398 Mr. Faraday on the mutual Action of Sulphuric Acid 
by further heat was entirely dissipated. Its general habits 
were those of ammoniacal salts. When its solutions, though 
previously rendered alkaline, were evaporated to dryness at 
common temperatures, and exposed to air, the salt became 
strongly acid to litmus paper. This however is a property 
common to all soluble ammoniacal salts, I believe, without 
exception. 
Baryta. It is easy by rubbing carbonate of baryta with 
solution of the impure acid, to obtain a perfectly neutral so- 
lution, in which the salt of baryta, containing the acid already 
described, is very nearly pure. There is in all cases an un- 
dissolved portion, which being washed repeatedly in small 
quantities of hot water, yields to the first portions a salt, the 
same as that in the solution. As the washings proceed, it is 
found, that the salt obtained does not burn with so much 
flame on platina foil, as that at first separated; and the fifth 
or sixth washing will perhaps separate only a little of a salt, 
which when heated in the air, in small quantities, burns with- 
out flame in the manner of tinder. Hence it is evident that 
there are two compounds of baryta, which as they are both 
soluble in water, both neutral, and both combustible, leaving 
sulphate of baryta, differ probably only in the quantity of 
combustible matter present, or its mode of combination in the 
acid. 
It is this circumstance, of the formation of a second salt in 
small but variable quantities with the first, which must be 
guarded against, as before mentioned, in the preparation of 
salts from the impure acid. It varies in quantity accordin 
to the proportions of materials, and the heat employed : a 
I have thought that, when the naphthaline has been in large 
quantity, and the temperature low, the smallest quantity is 
produced. When the impure acid is used for the preparation 
of the salts now under description, a small portion of it should 
be examined by carbonate of baryta, as above, and rejected, 
if it furnish an important quantity of the flameless salt. 
These bodies may be distinguished from each other provi- 
sionally, as the flaming and the glowing salts of baryta, from 
their appearances when heated in the air. The latter is more 
distinctly crystalline than the former, and much less soluble, 
which enabled me by careful and repeated crystallizations, to 
obtain both in their pure states. 
The flaming salt (that corresponding to the acid now under 
description) when obtained by the slow evaporation of the 
saturated solution, formed tufts, which were imperfectly cry- 
stalline. When drops were allowed to evaporate on a glass’ 
plate, the crystalline character was also perceived; but when 
the 
