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same in all the cases. These circumstances give reason to suspect, that the cause of diminution is, in reality, the same in all the cases. What this cause is, may, perhaps, appear in the next course of observations.

SECTION VIII.

Of the Effetl of the Calcination of Metals, and of the Effluvia of Paint made with White-lead and Oil, en Air.

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

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

K 3 cocopiously. I presently perceived a dlminu? tion 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 observed before, and which I account for, by supposing that, in other cases, there was not only a cause of diminution, but causes ot addition also, either of fixed or inflammable air, or some other permanently elastic matter, but that the effect of the calcination of metals being simply the escape of phlogiston, the cause 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 allo, like that which had been insected with the fumes of charcoal, was in the highest degree noxious, made no efservescence 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 washing in water.

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

be be owing to some fumes peculiar to that metal; but I found no sensible 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. Also a thin and whitish pellicle covered both the surface of the water, and likewise the sides of the phial in which the calcination was made $ insomuch that, without frequently agitating the water, it grew so opaque by this constantly accumulating incrustation, that the fun-beams could not be transmitted through it in a quantity sufficient to produce the calcination.

I imagined, however, that, even when this air was transferred into a clean phial, the metals were not so easily melted or calcined as they were in fresh air; for the air being once fully saturated with phlogiston, may not so readily admit any more, though it be only to transmit it to the water. I also suspected that metals were not easily 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 suffered any change by this operation nor was there any precipitation of lime, when charcoal was heated in any of these kinds of air standing in lime-water. This furnishes another, and I think a pretty decisive proof, that, in the precipitation of lime by charcoal, the fixed air does not come from the charcoal, but from the common air. Otherwise it is hard to assign a reason, why the same degree of heat (or at least a much greater) should not expel the fixed air from this substance, though surrounded by these different kinds of air, and why the fixed air might not be transmitted through them to the lime-water.

Query. May not water impregnated with phlogiston from calcined metals, or by any other method, be of some use in medicine? The effect of this impregnation is exceedingly remarkable; but the principle with which it is impregnated is volatile, and intirely escapes in a day or two, if the surface of the water be exposed to the common atmosphere.

* I conclude from the experiments of M. Lavoisier, which were made with a much better burning lens than I had an opportunity of making use 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 slight degree of calcination in any given quantity of common air.

It should seem that phlogiston is retained more obstinately by charcoal than it is by lead or tin; for when any given quantity of air is fully saturated with phlogiston from charcoal, no heat that I have yet applied has been able to produce any more effect upon it; whereas, in the same circumstances, lead and tin may still be calcined, at least be made to emit a copious fume, in which some part of the phlogiston may be set loose. 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 substance, and well deserves to be examined. I shall endeavour to do it at my leisure.

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

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

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