fame in all the cafes. These circumstances give reason to suspect, that the cause of diminution is, in reality, the fame in all the cafes. What this caufe is, may, perhaps, appear in the next course of obfervations.

SECTION VIII.

Of the Effect of the CALCINATION of METALS, and of the EFFLUVIA of PAINT made with WHITE-LEAD and OIL, en AIR.

Having been led to fufpect, from the experiments which I had made with charcoal, that the diminution of air in that case, and perhaps in other cafes also, was, in fome way or other, the confequence of its having more than its usual quantity of phlogiston, it occurred to me, that the calcination of metals, which are generally fuppofed to confift of nothing but a metallic earth united to phlogiston, would tend to ascertain the fact, and be a kind of experimentum crucis in the cafe.

Accordingly, I fufpended pieces of lead and tin in given quantities of air, in the fame manner as I have before treated the charcoal; and throwing the focus of a burning mirror or lens upon them, fo as to make them fume

copiously. I prefently perceived a diminution of the air. In the first trial that I made, I reduced four ounce measures of air to three, which is the greatest diminution of common air that I had ever obferved before, and which I account for, by fuppofing that, in other cafes, there was not only a caufe of diminution, but caufes of addition alfo, either of fixed or inflammable air, or fome other permanently elastic matter, but that the effect of the calcination of metals being fimply the efcape of phlogifton, the caufe of diminution was alone and uncontrouled.

The air, which I had thus diminished by calcination of lead, I transferred into another clean phial, but found that the calcination of more lead in it (or at least the attempt to make a farther calcination) had no farther effect upon it. This air alfo, like that which had been infected with the fumes of charcoal, was in the highest degree noxious, made no effervefcence with nitrous air, was no farther diminished by the mixture of iron filings and brimstone, and was not only rendered innoxious, but also recovered, in a great measure, the other properties of common air, by wafhing in water.

It might be fufpected that the noxious quality of air in which lead was calcined, might

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be owing to fome fumes peculiar to that metal; but I found no fenfible difference between the properties of this air, and that in which tin was calcined.

The water over which metals are calcined acquires a yellowish tinge, and an exceedingly pungent smell and taste, pretty much (as near as I can recollect, for I did not compare them together) like that over which brimstone has been frequently burned. Alfo a thin and whitish pellicle covered both the furface of the water, and likewise the fides of the phial in which the calcination was made; infomuch that, without frequently agitating the water, it grew fo opaque by this conftantly accumulating incrustation, that the fun-beams could not be tranfmitted through it in a quantity fufficient to produce the calcination.

I imagined, however, that, even when this air was transferred into a clean phial, the metals were not fo eafily melted or calcined as they were in fresh air; for the air being once fully faturated with phlogifton, may not so readily admit any more, though it be only to transmit it to the water. I alfo fufpected that metals were not eafily melted or calcined in inflammable, fixed, or nitrous air, or any kind of

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diminished air *. None of these kinds of air fuffered any change by this operation; nor was there any precipitation of lime, when charcoal was heated in any of these kinds of air ftanding in lime-water. This furnishes another, and I think a pretty decifive proof, that, in the precipitation of lime by charcoal, the fixed air does not come from the charcoal, but from the common air. Otherwife it is hard to affign a reason, why the fame degree of heat (or at leaft a much greater) fhould not expel the fixed air from this fubftance, though furrounded by thefe different kinds of air, and why the fixed air might not be tranfmitted through them to the lime-water.

Query. May not water impregnated with phlogifton from calcined metals, or by any other method, be of fome ufe in medicine? The effect of this impregnation is exceedingly remarkable; but the principle with which it is impregnated is volatile, and intirely efcapes in a day or two, if the furface of the water be expofed to the common atmosphere.

I conclude from the experiments of M. Lavoifier, which were made with a much better burning lens than I had an opportunity of making ufe of, that there was no real calcination of the metals, though they were made te fume in inflammable or nitrous air; because he was not able to produce more than a flight degree of calcination in any given quantity of common air.

It should seem that phlogiston is retained more obftinately by charcoal than it is by lead or tin; for when any given quantity of air is fully faturated with phlogifton from charcoal, no heat that I have yet applied has been able to produce any more effect upon it; whereas, in the fame circumstances, lead and tin may ftill be calcined, at least be made to emit a copious fume, in which fome part of the phlogifton may be fet loofe. The air indeed, can take no more; but the water receives it, and the fides of the phial also receive an addition of incrustation. This is a white powdery fubstance, and well deserves to be examined. I fhall endeavour to do it at my leifure.

Lime-water never became turbid by the calcination of metals over it, the calx immediately feizing the precipitated fixed air, in preference to the lime in the water; but the colour, fmell, and tafte of the water was always changed and the furface of it became covered with a yellow pellicle, as before.

When this procefs was made in quickfilver, the air was diminished only one fifth; and upon water being admitted to it, no more was abforbed; which is an effect fimilar to that of a mixture of nitrous and common air, which was mentioned before.

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