PH O
*
( 217 ) PHO
ftrongly of phofphuretted hydrogen, and fome-times actually takes fire. In an open veflelthefe bubbles kindle the inftant they reach thefurface. By continuing the heat fora confide-rable time, all the fuperfluous phofphorus isburnt off", and the liquor is now converted intophofphoric acid.
The phofphoreous acid, though not faturatedwith oxygen, does not abforb it, either whenexpofed to the air or to oxygen gas, at leaft nottill after a confiderabie length of time, which,together with the great difficulty of expellingthe excefs of balls by heat, {hews the very ftrongaffinity between pholphorus and phofphoric acid,and feem to confirm the opinion already Hated,with regard to the conftitution of the phof-phoreous acid. By boiling with nitric acidhowever, the excefs of phofphorus is compleatlyoxygenated, and the whole becomes phofphoricacid.
Phofphoreous acid added to thofe of themetals that difengage hydrogen during folution,fuch as iron and zinc, dillolves them with theprodudtion of a fetid gas, luminous in the dark,but not inflammable per fe, which is hydrogen,holding a very final 1 portion of phofphorus infolution, or what may be termed fub-phofphu-retted hydrogen. The refidue is the phol'phatof the metal employed.
This acid mixes with water in all proportions,and cannot be rendered folid by evaporationwithout lofing the excefs of phofphorus, whichdiflinguifhes it from the phofphoric.
The combinations of this acid with the feveralbafes, form tli ephofphites, which will be prefentlydefcnbed.
- This acid is found native in feveral ftates ofcombination, particularly with lead and withlime, and it compofes a great part of the earthof bones (whence it may be extracted in theway already mentioned), but it is never foundnative uncombined with any bafe. There aretwo methods of preparing it, one direftly fromphofphorus, the other by the decompolition ofphofphat of lime. The former gives it morecertainly in a ftate of abfolute purity.
Phofphoric acid is produced by the rapidcombuflion of phofphorus in oxygen, whichmay be done either. , ter combuftion, wlm
lig it, the acid fublimes in white cryftalline flakes,
a concrete on the inner furface of the veflel,an w 11c oon deliquefee into a very ftrongacid. If this is done over water, it {hould beover a very thin ftream of this fluid, and thenby wafting out the jar with it, all the acid
VOL. II.
lorus m oxygen,over mercury or water,which gives intenfe heat and
mixes with the water, and may be concentrated
by evaporation.
Another and very convenient way of preparingit is, by compleating the acidification of phof-phoreous acid by the nitric, as mentioned in theformer article. Let phofphoreous acid be pre-pared either by the flow combuftion of-phof-phorus, or elfe in the following way, which isvery convenient. Put a quantity of pure phof-phorus in a fmall cup, which will not crackby the heat, Handing in a plate, the bottom ofwhich is ’ covered with water: kindle it, andimmediately whelm over it a large bell-glafs,which, as in the combuftion in oxygen, willfpeedily be lined with the concrete acid. When-the flame has gone out and the veflel is cold, liftup the jar to change the air within it, andagain fire the phofphorus as before. When thishas been repeated a number of times, waft allthe infide of the jar with the water of the plate,,and evaporate this in the open air till it beginsto fend forth bubbles which flafli as they break;.,then add nitric acid till no more red nitrousvapours are perceived, and complete the eva-poration nearly to drynefs. The refidue is purephofphoric acid. The oXymuriatic acid may'be uied inftead of the nitric.
Phofphorus alfo may be compleatly oxygen-ated by boiling with the nitric acid without theafliftance of previous combuftion, and this isperhaps the fllorteft, and belt way of preparingit. For this purpofe put about fix parts ofmoderately ftrong nitric acid into a retort, heat'it arid then drop in one part of pure phofphorus.A violent dilengagement of red nitrous va--pour takes place, and when it has ceafed, boiLthe liquor till all a£fion between the two ap--pears to be over. Then evaporate the folutionnearly to drynefs, and the refidue is pure phos-phoric acid.
This acid may alfo be prepared from pliof-pbat of lime in confiderabie purity, by means-of fulphuric acid and ammonia in the way givenby Dr. Higgins, and already mentioned underthe article Phofphorus.
It was found by Lavoifier that ioo parts ofphofphorus, during combuftion in oxygen gas,abforbed iyt of oxygen, and produced 254 ofconcrete phofphoric acid; which proportionshave been confirmed, or nearly fo, by otherchemifts.
Phofphoric acid, in whatever way prepared, ,has the following properties: when concen-trated as much as poflible, retaining the liquidform, it is a thick un&uous-lookirig fluid ofthe fpecific gravity of 2, or fometimes more,ftrongly acid but with no other tafte, nor any2 E