times through quickfilver, into different kinds of air, and always found that the blackest parts affumed a florid red colour in common. air, and more efpecially in dephlogisticated air, which is purer and more fit for refpiration than common air (and accordingly the blood always acquired a more florid colour, and the change was produced in lefs time in this than in common air) whereas the brightest red blood became presently black in any kind of air that was unfit for refpiration, as in fixed air, inflammable air, nitrous air, or phlogisticated air; and after becoming black in the laft of thefe kinds of air, it regained its red colour upon being again expofed to common air, or dephlogisticated air; the fame pieces becoming alternately black and red, by being tranfferred from phlogifticated to dephlogisticated air; and vice versa.

In these experiments the blood must have parted with its phlogiston to the common air, or dephlogisticated air, and have imbibed it, and have become faturated with it, when expofed to phlogisticated, nitrous, inflammable, or fixed air. The only difficulty is with refpect to the fixed air; for all the other kinds certainly contain phlogifton. But, as I have obferved in the account of my experiments on vitriolic acid air, phlogifton feems to be ne

ceffary

ceffary to the conftitution of every kind of air; and befides, the blacknefs of the blood may arife from other causes than its acquiring phlofton. Gaber, for inftance, obferves, that blood becomes black when it begins to putrefy, as it does alfo whenever it is dried and hardened near the fire. Father Beccaria also found, as I have obferved, that red blood continued (and he could hardly fail to obferve also, that it became) black in vacuo, where it could not have imbibed phlogiston. This I found to be the cafe when the blood was covered two inches and a half with ferum; but it regained its florid colour when it was expofed to the open air,

In general, however, it cannot be expected, that when blood has become black without having received phlogifton ab extra, it will recover its florid colour by being exposed to the air. For the delicacy of its texture, and confequently its capacity of being easily affected by phlogiston, may be effentially altered by internal caufes of black nefs. This is even the cafe when blood has become black by being exposed to nitrous and inflammable air, though this change is probably effected by its imbibing phlogiston.

;

I expofed pieces of the fame mass of red blood to these two kinds of air, and alfo to fixed air at the fame time. They all became black but that which was in the inflammable air was the least so, and none of them recovered their florid colour in the open air. But at another time, a piece of craffamentum, which had become black in fixed air, did, in fome measure, and very flowly, recover its florid colour in dephlogisticated air. Perhaps the pieces that had loft their colour in the nitrous and inflammable air might have recovered it by means of this more powerful menftruum.

Since, however, blood, after becoming black in phlogisticated air, is always capable of refuming its red colour on being again exposed to pure air, it may be concluded, that the preceding blackness, discharged in the pure air, and producing the conftant effect of phlogifton, in depraving the air, was owing to the phlogifton it had imbibed in the former fituation, and which it parted with in the lacter. And this is remarkably the cafe when blood is transferred from phlogisticated into dephlogifticated air. Even the circumstance of the deeper colour is fufficient to give a chemist a fufpicion that it contains more phlogiston than blood of a lighter colour.

When

When I had found how readily pieces of blood changed their colour, according to the quality of the air to which they were expofed, I proceeded to examine the state of that air, in order to obferve what change had taken place in it; and as dephlogisticated air admits of a more fenfible change of quality than common air, I gave it the preference in this expe'riment; putting a piece of craffamentum, about the bigness of a walnut, into the quantity of about five ounce meafures of this air.

This process I continued for the space of twenty-four hours, changing the blood about ten or twelve times; after which I found the air fo far depraved, that whereas, at the beginning of the experiment, one measure of it and two of nitrous air occupied the space of no more than half a measure, the fame mixtures afterwards occupied the fpace of a measure and a half. Now fince air is univerfally depraved by phlogifton, and in this sense, I believe, by nothing elfe, it is evident, that this black blood muft have communicated phlogifton to the air; and confequently its change of colour from black to a florid red must have been occafioned by the feparation of phlogifton from it.

The

The next day, when, of course, the blood was nearer to a state of putrefaction, in which every kind of substance, without exception, will injure refpirable air, I put a quantity of red blood, tinged in a few places with black, which I could not eafily feparate from it, to about the fame quantity of the fame dephlogifticated air, and fuffered it to stand, without changing, for the fame fpace of time; when it was fo little injured, that the measures above mentioned occupied the space of only two-thirds of a measure.

That blood has a power of taking phlagifton from air, as well as imparting phlogifton to air, I fatisfied myfelf by expofing blood of a very beautiful florid colour to nitrous air, inflammable air, and phlogisticated air. The two firft-mentioned kiuds of air were confiderably diminished by the process, which was continued two days, during which time the blood had been changed five or fix times.

The nitrous air, by this means, loft a great proportion of its power of diminishing, that is, phlogisticating, common air. For now two measures of common air and one of this occupied the space of 24, inftead of 1 meafures. The inflammable air, though ftill inflammable, was rendered in fome degree

whole