[ *73 ] 
Experiment XV. 
Dr. Black fays, that, by expofing a folution of fait of 
tartar for a long time to the open air, fome cryftals 
were formed in it, which feemed to be nothing elfe 
than the vegetable alcali united to more than its ufuah 
proportion of fixed air. This induced me to try, 
whether I could not perform the fame thing more 
expeditioufly, by furnifhing the alcali with fixed air 
artificially j which I did in the manner reprefented in- 
Fig. 6 : where A reprefents a wide-mouthed bottle, 
containing a folution of pearl allies; B b reprefents a 
round wooden ring; fafiened over the mouth of the 
bottle, and fecured with luting; C is a bladder bound 
tight over the wooden ring. This bladder, being firft 
prefied clofe together, fo as to drive out as much of the- 
included air as pofiible, was filled with fixed air, by 
means of the bent tube D ; one end of which is fixed' 
into the wooden ring, and the other fafiened into the 
mouth of the bottle E, containing marble and fpirit of 
fait. By this means the fixed air thrown into the- 
bladder mixed with the air in the bottle, and came in - 
contact with the fixed alcali. The fixed air was by 
degrees abforbed, and cryftals were formed on the 
furface of the fixed alcali, which were thrown to the 
bottom by (baking the bottle. When the alcali had 
abforbed as much fixed air as it would readily do, the 
cryftals were taken out and dryed on filtered paper, 
and the remaining folution evaporated ; by which 
means fome more cryftals were procured. 
N. B. It feemed, as, if not all the air difcharged from 
the marble was of a nature proper to be abforbed by 
the alcali, but only part of it ; for when the alcali had 
abforbed 
