produce of the loom with his varied and beautiful devices.

It is the business of the manufacturer to select from the vast multitude of coloured compounds which his art can thus produce, such as are at once the most pleasing, economical, and permanent; for permanency of colour is here a most essential requisite. The dyes employed must be such as will be subsequently unaffected by washing in hot or cold water, and totally unalterable under all the casualties to which, in the course of fair wear, they are likely to be exposed. These several points are attained in a very simple manner.

The metallic solutions, or mordants, being previously inspissated with starch, gum, pipe-clay, or other appropriate thickener, are deposited in their respective places upon the cloth to be printed by means of a machine of great cost and complexity ; indeed, its construction is so very intricate, that it would be difficult to attempt a description of its parts within the limits of this paper.

After the impression is thus given, the cotton is allowed a short time to dry. It is then immersed in the dye-bath, a vessel containing a suitable reagent. As the decoction penetrates the cloth it encounters the mordants, producing a number of coloured precipitates so closely entangled amongst its fibres, and incorporated with its substance, as to be perfectly proof against all subsequent washing.

The details of this branch of manufacture extend over so wide a field, and the niceties of manipulation required in its practice are so great, that a very brief and cursory notice must suffice.

The most important mordants in use are the acetate of alumina, the acetate of iron, and the protomuriate and permuriate of tin.

The colouring matter of cochineal is fixed by means of tin with the production of a scarlet, and by the addition of alum the scarlet is changed to crimson. A decoction of madder may be substituted for cochineal with but little inferiority in the result. A beautiful yellow is obtained by printing with acetate of lead, and subsequently immersing the cloth in bichromate of potassa. The ammoniacal solution of sulphuret of arsenic is also productive of a good yellow; so is a decoction of weld, of quercitron bark, of fustic, sumach, Persian berries, &c. ;—the mordants in these cases being the salts of alumina and of tin. The first object of the chemist in every instance is to render the colouring matter soluble, so that it may thoroughly penetrate the pores of the cloth ; and then, when the latter is perfectly saturated, to render it insoluble, and thereby fix it.

Blue is readily obtained by printing with an iron mordant, and afterwards passing the cloth through an acidulated solution of ferrocyanate of potassa ; but indigo is more commonly employed for

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