analogy, similar to that in which it holds dissolved the vegetable acids, which are admitted to be ternary compounds of carbon, hydrogen, and oxygen. The opposite view applies only to that portion of water considered as essential to the body in an insuÎated state, and in which it is combined in a definite proportion, observing in its relations, or the relations of its elements, equivalent proportions to other bodies.

In the last place, considering this opinion in relation to the two opposite views which have been maintained with regard to the constitution of oxymuriatic and muriatic acids, while it has. all the evidence in its favour from which the existence of water in muriatic acid gas is inferred, and all the analogies by which this is confirmed; it has the support which the doctrine of the undecompounded nature of chlorine derives from the relations of sulphur, iodine, and cyanogen; and from the induction that hydrogen, as well as oxygen, communicates acidity. It avoids, at the same time, the improbability which attends that doctrine, in its leading principle, that muriatic acid contains no combined water, though other powerful acids are held to contain it,and though it affords water by the very same processes by which they yield it; and in the still greater violation of analogy (the most extraordinary perhaps ever admitted in chemical reasoning), involved in the conclusion that the compounds which this acid forms with salifiable bases, though the same in all generic properties with those formed by other acids, are not of similar constitution, and are not even of a saline nature. It unites the advantages, therefore, of both doctrines, and connects, under one system, facts which are otherwise insulated, and partial generalisations, which, instead of having any relation, seem opposed to each other.

The same general view, I have still to add, may be further extended. Alkalinity, Alkalinity, as well as acidity, is the result apparently of the action of oxygen; the fixed alkalies, the earths, and the metallic oxides, which all contain it as a common element, forming a series in which it is difficult to draw any well defined line of distinction. Ammonia alone remains an exception: it contains no oxygen, and yet possesses in a very marked degree all the alkaline properties-an anomaly so great, as to have led almost every chemist to infer that oxygen must exist as an element in one or other of its constituent principles; and as nitrogen is the one apparently least elementary, it has been supposed to be a compound containing oxygen. The result may be accounted for, however, on a very different principle. As hydrogen, in some cases, gives rise, as well as oxygen does, to acidity, so it may, in other cases, give rise to alkalinity. Under this point of view, ammonia is a compound of which nitrogen is the base, deriving its alkaline power from hydrogen; it stands, therefore, in the same relation to the other alkalies that sulphuretted hydrogen does to the acids. And thus the whole

[APRIL, speculation with regard to the imaginary metallic base ammonium, and the existence of oxygen in ammonia and in nitrogen falls to the ground, while the anomaly presented by this alkali is removed. If the claim of the lately discovered principle in opium, Morphia as it has been named, to the distinction of an alkali be established, as from its origin it must probably have a compound base, it may, if it contain hydrogen, bear the same relation to the other alkalies that prussic acid does to the acids; or if it contain oxygen, it will be analogous to the vegetable acids.

The fixed alkalies and the alkaline earths are considered as containing water in intimate combination in a definite proportion; and it is doubtful if they can be obtained free from it in an insulated state, retaining at the same time their alkaline properties. It is obvious, however, that the elements of water may exist in combination with the base; that potash, for example, is not a compound of an oxide of potassium with water, but of potassium, oxygen, and hydrogen. Hence when, on adding water to peroxide of potassium, potash is produced, and oxygen gas is disengaged; this is not owing, as has been supposed, to the excess of oxygen in the peroxide being expelled, and the water taking its place; but to the water being decomposed, and a portion of its hydrogen entering into the combination, to form the alkali, while the corresponding oxygen is liberated. If hydrogen were brought to act on peroxide of potassium, the alkali would in like manner be formed. With the peroxide of barium, this very change, from the action of hydrogen, takes place; the hydrogen, according to the usual explanation, combining with its cxygen, and forming water, which unites with the real earth, forming the hydrate; in other words, and according to the strict expression of the fact, the hydrogen entering into the composition, and forming the barytes; a result perfectly analogous to the formation of muriatic acid from oxymuriatic gas by the agency of hydrogen.

The evidence in support of this doctrine, it is evident, is of the same kind as that with regard to the doctrine applied to the acids. There is the same superior probability in favour of the conclusion that the elements of water rather than water itself exist in these compounds, from the consideration that modifications of properties so important are more likely to arise from the agency of these elements than from any action which water can exert. And that water does not exist in them in consequence of the strength of attraction which the real alkali, as it has been considered, exerts towards it, is evident from this, that on the same principle ammonia ought to contain combined water in its insulated form, which is not the case. The combination of water, therefore, or rather of its principles, in these compounds, depends on relations subsisting among the ultimate elements, not on an affinity exerted by the alkali itself; and this

adds confirmation to the conclusion, that these elements are in ternary union.

Their superior alkaline energy compared with the common metallic oxides may obviously arise from the joint action of the hydrogen and oxygen, in the same manner that the acidity of the ternary compared with the binary acids is increased by a similar constitution. Thus the class of alkalies will exhibit the same relations as the class of acids. Some are compounds of a base with oxygen: such are the greater number of the metallic oxides, and several, probably, of the earths. Ammonia is a compound of a base with hydrogen. Potash, soda, barytes, strontites, and, probably, lime, are compounds of bases with oxygen and hydrogen; and these last, like the analogous order among the acids, possess the highest power. Many of the metallic oxides, however, in the state in which they combine with the greatest facility with the acids, are hydrates; that is, supposed compounds of the oxide with water, but probably ternary compounds of the metal with oxygen and hydrogen; and their facility of combination may depend on this constitu tion. The same principle explains the necessity, not otherwise easily accounted for, of the presence of water, to enable some of the earths, as barytes, to combine with acids.

There are two views under which the neutral salts may be considered in the preceding theory. It has been shown, that when water is obtained in the action of a salifiable base, whether alkali, earth, or metallic oxide, there is reason to infer that this water is formed by the hydrogen and part of the oxygen of the acid entering into binary combinations; and when water is obtained from an alkali by the action of an acid, there is the same reason to believe that it is formed by the combination of the hydrogen of the alkali with a portion of its oxygen. In these cases it may be supposed, that the radical of the acid combines with its remaining oxygen, forming a binary compound, which may still be considered as an acid; and that the radical of the alkali combines with its remaining oxygen, forming a binary compound, which may be regarded as an alkali; and these two compounds may unite with each other, forming the neutral salt. This is conformable nearly to the common doctrine. But there is another point of view under which the subject may also be considered. A ternary combination, into which oxygen and hydrogen enter, gives rise apparently to a higher state of acidity, and to a greater degree of alkaline energy than is acquired from a mere binary combination into which oxygen enters. It is doubtful, therefore, if such binary compounds were formed, if they would constitute either acid or alkali. And there is at least no proof of their formation. In all these cases, while the hydrogen present combines with the requisite proportion of oxygen forming water, the radical of the acid and the radical of the base may enter into union with the'

remaining oxygen, and form a ternary compound. And where hydrogen is not present, such a combination may be at once established.

It is not easy to determine which of these opinions is just. The reason above stated renders the latter, perhaps, more probable; and the view which leads to the conclusion, that in the constitution of the acids and alkalies the three elements, when present, are in simultaneous combination, leads also to a similar conclusion with regard to the constitution of the neutral salts. If this be adopted, neutralization is not the saturation of acid with alkali, and the subversion of the properties of the one by the opposed action of those of the other, but is the change of composition of both, and the quiescence of the elements in that proportion in which their affinities are in a state of equilibrium without any excess. The compounds, therefore, have little activity; and energy of action is restored only by the reproduction of substances, which, by their mutual attractions, tend to the same state of quiescence.

All these results display more fully the extensive relations of the two elements, oxygen and hydrogen. They do not act merely in opposition, as had been imagined, but more frequently in union, producing similar effects. Hydrogen is of nearly equal importance with oxygen; and the principal details of chemistry consist in their modified action on inflammable and metallic bodies.

ARTICLE VI.

On the Persulphates of Iron. By Mr. Cooper.

(To Dr. Thomson.)

DEAR SIR, 89, Strand, March 16, 1819. I FEEL very happy in being able to confirm your analysis of the persulphates of iron contained in Vol. X, No. LVI, of your Annals of Philosophy, and also to verify the conjecture you have thrown out of there being a persalt of iron containing an excess of sulphuric acid. This salt I have formed by boiling recently precipitated peroxide of iron (from nitric acid by ammonia) in a considerable excess of sulphuric acid; the solution goes on but slowly; but when obtained, if it be evaporated to the consistence of syrup, it will in a few days deposit crystals: these are the bipersulphate of iron. I find the crystallization to succeed better when a small excess of acid is present. The form of the crystal is that of an octohedron; some of the solid angles are truncated, some of the edges are bevelled, and in others the edges are truncated, while others of them are the perfect octohedron. These crystals are permanent, and are perfectly transparent and

colourless; they do not at all indicate the presence of iron by the taste, which exactly resembles that of alum. They are very soluble in water, and contain a considerable quantity of that fluid combined, as will be shown in the sequel. They undergo the watery fusion; and, when fused in their own water of crystallization, immediately change their colour and become red; and if the whole of the water be driven off by heat, the dry mass is converted into two substances, one of which is soluble, but the other and greater portion insoluble in water: this latter, however, is readily taken up by the addition of muriatic acid. Three hundred and eighty grains of it were dried at a temperature of about 300° Fahr. and lost 200 gr. of water: the whole, being redissolved by the addition of a small quantity of muriatic acid, the oxide of iron precipitated by ammonia, and the sulphuric acid by muriate of barytes, gave of

Peroxide of iron.

60 gr.

Sulphate of barytes 352 gr. = sulphuric acid.. 120
Water

... 200

380

This is the mean of three experiments; and from these data, you will perceive the composition of this salt to be

1 atom of peroxide of iron. 2 atoms sulphuric acid.. 15 atoms water....

considering hydrogen as unity, and oxygen 8.

I feel confident of the existence of another salt of iron containing a still larger quantity of acid; and I have little doubt that I shall be able to obtain it in a distinct form. If I should be successful, I shall send you the results; but hitherto I have obtained it only in very minute crystals, and which it is difficult to free from the adhering excess of acid. The way in which I have formed this salt is as follows:-After separating two crops of crystals of the bipersulphate, I added sulphuric acid to the mother liquor, and evaporated till a pellicle formed on the surface on cooling, the salt in question separated. These crystals appear, when very highly magnified, in the form of quadrangular plates. They are slightly deliquescent (but this probably may be owing to the adhering acid); they are perfectly white, and have a pearly lustre; their taste is very acid, but not so astringent as the former salt; when caustic alkalies are added to them, they immediately indicate the presence of peroxide of iron. Owing to the above-mentioned circumstance of my hitherto not being able to get this substance in a form fit for analysis, I