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with zinc filings, the two bodies immediately act, and a hea above that of boiling water is produced.

Zinc is not the only common metal which thus rapidly decomposes chloride of silver, in the dry way. Tin acts even more powerfully when triturated with it: and copper and iron have both of them affinities for chlorine strong enough to produce the same effect.

There is therefore no occasion to assume hydrogen as the decomposing agent, when chloride of silver is reduced in contact with zinc or iron (iron acts as zinc does in all these experiments, though not so powerfully); for the metals, by their attraction for chlorine, are sufficiently energetic to produce the effect. Yet, as I had supposed, from general opinion, that hydrogen could, by its attraction for chlorine, separate that element from silver, I endeavoured to ascertain in what circumstances it had the

power of doing so. If a stream of hydrogen, rapidly generated from iron or zinc, be sent against moist chloride of silver, in a dark place or by candle-light, it appears to alter it; but this effect must be due to metals or impurities held in solution, for when purified, it has no power of changing it out of day-light; nor have I been able, even in the sun-shine of this month, to make hydrogen act on chloride of silver in several hours.

Still, however, hydrogen may be allowed in certain circumstances to have the power of decomposing chloride of silver, but the circumstances are not such as were, I believe, generally supposed to have place in the experiment first referred to. When zinc, iron, tin, &c., are thrown into moist chloride of silver, the first decomposition is occasioned by the action of the zinc on the chloride, afterwards a voltaic circle is formed by the zinc, the reduced silver and the water; water is decomposed, the zinc takes oxygen, the hydrogen liberated at the surface of the silver takes the chlorine from the chloride in its immediate neighbourhood, and thus the reduction will go on to the distance of an inch or more from the piece of zinc, and the consequent products are silver and solution of muriate of zinc. But as this is a case of decomposition entirely different to the supposed one of the reduction of chloride of silver, by hydrogen, any

denial of the latter is not at all invalidated by the truth of the former.

8. On a particular Substance from the Thermal Waters of Ischia.-Extract from a Letter from Sig. Carlo di Gimbernat, Counsellor of Legation, &c., to Count Moscati, at Milan, dated Naples, Feb. 4, 1819.

"The bottle received with this sheet, contains the substance which is formed by the vapours of the thermal waters of Ischia,and which I discovered last October. It is perfectly identical with that I found at Bader. This substance covers like an integument many rocks in the valleys of Senagalla and Negroponte, at the foot of the famous mountain Epomeo, beneath which the poets confine Typhon; and it is curious to find, exactly in this place, a substance very similar to skin and human flesh *. One part of the mountain, covered with this substance, was found, on measurement, to be forty-five feet long, and twenty-four feet high. The trials I have made to ascertain its nature, gave me, by distillation, an empyreumatic oil; and, by boiling, a gelatine, with which paper might be sized. These results are the same as those I obtained at Baden, and appear to me to characterize its animal nature. The presence, therefore, of an animal principle in thermal waters is confirmed, which being volatilized by heat, is found near the sources in those places where the vapours, by the concurrence of favourable circumstances, are condensed and deposited. This principle I have called Zoogene."

9. On Gluten, and its Action on Guaiacum.-The following observations are contained in a Letter from the Marquis Ridolfi to Dr. Brugnatelli:

*The Editors of the Giornale di Fisica, from whence this extract is taken, state, that they have seen the substance obtained by M. Gimbernat, and that they can assure (their readers) that when observed externally, it appears like true flesh, covered with skin. They refer also to Breislak's Voyages, &c., where there is an account of a substance, collected by M. Pilhes, from the mineral waters of Aix and Ussat, and which was analyized by Vauquelin. It had many analogies with albumen and mucilaginous matter.-Giornale di Fisica, 11. p. 178.

"D. Taddei, my companion in Natural Philosophy, having undertaken researches in fermentation, and particularly in that of grain and pulse, in various cases has ascertained that the gluten of wheat is composed of two substances, perfectly distinct from each other, one of which he has named gloiodina, and the other zimoma. The first of these gives to gluten its elasticity; and the second is the cause of the fermentation which takes place in the mixtures of gluten with other vegetable substances. D. Taddei had occasion to mix various gums, gum resins, and resins, with the different kinds of flour. Amongst the mixtures it was found, that that of the powder of the resin of guaiacum with wheat flour, became of a very fine blue, as soon as it was well kneaded with water, in contact with the air. Various colours were produced with the flour of other kinds of grain; and it appeared, that the shade of blue colour, produced by the various mixtures, corresponded to the quantity of zimoma contained in them. D. Taddei has made me acquainted with these facts, and whilst he has been engaged on other things, I have pursued them somewhat farther. I have observed that guaiacum does not give any blue colour to pure starch, however mixed with it, nor to any other vegetable substance, not containing zimoma; that it is not at all, or very slightly, tinged by flour poor in gluten, and that it is not tinged by that in which the gluten has been much altered. But when guaicum is mixed with pure gluten, or pure zimoma, the colour instantly appears, and is a most superb blue. Guaiacum does not, however, become at all coloured by mixture with zimoma, unless contact with the oxygen of the air be allowed.

The powder of guaiacum is, therefore, a re-agent, capable of discovering the alteration which flour may have undergone by fermentation in magazines, ships, &c.; and also of ascertaining if it be mixed with the flour of other seeds deficient in gluten. It will also test the purity of starch. The flour of grain is consequently too a test of the purity of the resin guaiacum, which in commerce is almost always adulterated and falsified.-Giornale di Fisica, I. p. 168.

10. New Vegetable Alkali.-M. M. Pelletier and Caventon, give the following account, in a note, of a new alkali, discovered in the seeds of the Veratrum Sabadilla:

"Whilst pursuing our researches on those vegetable substances, which have a marked action on the animal system, we were induced to examine the Ciradille, and we found in them a crystallizable alkaline matter, extremely acrimonious. We as yet are only imperfectly acquainted with this substance; but being aware that many chemists are engaged at present in the analyses of vegetable substances, we have thought right to announce this particular principle.

"We have not been so fortunate in the analyses of hemlock, on which we have lately been engaged; this plant has not offered any thing particular. We have not been able to isolate its active principle, which appears extremely fugacious. Some, traces of it may be found in the ethereal tincture of hemlock.

11. Acids of Arsenic.-The last determination of M. Berzelius, respecting the composition of these acids, is as fol

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The proportions of the oxygen in the two acids being as

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Sub-arseniate of lead.. 100 arsenic acid.

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12. Oxides of Mercury.—Mr. Donovan has, in his late experiments on the compounds of mercury, given the following as the only proportions he could obtain of the elements in the two oxides; though the experiments were frequently and carefully repeated:

Black oxide. Mercury...... 96.04

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It is observed also, that mercury perhaps presents an exception to the general law, that the second dose of oxygen is retained by a metal with less force than the first. If red oxide of ́ mercury, in grains, be exposed nearly to a red heat, it becomes black; but it is still peroxide, for if immersed in mercury, or water, so as to exclude oxygen, it comes out, when cold, even a brighter red than before. The red oxide bears a much higher heat than the black. When black oxide is exposed to a moderate heat, part is reduced, and part is raised to the state of peroxide; which then bears even a low red heat unaltered. It is true that light renders red oxide black; but this is only in its progress to the metallic state. All these facts appear to support the exception.-Thom. Annals xiv. p. 246.

13. Volatility of Oxide of Lead.-Dr. Thomson states the volatility of this substance to be such, that it rises in a common red heat. In the conversion of lead into litharge, and the litharge into lead again, by the manufacturer, a loss of about 10 per cent. is suffered from this cause.

14. New Acetate of Lead.-Dr. Thomson has published the analysis of a new acetate of lead. It was obtained at the manufactory of Charles Macintosh, esq., in Glasgow, in large flat rhomboidal crystals. The crystals were white and translucent, and consisted of flat rhomboidal prisms, with angles of

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