3); in order, however,
that I may properly prevent the escape of the air it is necessary to
scratch the neck of the retort somewhat at this place with a flint.
(Retorts which I employ for investigations of this kind I have blown not
larger than to be capable of holding only from one half to three ounces
of water, but which have at the same time a neck which is about half an
ell long, and that for this reason that the attached bladder may not be
destroyed during the operation by the heat of the furnace or by the hot
vapours.) Into this bladder I pour some milk of lime (Sec. 22), and press
the air out as fully as possible. This lime will absorb the aerial acid
during the distillation, and leave the phlogisticated acid of nitre
untouched. (_c._) In exactly the same way as is described in _a_ I also
collect aerial acid and the inflammable air of sulphur (of which I shall
speak further on). But if the bladders are moist, or even if only the
air surrounding them is so, both these kinds of air penetrate completely
through the bladders in a few days; if the bladders and air are dry,
however, this does not take place. I obtain inflammable air from the
metals, as iron or zinc, in exactly the same way, except that I place
the bottle in warm sand. This air is still more subtle than the
preceding; it penetrates through the fine pores of the bladder in a few
days, although air and bladder are dry. I frequently experienced this
to my vexation. (_d._) I not infrequently catch air in bladders, without
any bottles. I place in a soft bladder (AA, Fig. 4) the material from
which I intend to collect the air, for example, chalk; above this chalk
I draw the bladder together with twine BB; I then pour above it the acid
diluted with water and press out the air as completely as possible; I
finally tie up the bladder above at CC. I then untie the twine B, when
the acid runs upon the chalk; it immediately drives out the aerial acid,
whereupon the bladder must expand. (_e._) When I require to get an air
out of the bladder into a flask, glass, retort, or bottle, I fill such
apparatus with water and place in it a tightly fitting cork; I then tie
the bladder which contains the air, that is, the opening from C to D
(Fig. 4), very firmly over such bottle; I then invert the bottle so that
the bladder comes below and the bottle above, whereupon I hold the
bottle with the left hand and with the right I withdraw the cork; I hold
this cork firmly between both