PICRIC ACID or LYDDITE, is a yellow crystalline solid melting at 122.5° C and owes its name to the intensely bitter and persistent taste of its yellow aqueous solution. As experiments regarding the use of picric acid as a high explosive were once conducted at the town of Lydd (Sussex), the name Lyddite became attached to the acid. The term melinite is employed in France and Germany, and shinosite indicates the Japanese variety of this explosive. As its graphic formula (inset) indicates, it is a ben zenoid derivative with three hydrogen atoms of the benzene nu cleus replaced by nitro (NO2) groups, and one replaced by a hydroxyl (OH) group, whence its name and formula 2 :4 :6 tri nitrophenol It is a typical example of a nitro compound of the aromatic series, and was formerly employed as an acid dyestuff giving yellow shades on wool or silk. Although acting as a poison when administered in quantities exceeding a few milligrams, its saturated aqueous solution may safely be applied to burns and scalds with the object of subduing the pain, which it effects by paralysing the sensory nerve endings.
The ordinary method of manufacturing picric acid is to dissolve phenol in strong sulphuric acid thus making a phenolsulphonic acid, which on treatment with a moderate excess of nitric acid regenerates sulphuric acid, and at the proper temperature gives finally trinitrophenol. This separates as an oily liquid which becomes solid on cooling. It is washed with cold water to remove excess of acids and then dissolved in hot water, from which it crystallizes in yellow scales, plates or needles, depending on the concentration of solution. The picrates of ammonia and of the alkali metals sodium and potas sium are most soluble, whereas the lead salt and a few others are sparingly soluble in water. Certain organic bases, such as acridine, give more insoluble salts than metals. When heated above its melting point picric acid puffs or burns with a smoky flash : a small amount may sublime as a yellow powder. This burning does not appear to be dangerous when only small quantities are dealt with unless the external heating be rapid, and extensive, so that the whole bulk under experiment is heated up ; in that case the simple burning passes over into a kind of detonating decomposition. The metallic salts of picric acid are most liable to this violent form of explosion, in fact it is almost the only form of explosion they exhibit either on heating or under percussion. The picrates of lead, bismuth and other heavy metals are extremely sensitive to friction or percussion as well as heating, especially when dry. The pure acid itself is not dangerously sensitive to percussion, although a powerful and quick glancing blow will often induce decompo sition. The explosion of a very small amount of a picrate—as
the lead or potassium salt—in contact with the acid, especially if the latter be in a compact state, generally induces a complete detonation. At one time picric acid was much in favour either as an ingredient of an explosive mixture with some special objective use or alone. Among the mixed explosives containing picric acid may be mentioned Brugeres powder, a mixture of 54 parts of ammonium picrate and 45 of potassium nitrate; Designolles' pow der, potassium picrate, potassium nitrate and charcoal; and Emmensite, of similar composition, suggested by Mr. Emmens, U.S.A. Many others were designed for blasting or similar pur poses. (W. R. Ho.) PICRITE, in petrology, an intrusive igneous rock of ultra basic composition somewhat similar to peridotite (q.v.). The name, given by Tschermak in 1866, is derived from rucp6s, "bit ter," owing to the high percentage of magnesia (bitter earth) these rocks contain. The picrites are dark, heavy rocks largely composed of olivine and augite, with a small but variable amount of plagio clase felspar. Hornblende and biotite may also be present. The picrites usually occur in sills or sheets, and seldom in large plu tonic masses (unlike peridotites). Varieties of picrite include augite, enstatite and hornblende picrite.
The minerals of picrites are very frequently decomposed. Ser pentine partially or wholly replaces olivine, while hornblende (pil ite), talc and chlorite appear as secondary products after the same mineral. Augite passes into hornblende or chlorite, and the essen tial felspar is often represented by epidote, prehnite and white mica. In some picrites, as in the peridotites, a "lustre mottling" is given by the poikilitic inclusion of olivine grains in large crystals of augite or hornblende. In the porphyritic varieties a residual brown glass may be present. Such rocks are known as picrite porphyrites.
Many picrites clearly possess alkaline affinities. Thus the augite picrites of the Midland valley of Scotland and of the type locality in Moravia contain interstitial analcime and are closely related to the teschenites with which they are associated. Other picrites are more clearly calc-alkaline, being associated with diorites or dole rites. The hornblende picrites of Carnarvonshire, Anglesey and other places belong here. In the Devonian rocks of the Fichtelge birge and Nassau picrites accompany dolerites and proterobases, as they do also in Cornwall and Devon. The term picrite-basalt is reserved for felspar-poor basalts rich in olivine. (C. E. T.)