which his trials were to be made, and the oxyde of mercury; in this mixture he made a cavity, in which he poured a globule of mercury, and then by the action of the battery it was converted into the amalgam required. The metallic bases thus obtained, in general resemble one another: they are solid, excepting at high temperatures: they are, however, of a greater specific gravity than water; have a light me tallic lustre, resembling that of silver, and require a considerable force to flatten them. When exposed to oxygen, they absorb it greedily and return to their native earths.

With silex, argil, zircon, and glucine, the success was much less perfect; but when they were submitted to the action of galvanism, combined with potass and soda in fusion, appearances were obtained which indicated their decomposition, and the production of bases of a metallic nature. Hence it is inferred that all the earths are compounds of bases somewhat analogous to those of the fixed alkalies.

We may give the analysis of the earth barytes as an illustration of the methods adopted in the case of the others. Sir H. Davy, by placing barytes slightly moistened, either alone or mixed with potass, in the galvanic circuit, obtained indistinct appearances of metallization; but the complete reduction was best obtained, as has been remarked, in the mode of experiment performed by Berzelius and Pontin, negatively electrifying the earth in contact with quicksilver. On repeating this experiment with a powerful galvanic battery, it was found that the mercury became less fluid: on exposing this amalgam to the air, it was covered with a film of barytes; and when thrown into water, hydrogen was disengaged, and barytes formed: a similar result was obtained from some of the saline combinations of barytes, as the muriate.

Sir H. Davy, having found that the presence of an oxyde of mercury favoured the decomposition of barytes, combined this method with that of the Swedish chemists. The earth slightly moistened, and mixed with one-third of red oxyde of mercury, was placed on a plate of pla tina, having a cavity to receive a globule of mercury; the whole was covered by a film of naphtha, and the plate was made positive, the mercury negative, by communication with a very powerful galvanic battery. An amalgam was thus obtained; and to procure from this the base of the barytes it was distilled in a tube of glass, bent in the middle, and enlarged, and rendered globular at each extremity, so as to form a kind of retort and receiver; a little naphtha too having been introduced into the tube, and boiled, so as to exclude the air. The quicksilver rose pure by distillation; but it was very difficult to obtain a complete decomposition, a red heat being required for this, and at this heat the base of the earth acted on the glass. The result, therefore, was less perfect than those with regard to the bases of the alkalies; and there remained the uncertainty but that the metallic base procured might have retained in combination a little quicksilver. To the metallic matter obtained from barytes, Sir H. Davy has given the name of barium.

Barium, as obtained by the experiment above described, appeared as a white metal of the colour of silver. It was fixed at all common temperatures, but became fluid at a heat below redness, and did not rise in vapour when heated to a redness in a tube of glass. It acted, however, violently on the glass, producing a black mass, which seemed to contain barytes and a fixed alkaline base in the first degree of oxygen

ation. When exposed to air, it rapidly tarnished, and fell into a white powder, which was barytes; and when this process was conducted in a small portion of air, oxygen was absorbed. When the base was dropt in water, it acted on it with great violence, sunk to the bottom, producing barytes, and generating hydrogen gas. It not only sunk in water, but even in sulphuric acid, though surrounded by globules of hydrogen: hence it is concluded that it cannot be less than four or five times heavier than water. Whether it owe this in part to a little mercury combined with it, is uncertain. It flattened by pressure, but required for this purpose a considerable force.

Our chemist endeavoured to ascertain the proportions of base and of oxygen in the composition of barytes, but without success. He ascertained, however, that when burned in a small quantity of air, it absorbed oxygen, gained weight in the process, and the earth produced was in the driest state. The conclusion follows, therefore, that barytes is a compound of this base with oxygen,

:

The decomposition of strontites was effected by the same modes of galvanic analysis, as those applied to the other earths. By negatively electrifying it in contact with mercury, the phenomena denoting the decomposition of the earth, and the addition of metallic matter to the mercury, rapidly took place; and by employing the process which has just been described with respect to barytes, the metallic base of strontites was obtained. Strontium, for so this base was named, has the general characters of the base resulting from barytes; and by exposure to the air, it absorbs the oxygen from it, gains weight, and is converted to the original earth. It is of a considerable specific gravity. Without pursuing the analysis of the other earths, which would be but a repetition, or at least with very little variation, of what has already been said, we may conclude our account of the brilliant discoveries of Sir Humphery, which do so much honour to the present era, with observing, that one object of research was suggested to our author by a very important experiment of the Swedish chemists, already mentioned:"These ingenious philosophers found that mereury, placed in contact with a solution of ammonia, and negatively electrified, expands in volume, and becomes a soft solid; that this solid, on exposure to air, absorbs oxygen, and reproduces ammonia and mercury; that water is decompounded by it, giving out hydrogen gas, and leaving a solution of ammonia and mercury. The conclusion naturally drawn from this curious experiment was, that ammonia is, as Sir H. Davy himself had formerly supposed, an oxyde with a double basis, composed of hydrogen and nitrogen; but it seems to shew also, that this double basis possesses metallic properties. So unexpected a light could not fail to attract the quick and discerning eyes of our author; and he lost no time in pursuing the track into which it plainly led him. His first repetition of the Swedish experiment suggested a very material improvement on it the substitution of neutral salt of ammonia, whereby the deoxygenation and amalgamation are effected in the nascent state of that alkali, and are consequently more easily performed. His process was thus the same with that formerly described for deoxygenating the earths; only, that instead of sulphates or muriates of those earths, he exhibited muriate of ammonia. "The action,' says he, of the quicksilver on the salt was immediate. A strong effervescence, with much heat, took place. The globule in a few minutes had enlarged

to five times its former dimensions, and had the appearance of an amal gam of zinc; and metallic chrystallizations shot from it as a cen tre, round the body of the salt. They had an arborescent appearance; often became coloured at their points of contact with the muriate; and, when the connection was broken, rapidly disappeared, emitting ammoniacal fumes, and reproducing quicksilver.' Carbonate of ammonia gave the same result; only that a manifest decomposition of the acid, and production of carbonaceous matter, accompanies the other phenomena in this case. The bases of the alkalies and earths, united with mercury, and exhibited in this state to ammonia, supplied the place of electricity, and formed an amalgam of the bases of ammonia and mercury. A little of the bases here used for the purpose of deoxygenating the ammonia, adhered to it in the amalgam; but, independently of this consideration, our author seems to think that the experiment in question unites more of the ammoniacal basis to mercury than the process of deoxygenation by electricity. He does not mention, though we must presume, that, in this ingenious and beautiful experiment, the fixed alkalies or earths are produced.

"The singular amalgam discovered by the Swedish chemists may thus be obtained with great ease either by the agency of electricity, or by double elective affinity. But our author preferred the former method, because it is not attended with the admixture of any third substance, giving the amalgam composed solely of mercury, and the bases of ammonia. Having procured a sufficient quantity of it in this way, he examined it by various simple and satisfactory trials. Its principal properties are the following:-At 70° or 80° this body has the consistence of butter; at the freezing point it hardens and chrystallizes; it is not quite three times heavier than water. In water it absorbs oxygen, causing hydrogen gas to be evolved. In air it likewise absorbs oxygen; and, in both cases, ammonia and quicksilver are reproduced. In sulphuric acid it becomes coated with sulphate of ammonia and sulphur. Sixty grains of mercury are amalgamated by onetwo-hundredth part of a grain of the compound basis, or one-twelvethousandth of the weight of the mercury."-Phil. Trans. part ii. 1808.

This valuable paper our author concludes with some general speculations concerning the theory of alkaline and earthy bodies, as elucidated by the discoveries which we have now considered. His observations are always ingenious; and whatever comes from so great a discoverer, one so strict in his experimental investigations, and so successful in generalizing them, ought to be received with singular respect. Nevertheless, we shall not follow him through the whole of his queries and reflections, highly useful as they are likely to prove. We shall only state what we conceive to be the legitimate inferences from his experiments, and then notice a few of his most prominent observations.— It is clearly proved that the fixed alkalies, and the alkaline earths, are metallic oxydes; and the proportion of their bases is nearly as well ascertained as those of several metals known for ages to philosophers, and in common life. That alumina, zircon, glucine, and silex, are also metallic oxydes, seems highly probable; but their decomposition has not yet been so completely effected as to render this point altogether certain; and respecting the metals which seem to constitute their bases, we can scarcely be said to know any thing with precision. demonstrated that ammonia is a compound of oxygen, with hydrogen

It is

and nitrogen; ana that when the oxygen is removed, the hydrogen and nitrogen are capable of entering into a true chemical union with mercury, forming a substance in all respects similar to the amalgams of that body with other metals. It is highly probable, that the hydrogen and nitrogen are united together as a chemical compound, which thus unites with mercury; and that the same compound unites with oxygen to form ammonia. The appearance of amalgamation, as well as the nalogy of the other alkaline bodies, leads us to suspect that this compound basis is truly of a metallic nature, and that the volatile, like the fixed alkalies and the alkaline earths, is a metallic oxyde; but this basis has not yet been separately exhibited. Such, in general, is the state of our knowledge upon the constitution of the alkalies and earths, as extended by the late wonderful discoveries; and such is the line to be drawn between what we have strictly learned as physical truths, and what we have been taught to conjecture upon evidence of a lower nature than that of legitimate induction.

The last of these wonders, the constitution of ammonia, gives rise to various hypotheses. To account for the phenomena of amalgamation with mercury and the reproduction of the alkali, three different theories have been stated. Mr. Davy himself seems to think it possible, that hydrogen and nitrogen are both metals, aëriform at common temperatures, as zinc and mercury are when ignited. Mr. Berzelius suggests, that they may be simple bodies, not metallic, but forming a metal when united, without oxygen; and an alkali, when united and oxygenated. Mr. Cavendish has submitted a third conjecture, that these gases, in their common form, may be oxydes, which, when further oxygenated, become metallic.

The labours of Sir Humphrey Davy in this department of science have been unwearied, but they have been crowned with a degree of success, and with discoveries of such importance as no one could have anticipated. Let us consider what we should have said, had such a contribution to chemical knowledge (as that in the Phil. Trans. 1809) fallen in our way ten years ago-had we for instance heard that the basis of the boracic acid had been discovered, that hydrogen had been detected in sulphur and phosphorus, and oxygen in azote? The whole world of letters would have been in commotion, and it would have been universally allowed, that, since the establishment of modern chemistry, no such steps had been made towards its perfection. If we now think less of these improvements, or even receive them with coldness, it is because we have been spoiled with the abundance of capital discoveries in which we have been revelling-and it is Davy himself who has spoiled us. His grand and numerous inventions, together with the two unexpected and important steps made by the French and Swedish chemists, have, for a while, so completely satiated the curiosity of the scientific world, that scarcely any new fact would now excite astonishment.

CHAP. IX

DYEING.

THE substances commonly employed for clothing may be reduced to four: wool, silk, cotton, and linen.

Permanent alterations in the colour of cloth can only be produced two ways; either by producing a chemical change in the cloth, or by covering its fibres with some substance which possesses the wished-for colour. Recourse can seldom or never be had to the first of these methods, because it is hardly possible to produce a chemical change in the fibres of cloth without spoiling its texture, and rendering it useless. The dyer, therefore, when he wishes to give a new colour to cloth, has always recourse to the second method.

The substances employed for this purpose are called colouring matters, or dye stuffs: they are for the most part extracted from animal and vegetable substances, and have usually the colour they give to the cloth. Hence as the particles of colouring matter, with which cloth when dyed is covered, are transparent, it follows, that all the light reflected from dyed cloth must be reflected, not by the dye stuff itself, but by the fibres of the cloth below the dye stuff. The colour, therefore, does not depend upon the dye alone, but also on the previous colour of the cloth. If the cloth be black, it is clear that we cannot dye it any other colour whatever because as no light in that case is reflected, none can be transmitted, whatever dye stuff we employ. If the cloth were red, or blue, or yellow, we could not dye it any colour except black, because as only blue or red or yellow rays were reflected, no other could be transmitted. Hence the importance of a fine white colour when cloth is to receive bright dyes. It then reflects all the rays in abundance, and therefore any colour may be given, by covering it with a dye stuff which transmits only some particular rays.

If the colouring matters were merely spread over the surface of the fibres of cloth by the dyer, the colours produced might be very bright, but they could not be permanent; because the colouring matter would be very soon rubbed off; and would totally disappear whenever the cloth was washed, or even barely exposed to the weather. The colouring matter then, however perfect a colour it possesses, is of no va lue, unless it also adheres so firmly to the cloth that none of the substances usually applied to cloth, in order to clean it, &c. can displace it. Now this can only happen, when there is a strong affinity between the colouring matter and the cloth, and when they are actually com bined together in consequence of that affinity.

Dyeing then is merely a chemical process, and consists in combining a certain colouring matter with fibres of cloth. This process can in no instance be performed, unless the dye stuff be first reduced to its integrant particles; for the attraction of aggregation between the particles of dye stuffs is too great to be overcome by the affinity between them and the cloth, unless they could be brought within much smaller distances than is possible while they both remain in a solid form. It is necessary, therefore, previously to dissolve the colouring matter in some liquid or other, which has a weaker affinity for it than the cloth has.