the stopper of each; then place the neck of the bottle of hydrogen close against that of common air, and transfer, as you would do in the pneumatic trough, the hydrogen into the bottle of air; thus it will rise through the air, displace it, and fill the bottle, as a lighted candle applied to its mouth will show you directly, by an explosion; but if you similarly test the bottle that first contained the hydrogen, you will have no explosion, because it is now filled with air, in which, of course a candle will quietly burn.

Hydrogen is the lightest substance with which the chemist is acquainted,—indeed so light, that formerly its ponderable nature was doubted by some philosophers, for good balances could not detect its weight, even when in very large volume. The chemist, however, invoked the aid of the mechanist, who produced a balance capable of being affected by this highly-attenuated elementary form of matter, 100 cubical inches of which were found to weigh about two grains; still further perfecting the balance, hydrogen was found to be yet a little heavier, until at the present day its weight is determined with all the accuracy that can be ensured, and it is found that 100 cubical inches weigh 2118 grains. Such is the perfection of mechanical skill, that some balances are actually affected by the weight of one cubical inch of hydrogen!

Besides inflammability and levity, hydrogen has another remarkable property, namely, that of producing musical sounds during its combustion, and the mode of experimenting to obtain the result is sufficiently simple. Obtain a common brass blowpipe, heat its smaller end red-hot in the fire, suffer it to cool, and then make it perfectly straight, (you must not attempt to do this whilst it is hot, for hot brass is very brittle;) substitute this for the tobacco-pipe in the "philosophical candle," and proceed to evolve hydrogen, which, after the lapse of a minute or two, kindle at the end of the brass tube. You must not have a very rapid evolution of the gas, therefore do not pour in much acid, neither let the zinc be too small,—indeed a lump or two is better than a number of small pieces, for you do not require much hydrogen, and only want a very small flame. Having succeeded in obtaining this, hold over it so as to enclose it, a tube of almost any material-glass, brass, tin, or copper, about an inch in diameter and eighteen inches or two feet long,—and a singular musical sound is very soon produced. By partially closing the tube at top, or by employing various-sized tubes, a great variety of tones may be produced.

This effect is not, however, peculiar to hydrogen, for other inflammable gases also produce it, but none in such perfection as hydrogen; there is no magic in the matter, for it simply depends on the infinite number of small explosions of hydrogen with the common air, and these succeeding each other with vast rapidity, and being reverberated by the sides of the tube, combine to produce a musical sound.

If, in order to produce a musical sound by the combustion of hydrogen, you happen to hold a glass tube over the flame, you will find its interior very soon covered with a sort of mist or dew; or if you hold a clean, dry, and cold bell-glass over it, although you obtain no sound, yet you still find this deposition of moisture on its interior.

I have now to point out to you the cause of this remarkable ap

pearance, which at first sight you would naturally enough conclude was referrible either to the dampness of the glass, or to some steam arising from the effervescent mixture in the bottle to which the jet is attached. In order to satisfy you that the appearance of the moisture is not accidental, it may be worth while to prepare and burn some perfectly dry hydrogen; and this experiment will give you a notion of the way in which some gases are desiccated.

B

Arrange a little apparatus like the annexed figure. Two bottles, each fitted with a cork, perforated so as to allow the tube from A to pass to the bottom of B, from whose cork a straight tube arises: a piece of tobacco-pipe will answer the purpose, or the brass tube already spoken of. Pour some strong sulphuric acid into B, so that the end of the bent-tube (which should be of copper or glass) just dips beneath it; you can easily judge how much will be necessary by previous a outside admeasurement; then put in the cork, and make it fit tight with melted wax. Into A put the zinc, water, and acid, and then quickly make its cork tight also. Now you will observe that as the hydrogen evolves in A, the bent-tube conducts it into в, and as it bubbles through the sulphuric acid, should any steam or moisture be hurried over by the heat of the effervescence, it will be absorbed and arrested by the acid, which is exceedingly hygrometric, and has a strong attraction for water; dry hydrogen, therefore, will now fill B, and issue from the jet, and when you judge that all the common air is expelled from both bottles, you may the hydrogen; hold the dry bell-glass over its flame, you will observe that although the gas is dried, yet its flame still deposits moisture on the interior of the cold glass. The reason why this takes place is as follows. You very well know that a combustible body cannot burn without some supporter of combustion being present,-that a candle burns in air because the oxygen of the air supports its combustion; such also is the case with hydrogen; its flame is in this experiment supported by the oxygen of the air; the hydrogen and the oxygen, therefore, have a mutual affinity for each other; they combine, and, however strange it may appear, the result of the combination is water. This is an excellent instance of chemical affinity, and shows you in a most striking manner how completely chemistry is a science of experiment. Who would imagine that water consisted of two invisible gases? that this, however, is the case, has been incontestibly proved, and it is found that whenever hydrogen has its flame supported by oxygen, nothing but water is the product of he

combustion.

kindle

Then what is the product of the combustion of a candle, a lamp, or a fire? Why this is rather more complicated, but I must just mention it here, for I think you will understand it. The wax or tallow of the candle, the oil of the lamp, and the coals of the fire, consist of hydrogen and carbon, both combustible bodies, and when these enter into combustion, the hydrogen unites with the oxygen to produce water, and the carbon with the oxygen to produce carbonic acid, so that when you say that the candle, the lamp, or the fire are burnt out, you, in fact, mean

that they are resolved into water and carbonic acid. Considering the enormous quantities of fuel hourly consumed, either for the sake of its heat or of its light, you may very naturally put the question, what becomes of all the water and carbonic acid? why are we not drenched with showers of the one, or suffocated by the deleterious vapour of the other? For this reason, because air has the remarkable property of dissolving watery vapour, and therefore as it pours into the air, it is rapidly wafted along with the carbonic acid, and so diffused throughout the enormous bulk of the atmosphere, that we never find any inconvenient accumulation of these products of combustion.

To show that water is produced by the hydrogen of a burning candle, hold a cold and dry bell-glass over it, and you get the watery vapour condensed on the cold surface directly; now close the mouth of the bell-glass with a card or plate, turn the mouth uppermost; remove the card, and quickly pour in a little lime-water, a perfectly clear liquid, but it instantly becomes turbid and milky, upon meeting with the contents of the jar; lime-water is a test of carbonic acid,—it unites with it to form carbonate of lime or chalk, which is the cause of the turbidness. Try a similar experiment with the flame of pure hydrogen,-you only obtain water, and the lime-water is not troubled.

The discovery of the composition of water was made by the celebrated Mr. Cavendish, and its composition is now universally admitted to be oxygen and hydrogen, in the proportions by weight of eight parts of oxygen to one part of hydrogen = nine parts of water; you will remember that eight and one are the equivalents of oxygen and hydrogen, consequently nine is the equivalent of water. Now there are many other experiments that I might adduce which would show you the composition of water synthetically, but they require more manipulation and apparatus than I presume you are yet master of; I shall, therefore, content myself with adducing two instructive experiments to show its decomposition, or analysis. The materials in the "gas-bottle" or the "philosophical candle" apparatus present you with the first; there you have zinc, water, and sulphuric acid, and you evolvè hydrogen,—but why? Because the water consists of oxygen and hydrogen; the former is attracted by the zinc, forming oxide of zinc, with which the sulphuric acid instantly unites to produce sulphate of zinc, whilst the hydrogen escapes in the gaseous form. Its source, therefore, is the water which the zinc is enabled to decompose and rob of its oxygen through the agency of the sulphuric acid. If you wait till all the effervescence or evolution of hydrogen ceases, then pour off the clear liquid into an earthen basin, and evaporate it to about one-fourth its bulk, upon letting it cool, you will obtain perfect crystals of sulphate of zinc, which is a metallic salt, looking very much like Epsom salt, but is violently emetic and poisonous.

.

There are other metals that attract oxygen from water, simply by coming into contact with it, without requiring the intervention of an acid to call their affinities into play. Potassium is one of these: take a globule of it about the size of a pea, and put it in a bit of glass tube about half an inch long, closed at one end. Fill a bottle or jar with water, and invert it on the shelf of the pneumatic trough, hold the tube containing the potassium between the finger and thumb, so that its aperture may be VOL. II. 11

2 B

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 it 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.

RECENT INFORMATION ON

THE PREVENTION AND DETECTION OF SECRET AND ACCIDENTAL POISONING; PARTICULARLY

WITH ARSENIC.

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 expe

66

The late Dr. Parr. It was in his church, after morning service, that he announced and exhibited to his parishioners, Dr. Carmichael Smyth's celebrated mode of preventing and destroying contagion.

It is this principle which has dictated the notice or request distributed by the Royal Society, with the copies which are granted to a contributor of his own paper. Several other Societies in London are similarly costive.