closed, and then plunge it beneath the mouth of the bottle, keeping it closed until fairly beneath it; then gently withdraw the finger so as to let the water get at the potassium, -you will instantly find a copious evolution of gas ensue, which will rise into the bottle: it looks smoky at first, but agitate the bottle a minute or so, and it becomes clear; then, as usual, test it with a lighted taper, it inflames, in fact it is pure hydrogen. The oxygen of the water combining with the potassium to produce oxide of potassium or potassa, hydrogen is evolved and thus collected. Potassa dissolves in the remaining water of the bottle, and you can detect it by means of a bit of turmeric paper, which renders brown, because it is an alkaline oxide ; and now that you are convinced of the evolution of hydrogen, you can throw a bit of potassium on a very little water in a cup or glass; the metal floats on the water, evolving a beautiful rosecoloured flame; and the water being in smaller quantity than in the bottle and trough, a stronger alkaline effect is now manifest on the test-paper, because you will recollect that the small bit of potassium only decomposes a very little of the water, leaving much undecomposed, and capable of dissolving the potassa; but of course, if you put a globule of potassium, the size of a nut, into a few drops of water, it would all be decomposed, and the potassa be left in a solid state. The rose-coloured flame in this experiment is due to a little of the potassium combining with the nascent hydrogen, forming potassiuretted hydrogen, which inflames by the violent heat of the chemical action produced by the attraction of the oxygen for the principal part of the potassium. When you made the experiment under water instead of upon water, there was simply decomposition of the fluid, and no combustion of the nascent hydrogen, because no oxygen was there free to support it.

In these experiments you will remark that we have only succeeded in evolving the hydrogen of the water in a free state, the oxygen having entered into combination with the zinc or potassium ; and in all similar cases of the decomposition of water by metals, the oxygen is never evolved in a free state; if you wish to obtain both gases from water, you must decompose it by voltaic electricity; and the manipulations necessary for this experiment, as well as for the determination of some other facts connected with the history of hydrogen, and especially its combination with chlorine, forming muriatic acid, will form the materials of my next discussion.






ONE of the most beneficial victories of practical science is that by which the subtle and invisible agents which may have been used in the secret destruction of human life, are detected, seized, and exhibited. No matter how minute the atom, how mingled, how dissolved, the sagacity and skill of the modern chemist ascertains its presence, separates it from all possible combinations, fixes and exposes it, “palpable to sense.”

Too much notoriety cannot be given to this truth. Were the public-spirited vicar of Hatton living*, he would have felt it his duty to have promulgated it from the pulpit. The use of poisons, in this country at least, is now confined to the most ignorant classes ; and if the knowledge under consideration were thoroughly disseminated amongst them, almost the only motive to this species of murder would be taken away. A conviction of this truth has recently influenced the Society of Arts, and induced them to depart from the avaricious principle of hoarding up the scientific treasures which may have been intrusted to them until they can be published, either for emolument or vanityt.

In the early part of this year, a paper was presented to this Society, by Mr. James Marsh, of the Royal Arsenal, Woolwich, descriptive of A Method of separating small Quantities of Arsenic, from Substances with which it may have been mixed. The merit of this paper was estimated so highly, that the large gold medal of the Society was awarded to the author: and further, so admirable was the simplicity and efficacy of the process, so little the preparation and cost of apparatus necessary to make a most exquisite analysis, and so important to the public interest was the object of the process, that the Society ordered the instant publication of the paper, instead of imprisoning it in the pigeon-holes of the secretary, until the next succeeding volume could be published. Equally impressed with the importance and excellence of the process of Mr. Marsh, we propose to follow up this philanthropic intention of the Society of Arts, and to present it to our readers, satisfied that they will be struck with the beauty of this ingenious and practical application of chemical science. Mr. Marsh introduces the subject by stating that:

“Notwithstanding the improved methods that have of late been invented of detecting the presence of small quantities of arsenic in the food, in the contents of the stomach, and mixed with various other animal and vegetable matters, a process was still wanting for separating it expeditiously and commodiously, and presenting it in a pure unequivocal form for examination by the appropriate tests. Such a process should be capable of detecting arsenic, not only in its usual state of white arsenic, or arsenious acid, but likewise in that of arsénic acid, and of all the compound salts formed by the union of either of these acids with alkaline substances. It ought, also, to exhibit the arsenic in its reguline or metallic state, free from the ambiguity which is sometimes caused by the use of carbonaceous reducing fluxes. It appeared to me, that these objects might be attained by presenting to the arsenic hydrogen gas in its nascent state: the first action of which would be to deoxygenate the arsenic; and the next, to combine with the arsenic, thus deoxygenated, into the well-known gas called arsenuretted hydrogen. Being thus brought to the gaseous state, the arsenic would spontaneously (so to speak) separate itself from the liquor in which it was before dissolved, and might be collected for examination by means of any common gas-apparatus; thus avoiding the trouble, difficulty, and ambiguity of clarification and other processes whereby liquors, suspected of containing arsenic, are prepared for the exhibition of the usual tests, or of evaporation and deflagration, which are sometimes had recourse to in order to separate the arsenic from the organic substances with which it may have been mixed.

* The late Dr. Parr. It was in his + It is this principle which has dictated church, after morning service, that he an- the notice or request distributed by the nounced and exhibited to his parishioners, Royal Society, with the copies which are Dr. Carmichael Smyth's celebrated mode granted to a contributor of his own paper. of preventing and destroying contagion. Several other Societies in London are simi.

larly costive.

“I had the satisfaction of finding, on trial, that my anticipations were realized; and that I was thus able, not only to separate very minute quantities of arsenic from gruel, soup, porter, coffee, and other alimentary liquors, but that, by continuing the process a sufficient length of time, I could eliminate the whole of the arsenic in the state of arsenuretted hydrogen, either pure or, at most, only mixed with an excess of hydrogen. 6- If this

gas be set fire to as it issues from the end of a jet of fine bore into the common air, the hydrogen, as the more combustible ingredient, will burn first, and will produce aqueous vapour, while the arsenic will be deposited either in the metallic state, or in that of arsenious acid, according as it is exposed partially or freely to the air. The former condition is brought about by holding a piece of cold window-glass opposite to and in contact with the flame, when a thin metallic film will be immediately deposited on its surface; and the latter, by receiving the flame within a glass tube open at both ends, which, in half a minute, will be found to be dimmed by a white pulverulent sublimate of arsenious acid. By directing the flame obliquely within side of the tube, it strikes against the glass and deposits the arsenic partly in the metallic state. In this case, if the tube, while still warm, be held to the nose, that peculiar odour, somewhat resembling garlic, which is one of the characteristic tests of arsenic, will be perceived. Arsenuretted hydrogen itself has precisely the same odour, but considerable caution should be used in smelling to it, as every cubic inch contains about a quarter of a grain of arsenic.

“ The requisite apparatus is as simple as possible; being a glass tube open at both ends, and about three quarters of an inch in its internal diameter. It is bent into the form of a siphon (aa, fig. 1), the shorter leg being about five inches, and the longer about eight inches in length. A stop-cock b, ending in a jet of fine bore, passes tightly through a hole made in the axis of a soft and sound cork, which fits air-tight



into the opening of the lower bend of the tube, and may be

Fig. 1. further secured, if requisite, by a little common turpentine lute. To fix the apparatus when in use, in an upright position, a

Fig. 2. hole is made in the wooden block c for the reception of the lower part of the pillar d, and a groove is cut in the top of the same block, to receive the bend of the tube a a. Two elastic slips e e, cut from the

d neck of a

common bottle of India rubber, keep the tube firm in its place.

“ The matter to be submitted to examination, and supposed to contain arsenic, if not in the fluid state, such as pastry, pudding, or bread, &c., must be boiled with two or three fluid ounces of clean water, for a sufficient length of time.

“ The mixture so obtained must then be thrown on a filter to separate the more solid parts: thick soup, or the contents of the stomach, may be diluted with water and also filtered; but water-gruel, wine, spirits, or any kind of malt liquor and such like, or tea, coffee, cocoa, &c., can be operated on without any previous process.

“ When the apparatus is to be used, a bit of glass rod, about an inch long, is to be dropped into the shorter leg, and this is to be followed by a piece of clean sheet zinc, about an inch and a half long and half an inch wide, bent double, so that it will run down the tube till it is stopped by the piece of glass rod first put in. The stop-cock and jet are now to be inserted, and the handle is to be turned so as to leave the cock open. The fluid to be examined, having been previously mixed with from a drachm and a half to three drachms of dilute sulphuric acid (1 acid and 7 water), is to be poured into the long leg, till it stands in the short one about a quarter of an inch below the bottom of the cork. Bubbles of gas will soon be seen to rise from the zinc, which are pure hydrogen if no arsenic be present; but, if the liquor holds arsenic in any form in solution, the gas will be arsenuretted hydrogen. The first portions are to be allowed to escape, in order that they may carry with them the small quantity of common air left in the apparatus; after which the cock is to be closed, and the gas will be found to accumulate in the shorter leg, driving the fluid up the longer one, till the liquor has descended in the short leg below the piece of zinc, when all further production of gas will cease.

There is thus obtained a portion of gas subject to the pressure of a column of fluid of from seven to eight inches high: when, therefore, the stop-cock is opened, the gas will be propelled with some force through the jet, and, on igniting it as it issues (which must be done


quickly by an assistant), and then holding horizontally a piece of crown or window-glass (f, fig. 1) over it, in such a manner as to retard slightly the combustion, the arsenic (if any be present) will be found deposited in the metallic state on the glass; the oxygen of the atmosphere being employed in oxydizing the hydrogen only during the process. If no arsenie be present, then the jet of the flame as it issues has a very different appearance; and, although the glass becomes dulled in the first instance by the deposition of the newly-formed water, yet such is the heat produced, that in a few seconds it becomes perfectly clear, and frequently flies to pieces.

“ If the object be to obtain the arsenic in the form of arsenious acid, or white arsenic, then a glass tube, from a quarter to half an inch in diameter (or according to the size of the jet of flame), and eight or ten inches in length, is to be held vertically over the burning jet of gas, in such a manner that the gas may undergo perfect combustion, and that the arsenic combined with it may become sufficiently oxydized; the tube will thus, with proper care, become lined with arsenious acid in proportion to the quantity originally contained in the mixture.

“ When the glass tube is held at an angle of about forty-five degrees over the jet of flame, three very good indications of the presence of arsenic may be obtained at one operation; viz., metallic arsenic will be found deposited in the tube at the part nearest where the flame impinges,white arsenic or arsenious acid at a short distance from it,—and the garlic smell can be readily detected at either end of the tube in which the experiment has been made.

“ As the gas produced during the operation is consumed, the acid mixture falls into the short limb of the tube, and is thus again brought into contact with the zinc, in consequence of which a fresh supply is soon obtained. Tbis gas, if submitted to either of the processes before described, will give fresh indications of the presence of the arsenic which the mixture may have originally contained ; and it will be easily perceived that the process may be repeated as often as may be required, at the will of the operator, till no further proofs can be obtained.

“When certain mixed or compound liquors are operated on in this apparatus, a great quantity of froth is thrown up into the tube, which may cause a little embarrassment by choking the jet. I have found this effect to take place most with the contents of the stomach, with wine, porter, tea, coffee, or soup, and, indeed, with all mucilaginous and albuminous mixtures. The means I adopt to prevent this effect from taking

I place, or, at least, for checking it in a great measure, is to grease or oil the interior of the short limb of the apparatus before introducing the substance to be examined, or to put a few drops of alcohol or sweet-oil on its surface previously to introducing the stop-cock and its appendages. I have, however, found, if the tube be ever so full of froth in the first instance, that, in an hour or two, if left to itself, the bubbles burst, and the interior of the tube becomes clear without at all affecting the results.

“ In cases where only a small quantity of the matter to be examined can be obtained, I have found a great convenience in using the small glass bucket (g, fig. 2). Under such circumstances, the bent glass tube

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