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APPENDIX.

x. Preparation of Picric Acid.—Guinon arranges in a sand-bath twoseries of flasks, each containing an equal weight of nitric acid of sp. gr. r - 3.Crude carbolic acid is allowed to flow drop by drop into each from a vesselplaced above, through glass tubes, no external heat being applied. All theflasks communicate by means of glass tubes with a large stoneware vessel,which receives the acid fumes given off; part of them are condensed there,and the rest escape through the chimney. When the falling drops of carbolicacid undergo no further change, the supply is cut off, and a gentle heat isapplied to the sand-bath, to effedt the transformation of the resinous mass stillfloating in the flasks. The contents are finally poured into capsules where thepicric acid is deposited, on cooling, partly in coherent clots, and partly in finecrystals. It is placed in a funnel, the throat of which is loosely filled withfragments of quartz or of bricks free from lime. The nitric acid graduallydrains away, whilst the picric remains. It is still contaminated with a littleresinous matter, to remove which it is dissolved in boiling water containingtAs of sulphuric acid, in which the impurities remain insoluble. Perra pre-pares picric acid as follows :—600 parts by weight of nitric acid, of sp. gr. 1-3,are placed in a flask, and 100 parts of carbolic acid are gradually added. Thenitric acid volatilised during the readtion is condensed by means of a cohobator,and allowed to flow back upon the carbolic acid. The yield thus obtainedvaries from go to no per cent of the carbolic acid employed.

Carey Lea prepares picric acid from Australian “Yellow Gum,” the resin ofXanthorrhcea hastilis, as follows :—150 grms. of the resin, powdered and placedin a capsule containing 2 or 3 litres, are moistened with 300 grms. of nitricacid, of sp. gr. 1-42. As soon as the adtion begins 750 grms. of boiling water,which should be ready at hand, are added. Heat is applied, when the massswells up and threatens to run over, which must be prevented by the additionof small quantities of cold water and by a judicious regulation of the tem-perature. When the volume of the liquid has diminished one-half, 150 moregrms. of nitric acid are added, and the heating is continued till the liquid isreduced to its former bulk. From 120 to 200 parts of nitric acid are stillrequired to complete the operation. The liquid is then evaporated down tothe bulk of 120 to 150 c.c. On cooling the residue solidifies. The yield isfrom 25 to 50 per cent of the yellow gum employed.

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