Thus we have a red oxyd made in fifteen minutes; it produces a fine blue, with alkaline prussiates; with gallnuts its precipitate is black, or rather of a very deep blue'; in short, no chemist will be able to distinguish it from the oxyd of 56, at least by employing the means that are now practised for distinguishing between the oxyds of iron.

All these facts prove the wisdom and profundity with which the learned author of the Chemical Statics (relying not on the elective attractions, but on the properties of oxygen and of metals) has said, that the proportions of oxygen with metals may vary from the term wherein the combination is possible until that in which it arrives to the highest degree of which it is susceptible, and that a mul titude of circumstances may put a stop to, or promote, these proportions.

I have not pursued this experiment any further; but I believe that by favouring the division of the iron by all known means, and by opposing at the same time all obstacles to the action of the oxygen, we may obtain red oxyds of iron, with ,06 or,08 of oxygen. And possibly, by preventing entirely the action of this principle, we may obtain powders of iron that may be dissoluble without effervescence in acids, and which may possess the same properties as the oxyds. As for myself, I am the more ́persuaded of its possibility, because I am rather inclined to doubt the generally received principle, that metals should be previously combined with oxygen in order to combine with acids. I regard this oxydation rather as a consequence of the means that we employ to divide metals, and to give them that degree of fineness that is re

known to exist there; yet the separation may be easily effected, by digesting the oxyd for half an hour in muriatic acid, much diluted with water, which dissolves the green oxyd, or rather the iron that exists in it, and has no effect on the red oxyd,

quisite

quisite for dissolving them in acids, than as a condition indispensible to their dissolution.

I propose, in the sequel, to make some experiments on this subject; and then I shall relate the motives that force me doubtfully to dissent from this principle.

Before I conclude my account of the oxyds of iron by calcination, I must be permitted to make an observation ou the most interesting application of the oxyds of iron, that is, on their medicinal property. These oxyds are applied in numberless instances to the animal economy; but it is not yet known to which of the two principles of this combination their salutary effects are owing; and the faculty, until now, have been unable to make comparative and satisfactory observations on this subject, being unacquainted with the quantity of oxygen contained in the different oxyds of iron that are employed in medicine. Therefore they use indiscriminately, or with a blind preference, astringent saffron of Mars, operative saffron of Mars, Ethiop's Martial, vitriol of Mars, boules de Nancy, and many other preparations, in which the quantity of oxygen varies as much as two thirds. At present they know that all these saffrons, notwithstanding the uniformity of their colour, and their chemical properties, are of different degrees of oxydation, and that they will vary from some hundredths to beyond ,50. It is to be wished that some philosopher would make some observations tending to discover whether it is the oxygen or the iron that gives to this remedy its properties. Such a work would be very useful, since it would most probably enable them to augment or diminish, as convenient, the influence of the two substances.

Oxyds by Dissolution.

The solutions of iron have yielded results that are still more satisfactory, both because they confirm and render

more

more clear those obtained by calcination, and because they are calculated to throw much light on a number of manufactures, and to simplify the chemical theory of iron. But notwithstanding my own confidence in these results, I offer them only as conjectures; they are so diametrically opposite to the present opinions respecting the dissolutions of iron, and I am always fearful of being led

into error.

The inconstancy of the green oxyd that Lavoisier and M. Proust obtained when 100 grains of iron took up 37 of oxygen, and which, according to the experiments I have related, has varied from some hundredths to 32, should have led me to believe that the properties which distinguish this oxyd from the red oxyd were not owing to a fixed degree of oxygenation, but rather to a certain degree of density, which would permit the water or the acid, or both of them, to lodge in the interstices of each molecule; which occasions both the difference in the colour of these precipitates by the alkalies, by the prussiates of potash and galates, and their greater or less degree of solubility, which are the two sole properties that distinguish the green salt from the red.

This reasoning led me to examine not only the green and red oxyds, but at the same time the white oxyd lately announced by M. Thenard, respecting which I had advanced some doubts *. I am sorry to be obliged to

raise

*I could not believe, even before I undertook this work, that some hundredths of oxygen were capable of modifying the colour of the oxyds to such a degree as to change them from white to green, black, &c. All these facts being now within my knowledge, tend to disprove the existence of this property in the oxygene; consequently I was suspicious of all precipitates that were much unlike in colour that of the oxyds of the same metal obtained by calcina

tion.

raise doubts respecting the labours of philosophers whom I respect and esteem, but trust I shall promote their views if my observations are just.

Of the White Oxyd.

M. Thenard gives as the distinctive character of this oxyd, first, that it becomes green by exposure to the air; secondly, that the oxygenated muriatic acid changes it to green or yellow; thirdly, that when the precipi, tate is made in a bottle well corked, after having been stirred, in order to transform the white oxyd into green, an absorption will be perceived; which proves that a part of the oxygen of the air contained in the bottle. combines with the white oxyd, and changes its colour. I must be allowed to appreciate these three facts.

Since the labours of Messrs. Fourcroy, Proust, and Berthollet, M. Thenard and all the chemists of the present day are of opinion that the change of colour in the precipitates does not, for the greatest part, indicate a different degree of oxygenation. The white colour is that which very often disguises the true colour of the oxyds in almost all the metals which are said to be susceptible of oxygenation, such as tin, mercury, copper, silver, lead, bismuth, and probably manganese. That depends on the quantity of acid that the precipitates

tion. If the white colour, I say, is the result of the combination of iron, with some hundredths of oxygen, why, in the course of the calcination of the iron, does not this colour appear? Why, in the calcination of manganese, copper, and bismuth, do we not see the same phenomenon? Besides, the only well-established white oxyds are those of antimony, zinc, and arsenic, which are white, whether obtained by acids or by calcination; and from the moment that they become white the colour is unchangeable, even by a considerable surcharge of oxygen.

retain,

retain, and of which they are more or less easily deprived according to their nature, and still more according to the circumstances that take place during the precipitations. This is the case in which the white precipitate of iron appears to exist.

The conditions that are requisite in ord to precipitate a green salt of iron white are, first, that the solution be very much concentrated; secondly, that the precipi tating alkali be also sufficiently strong. These requisites did not escape the sagacity of M. Thenard, who advises, for the better success of the formation of the white oxyd, that some sulphuric acid, diluted with water, be boiled on an excess of iron filings. In fact, every time that an alkali a little concentrated is poured on a similar solution of iron, it seizes at first a part of the acid, and probably a little of the water; and it precipitates a white sulphate of iron, which often crystallizes, although irregularly, at the very moment even of the precipitation, and still preserves enough of acid to be soluble in water. It also invariably turns the syrup of violets green, and forms a red precipitate in turnsole water, as the salts of iron do with an excess of oxyd *.

It

* I have obtained several salts of iron which, when put in contact with the tincture of turnsole, immediately produce a red precipitate, and at the same time turns the syrup of violets green. The property of precipitating with the tincture of turnsole always indicates at least a neutral salt and we should not confound the simple colouration with that which is accompanied by a precipitate, because these are two results that are entirely different. In the simple colouration the small portion of alkali from the tincture of turnsole combines with an excess of acid, and abandons the colouring part that it would otherwise modify there is no doubt but that a salt which presents this phenomenon possesses an excess of acid, since it contains enough to saturate the alkali of the tincture without occasioning any precipitate; and it VOL. XI.-SECOND SERIES. G

is