P O T

P R U

( 150 )

It occurs in the sand of Ceylon , and hasmore recently been found among the substancesejected from Vesuvius .

This substance is at present ranked by Haiiyas a variety of Spinelle, with which it agrees inits crystalline form, and differs but little inother particulars.

POTASH, Pure , and IIydrat of. SeeAlkali Fixed.

POTASSIUM. See Alkaline Basis.

PRUSSIC ACIDS. Prussiats.

Some additions have been made to our know-ledge of this very singular class of substances,though the subject is still peculiarly difficult.What we have at present to offer relates toactual experiments and matters of fact.

Prussiat of Potash . The simple combi-nations'of prussic acid with the salifiable baseswere shewn by their eminent discoverer, Scheele ,to be peculiarly easy of decomposition, theprussic acid being volatilized and expelledfrom its bases by every other acid, and even bya boiling heat when in solution. Several chemistshowever have attempted to prepare a simpleprussiat of potash directly from the lixivium ofblood, without the intervention of any salt ofiron, and the following is the process of Kichter.*

Evaporate to dryness a strong solution of apound of pure salt of tartar, or subcarbonat ofpotash, with 3 pounds of dried and pulverizedox-blood. When quite dry, put the whole ina covered crucible and heat it in a moderate firetill it no longer puffs up : then raise the heat toredness, and continue it till no more flameissues from the crucible. Remove the contentsand lixiviate them with water. This liquor is■ quite limpid, alkaline, and gives ammoniacalfumes- Evaporate it considerably (during whichit gives out an intense bitter-almond smell,which always attends the volatilization of freeprussic acid) and then add acetite of lime aslong as there is any precipitate of lime producedby the excess of alkali, after which filter theliquid and boil it down till it skins over, andthen set it by in a cool place. In about twelvehours many small crystals are deposited of theshape of a cube truncated horizontally, andmore of the same crystals are yielded by a fur-ther concentration of the mother liquor. Collectthem all, and purify them by again dissolvingthem in water, evaporating, and crystallizingas before, but this is attended with some wasteof the prussic acid, as the same bitter-almondsmell is given out, and the liquor becomes“ An, Chem. torn. 51. p. 182. *

slightly alkaline. Therefore to avoid furtherloss, again purify this second crop of crystals bydrying them on blotting paper, washing themwith some alcohol mixed with an eighih ofacetic acid, and again drying them on paper.

By this process about an ounce and a half ofthe crystallized simple prussiat of potash is ob-tained. This salt has a caustic taste mixed withthe same, flavour of bitter almonds, and theodour of the same is perceived when the salt isdissolved in water and heated ; but not when itis heated by itself. When this salt is boiledwith prussian blue, a yellow solution is formed,which yields the common triple prussiat of pot-ash and iron.

To this process of Richter the following ob-servations are added by Bucholz. b The bestproportions of the material are two parts ofdried blood to one of alkali. No advantagewhatever is gained by using a perfectly causticalkali. The prussic acid is not very easily de-stroyed by dry heat, for much of it remains inthe alkali even when it begins to fuse: but theprocess of evaporation should be shortened asmuch as possible, as the prussic acid is con-stantly flying off during the whole time. Thereis always much uncertainty in this mode ofobtaining the prussic acid and a good deal ofwaste in every step of the operation, and eventhe treatment with alcohol and acetous acid,recommended at the end, does not merely satu-rate the excess of carbonat of potash, which isthe object of this addition, but liberates muchprussic acid which is thus lost.

Proust has the following observations on thesimple prussiat of potash. It may be obtainedeither by saturating potash with the prussic acidexpelled in the usual way from prussiat of mer-cury, or by digesting alcohol with a concen-trated lixivium of potash and calcined bloodprepared as above described. The alcohol gra-dually gains a bitter alkaline taste as it dissolvesthe prussiat. Generally a little hydrosulphuretof potash is contained in the lixivium, which isalso dissolved by the alcohol. .The prussiat ofpotash is then obtained in a solid state by eva-poration and crystallization.

When the watery solution of this simpleprussiat is boiled, it constantly parts with itsacid as already mentioned, and the vapour burnswhen a lighted taper is brought to it. Besideswhich, another part of the prussic acid is de-composed in the boiling lixivium itself, pro-ducing carbonat of ammonia, which appears in agradually encreasing quantity as the boiling is

Ibid, pagp 185. c Ibid. tom. CO.