PURIFICATION OP SULPHURIC ACID. 
31 
phiiric acid, and the temperature arises in proportion as the 
sohition becomes more concentrated. If it be concentrated 
too much, that is, if too much acid be distilled off, there 
would soon remain little more tlian the bisulphate, and, 
under these circumstances, the glass would be softened. 
The softening of the glass occurs even at an earlier stage of 
the process than this. In the second place, this solution of 
sulphate, or rather of bisulphate of potash in sulphuric acid, 
becomes less fusible in proportion as it contains less acid, 
and when the retort is to be again used, the risk of break- 
ing it is incurred in effecting the fusion of the solid mass. 
3. Of these two processes, 1 prefer the first; and although 
it may be sufficiently eflicacious, since those to whom 1 have 
shown it, and who have practised it, have been astonished 
at the facility with which the acid distils by this method, 
yet the process may be rendered more easy still by com- 
bining the two, that is to say, by putting into the cucurbit, 
both the sulphate and the quarzite. Then, except the eleva- 
tion of temperature, the sulphuric acid boils as easily, and 
with as much regularity, as water. 
It is- well not to distil the sulphuric acid too many times 
on the same quarzite, for the rough edges are blunted, a 
very fine powder is separated, which floats in the liquid, 
and after the lapse of a certain time, it no longer produces 
the same effect. Then, if the latter method be adopted, we 
must not wait until the residue of the distillation is quite 
cold before adding fresh acid, because the pieces o^ quarzite 
which would remain in the solid mass could not be changed. 
To effect this, it is necessary to add the acid before the re- 
frigeration is complete, to decant the liquid, take out the 
quarzite, and return the solution into the retort, in readiness 
for another operation, after having added to it some fresh 
pieces of the substance mentioned. The retort also should 
not be too full, and should be placed, if possible, in such a 
