384
ELEMENTS OF MATERIA MEDICA.
that (with the exception of small portions of these principles held insolution by the alkali) the filtered liquor is free from both fibrin andalbumen. A little acetic acid is now to be added, and the liquid boiled,by which any caseum present will be coagulated, and got rid of by filter-ing a second time. Sometimes the liquor is now found sufficiently freefrom organic matters to enable us to detect the arsenious acid very readilyby the ammoniaco-nitrate of silver. Dr. Christison says, that if this testact characteristically, that is, gives a copious yellow precipitate, theliquid is sufficiently free from foreign matter. If, however, it give noindication, or at least only imperfect ones of arsenious acid, evaporateto dryness by a gentle heat (as a water-bath), and boil the residue inrepeated portions of distilled water. We thus obtain a solution of arse-nious acid, which, after being acidulated with acetic or hydrochloricacid, is to be decomposed by passing a current of hydrosulphuric acidthrough it. 'The precipitated orpimeut (sesquisulphuret of arsenieum) isto be collected, and reduced in the way already described (p. 378).
Arsenious acid in organic liquids may sometimes be readily detected bythe developement of arseniuretted hydrogen when zinc and sulphuric acidare added to the suspected liquor {vide p, 381). But the frothing pro-duced by the organic matter creates considerable difficulty. I havealready pointed out the best methods of obviating it.
Composition.- —The following is the composition of arsenious acid:—
Eq.
Eq.Wt.
Per Cent.
Mitscherlich.
Arsenicum . . .
. 1 . .
. . 38 . .
. . 76 . . .
. 75-782 .
. . . 75-73
Oxygen.
. li . .
. . 12 . .
. . 24 . . .
, . 24-218 .
. . . 24-27
Arsenious Acid .
. 1 . .
. . 50 . .
. . 100 . . .
. . 100-000 .
. . . 100-00
Purity. —Powdered arsenious acid is sometimes adulterated withchalk or sulphate of lime. The fraud is readily detected by heat, whichvolatilizes the acid, but leaves the impurities.
Physiological Effects, {a.) On vegetables. —The effects of arseniousacid on plants have been studied by Jager [Biss. Inaug. Tubinga ;, 1808,quoted by Marx in his Die Lehre von den Giften, ii. 99) ; Marcet;Macaire, Mem. de la Soc. de Phys. et d’Hist. Nat. de Geneve t. iii.), andby others; and from their observations we leam that it is poisonous to allthe higher and most of the lower families of plants. It appears thatseeds which have been soaked in a solution of arsenious acid are incapa-ble of germinating, and that buds which have been plunged in it are nolonger capable of expanding. If roots or stems be immersed in this solu-tion the plants perish ; death being preceded by drooping and alterationof the colour of the leaves and petals. If the stem of the common bar-berry ( Berberis vulgaris) be placed in dilute hydrocyanic acid, or in anaqueous solution of opium, the stamens lose their remarkable contractilepower, but remain flexible. If, however, we employ a solution of arse-nious acid, the plant equally dies, but the stamens become stiff, hard, andretracted, and on any attempts being made to alter their position, theyreadily break. These curious facts appear to prove that the effects pro-duced by this acid on vegetables are very different from those caused byhydrocyanic acid and opium; for the latter seem to exhaust the irritabi-lity, while the former appear to give rise to a condition very analogous tothe spasm of animals. Jager has seen a small plant (supposed by De-candolle , in his Phys. Veg. p. 1329, to be Mucor imperceptibilis) grow in