158
ELEMENTS OF MATERIA MEDICA.
fire-place (fig. 41). 1681bs. of nitrate of potash are introduced at one
Fig. 41-
Fig. 42.
end of the cylinder, which is afterwards closed by a circular disk of ironperforated to allow of the introduction of a leaden funnel, through whichare poured 931bs. of strong sulphuric acid (sp. gr. 1'845). The tunnel sthen removed, the aperture closed, and heat applied. The vapours areconveyed into a row of five or six bottles of stone-ware filled to aboutone-sixth of their capacity with water. The acid obtained by this pro-cess is brown and fuming, and has a sp. gr. of T45. It is called in com-merce nitrous acid, or fuming nitric acid. To render it colourless it isheated in a glass retort, placed in a sand bath: the vapour whichescapes is passed through two stone-ware bottles to save any acid thatmay be expelled, and is subsequently conveyed into a chimney(fig. 42). The colourless acid remaining in the retort has a sp.
gr. of about T44, and con-stitutes the nitric acid ofcommerce. The residue inthe iron cylinder is a mix-ture of sulphate with a lit-tle bisulpliate of potash, andis sold as sal eniwum. It isemployed as a flux, and bythe alum-makers*.
The quantity of the in-gredients employed in theabove process is near-ly in the proportion ofeight equivalents or 392parts of strong sulphuricacid, and seven equivalentsor 714 parts of nitrate ofpotash. Some manufac-turers employ two parts byweight of nitrate of potash
Apparatus for the purification of coloured nitric acid.