346 | ‘Sequel fo Mr. Watr’s Thoughis 

watery and acid vapours which follow, a partial exhauftion will 
take place, and the receiving water will rife up into the retort’ — 
and break it, or at leaft fpoil the experiment. A common re- | 
ceiver, fuch as is ufed in diftilling {pirit of nitre, thould be 
applied, with a little water in it, toreceive the acid fteam; and 
_ it thould be kept as cool as can conveniently be done, as thefe 
fumes are very volatile. ‘This receiver fhould remain as lone — 
as the fumes are colourlefs; but when they appear, in the 
neck of the retort, of a yellow colour, it is a mark that the 
mereurial nitre will immediately produce dephlogifticated air > 
the receiver fhould then be withdrawn, and an apparatus placed 
to receive the alr. The reft of the procels has Sia fufficiently 
explained in my former letter. 
The phlogifticated nitrous acid, faturated by an alkali, will 
not cryftallize; and, if expofed to evaporation, even in’ the’ 
heat of the air, will become alkaline again, which fhews the 
weaknefs of its affinity with alkalies when diffolvedin water * ; 
a farther proof of which is, that it is expelled from them by 
all the acids, even by vinegar (which fact has been obferved 
by Mr. ScHeExe). I have obferved, that litmus is no teft of 
the faturation of this acid by alkalies; for the infufion of lit- 
mus added to fuch a mixture will turn red, when the liquor 
appears to be highly alkaline, by its turning the infufions of 
violets, rofe leaves, and moft other red juices, green. This 
does not proceed from the infufion of litmus being more fenfi- 
ble to the prefence of acids than other tefts; for I have lately 
difcovered a teft liquor (the preparation of which I mean to 
publifh foon) which is more fenfible to the prefence of acids 
* You have informed me, that Mr. Cavenptsu has alfo obferved this fact 5 
and that he has mentioned it in a paper lately read before the Royal Society 5, but I 
had obferved the fact previous to my knowledge of his paper. 
than 
