256 PHILOSOPHICAL TRANSACTIONS. [ANNO ] 782. 



for, he says, that in the combustion of inflammable and common air, no fixed 

 air is precipitated, 5 Pr. 124. He also seems inclined to admit another excep- 

 tion in the case of the combustion of sulphur. 



The questions that here arise are, first, whether the fixed air that appears in 

 these circumstances proceeded from the respirable air or not ? Secondly, if it 

 proceeded from the respirable air, whether it pre-existed in that air ; or whether 

 it was generated during the process that exhibits it ? and if so, what are its con- 

 stituent parts ? The first question is easily answered ; for in such phlogistic 

 processes as are attended with the destruction of the substances that are known 

 to contain fixed air, as those of the animal and vegetable kingdom, the fixed air 

 may be supposed to proceed in many cases, both from the decomposed substance 

 and from the respirable air ; and of this sort are the processes of combustion 

 of most animal and vegetable substances, and fermentation ; but the fixed air, 

 that appears in such phlogistic processes as are performed on substances that 

 contain no fixed air, must be deemed to proceed from the respirable air singly. 

 And of this case we have 4 clear instances ; the calcination of metals ; the de- 

 composition of nitrous air by respirable air ; the diminution of common air by 

 the electric spark ; and, lastly, its diminution by amalgamation. 



And first as to the calcination of metals, Dr. Priestley has observed, that by 

 this operation respirable air (and only respirable air) is diminished between J- and 

 -L, both in its weight and bulk ; but Mr. Lavoisier has demonstrated, that 

 nothing is lost or escapes through the vessels, as Mr. Scheele would have it ; for 

 the weight and materials continue undiminished when the operation is performed 

 in close vessels. That part, therefore, which the air loses, is taken up by the 

 metallic calx, which accordingly is found to gain the very weight which the air 

 loses. Now the air contained in the calx is fixed air ; for Mr. Lavoisier also ob- 

 served, that by the calcination of lead, by solar heat, over lime-water, the 

 water was rendered slightly turbid. It is true, that Dr. Priestley, in a similar 

 experiment, did not observe this turbidity ; but he accounts for this circumstance 

 very justly, by supposing that the calx of lead absorbed the fixed air preferably to 

 the lime. And this supposition is not gratuitous ; for metallic calces, and par- 

 ticularly those of lead, are known to attract fixed air as strongly as quick lime, 

 or rather more strongly : and what sets this matter beyond all doubt, the calces 

 of lead all yield fixed air by heat, and the grey calx of lead, in particular, which 

 was that produced by Dr. Priestley, in the experiment to which I allude, affords 

 by heat fixed air only. Other calces of lead after fixed air afford also dephlogis- 

 ticated air ; but this I shall show also to have been originally fixed air. If filings 

 of iron be mixed with water in close vessels, they will be converted into rust, 

 and the incumbent air diminished -l, as Mr. Lavoisier attests; but Dr. Priestley 

 has shown, that rust of iron yields scarce any other than fixed air, which may 



