VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 245 



an excess of acid; so that at the same time part would be supersaturated with 

 earth, and another with acid, if tinging vegetable blues be a mark of an excess 

 of acidity, which indeed in this case seems dubious. 



JOOgr. of alum, perfectly dried, contained 42.74 of acid, 32.14 of earth, 

 and 25.02 of water; but crystallized alum loses 44 per cent, by desiccation J 

 therefore 100 gr. of it contain 23.Q4 acid, 18 of earth, and 5 8. 06 of water. 



100 gr. of this pure earth take up, as far as I can judge, 153 of the mere 

 nitrous acid. Tbe solution still reddened vegetable blues; but after the addition 

 of this quantity of pure earth, I think it was, that an insoluble salt came to be 

 formed. The solution, when cold, grew turbid, and could not be wholly dis- 

 solved by 500 times its weight of water. The same quantity of pure earth 

 requires 173.45 of the mere marine acid for its solution; but the solution 

 still reddens vegetable blues. After this, an insoluble salt was formed; but the 

 beginning of its formation is difficultly discovered both in this and the former 

 cases. The specific gravity of argillaceous earth, containing 25 per cent, of 

 fixed air, I found to be 1.QQ01. 



Of plrfogistov. — Before proceeding to investigate its proportion in various 

 compounds, and particularly in plilogisticated acids, it will be necessary to sav 

 something of its nature. It is allowed on all hands, that fixed air, or the aerial 

 acid, as it is more properly called, is capable of existing in 2 states; the one 

 fixed, concrete, and unelastic, as when it is actually combined with calcareous 

 earth, alkalis, or magnesia; the other, fluid, elastic, and aeriform, as when it 

 is actually disengaged from all combination. In its concrete and unelastic state 

 it can never be produced single and disengaged from other substances; for the 

 moment it is separated from them, it assumes its aerial and elastic form. The 

 same thing may be said of phlogiston: it can never be produced in a concrete 

 state, single and uncombined with other substances; for the instant it is dis- 

 engaged from them, it appears in a fluid and elastic form, and is then commonly 

 called inflammable air. These different states of the same substance arise, 

 according to discoveries of Dr. Black, from the different portions of elementary 

 fire contained in such substance, and absorbed by it, while its sensible heat 

 remains the same, and hence called its specific fire. For want of attention to 

 these different states, the very existence of phlogiston as a distinct principle has 

 been frequently called in question, and chemists have been required to exhibit it 

 separate in its fixed state, without recollecting, that neither can fixed air be shown 

 separate in a concrete state, nor that phlogiston may also be in the same predi- 

 cament ; while others have totally mistaken the nature of inflammable air, and 

 imagined it to be a combination of acid and phlogiston. The reason why fixed air 

 cannot be separated from any substance in a concrete state is, because when it is 

 separated, for instance by means of an acid, there is always a double decomposi- 



