on the Specific Gravities , &c. of Saline Subfiances. 215 
calculation is nearly juft, will appear clearly in my next paper, 
by its coincidence with the quantity of phlogifton difcovered in 
lead by Dr. priestley and that which is contained very evi- 
dently in regulus of arfenic, filver, and quickfilver. 
OF THE QUANTITY OF PHLOGISTON 
IN FIXED AIR. 
Before I attempt to determine this quantity, it will be ne- 
ceflary to prove that it contains any ; and for this purpoie mi- 
nutely to examine its nature and origin. 
Dr. priestley firft difcovered that in all proceftes, wherein 
phlogifton is difen gaged from any fubftance, as in combuftion , 
refpiration , calcination of metals , putrefaction , decompofition of 
nitrous air by refpirable air , &c. fixed air is precipitated from 
the common or dephlogifticated air in which thefe proceftes are 
performed, and that thefe laft airs are dimimfhed both 111 
weight and bulk, and are afterwards lefs fit, or abfolutely un- 
fit, for thefe proceftes, according to the quantity of phlogifton 
that was fet loofe. Thefe facts are admitted by all, let their 
fyftems be what they may. However, Dr. priestley thinks 
he has feen one exception to this general rule ; for, he fays, 
that in the combuftion of inflammable and common air no 
fixed air is precipitated, 5 pr. 124. He alfo feems inclined to 
admit another exception in the cafe of the combuftion of 
fulphur. 
The queftions that here arile are, firft, wdiether the fixed 
air that appears in thefe circumftances proceeded from the 
refpirable air or not ? Secondly, If it proceeded from the refpi- 
rable air, whether it pre-exifted in that air ; or whether it was 
generated 
