PYRI′TES. A term applied to several native metallic sulphides. Iron pyrites is the best known of these.
PY′RO-. The term is applied to several acids that are obtained by the action of heat on other substances; as, PYROGALLIC ACID, PYROLIGNEOUS A., &c.
PYROACE′TIC SPIRIT. See Spirit (Pyroacetic).
PYROGAL′LIC ACID. HC6H5O3. Syn. Acidum pyrogallicum, L. Prep. 1. From either gallic or tannic acid, heated in a retort by means of an oil bath, and steadily maintained at a temperature of about 420° Fahr. as long as crystals are formed in the neck of the retort, or in the receiver, both of which should be kept well cooled. Pure. If a much higher heat is employed, the product consists chiefly of metagallic acid.
2. From Aleppo galls, in very coarse powder, heated in a dish covered with thin filtering
paper pasted to its edge, and connected with a well-cooled receiver. Dr Ure says that the so-called Chinese galls furnish, by dry distillation, a “very concentrated solution of pyrogallic acid, which, evaporated on the water bath, yields of brown crystalline pyrogallic acid nearly 15% of the weight of the galls.”
3. (Dr Stenhouse.) By sublimation from the dry aqueous extract of nut-galls, in a Mohr’s apparatus, in the same way that benzoic acid is obtained from benzoin resin, observing the precautions referred to in No. 1 (above). Nearly pure. The product is fully 10% of the weight of extract operated on.
Prop. Light crystals, which, when perfectly pure, are quite white; freely soluble in water, but the solution cannot be evaporated without turning black and suffering decomposition; it strikes a rich blackish-blue colour with the protosalts of iron, and reduces those of the sesquioxide to the state of protoxide; when heated to 480° Fahr., it is converted into METAGALLIC ACID and water.
Uses, &c. Pure pyrogallic acid is now very extensively employed in photography. A solution of the crude acid mixed with a little spirit is used to dye the hair, to which it imparts a fine brown colour, but has the disadvantage of also staining the skin when applied to it.
PY′ROGEN ACIDS. Those generated by heat.