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tamed feparate, the quantity of fulphurous acidmay be found, eftimating that ioo grains of ful-phat of potafh contain 45 of fulphuric acid,equal to 15 of fulphur, equal to 30 grs. or42.72 cubic inches of fulphurous acid gas.
Inftead of obtaining the fulphat of potafh fe-parate, which it is difficult to do exadfly, thealkaline folution, after the fulphite has becomefulphat, may be faturated with muriatic acid,and the quantity of fulphuric acid then efti-mated by muriat of barytes.
The accurate eftimation of fulphuretted hy-drogen is very difficult, particularly when mixedwith carbonic acid gas. The fulphuretted hy-drogen may indeed be expelled almoft entirelyby boiling, but when received over mercury itis partly decompofed by this metal, and whenby water it is partly abforbed by it, though thislaft inconvenience is in a good meafure pre-vented by warming the water.
1 here are two ways of eftimating it. Oneof them is the following: boil the water andreceive the gaffes over mercury in the ufualway. Tbefe may confift of carbonic acid, ful-phuietted hydrogen and azot, but not oxygen,this being incompatible with fulphuretted hy-drogen. It is neceffary in order to obtain thecarbonic acid pure that the fulphuretted hydro-gen be firft feparated, for the alkalies and thealkaline earths abforb both. For this, Mr.Kirwan advifes, firft to difplace any mercuryremaining in the jar by dipping it a little wayin hot water, whereby the mercury will falldown and the water take its place. Then dipthe jar into a fmall velfel full of moderatelyftrong nitrous acid, by which the acid will mixwith the water and come in contadf with thegaffes, and abforb very fpeedily or decompofethe fulphuret, and leave the carbonic acid andazot, which laft may afterwards be feparatedby an alkali.
But as all the fulphuretted hydrogen cannotbe expelled undecompofed from the water, theabove method is quite accurate only for theother gaffes. Mr. Henry fuggefts as an im-provement to the above method, inftead of ufingnitrous acid, to throw up oxymuriatic acidgas to the mixed gaffes Handing over water, tillthe jar is filled, doling the jar with a groundftopper, and allowing the mixture to remainfor twenty-four hours. Then on opening thejar under water, a quantity of this fluid willrufh in to fill the vacuum produced by the de-compofition of the fulphuretted hydrogen, andHanding about half an hour without agitation,
the excefs of oxymuriatic acid will be abforbed,and only the carbonic acid and azot will re-main. In this procefs the oxymuriatic acid gasfbould be quite pure, fuch as is obtained fromoxymuriat of potafh and muriatic acid, andequal in bulk to the other gas, but the requiredpurity of this gas is certainly an objection.
The mere quantity of fulphuretted hydrogencontained in any water is perhaps belt afeer-tained by decompofing this gas altogether whenin folution in the water, without attempting toexpel it by ebullition. The fubftances employedfor this may be either nitrous or oxymuriaticacid, which if properly managed precipitate thefulphur of the gas totally in its proper form.The theory of this decompofition has been ex-plained under the article Sulphuretted hydrogen.There are however two caufes of error to beavoided, one is, that the proportion of gas con-tained in natural water is fo fmall that a mo-derate quantity of nitric acid foon becomes toodilute to produce its complete^ffedt. The otherdifficulty (whichparticularly applies to the oxy-muriatic acid) is, that if an excefs of it be ufed,the precipitated fulphur again difappears, beingconverted into fulphuric acid. This laft cir-cumftance indeed might be taken advantage ofin the analyfis, by converting the whole of thefulphur to fulphuric acid, but it feldom happensthat any natural water is free from fome fulphator other, and the acid of thefe could not eafilybe extradited previoufly to the acidification ofthe fulphur, and of courfe would be confoundedwith it. The mod practicable method wouldbe firft to acidify the fulphur of the frefh waterand precipitate the whole by a barytic folution •,and then to colled the precipitate from anequal quantity of the water boiled and decom-pofed by the barytic folution, and the differencein the weights of the two precipitates of fulphatof barytes would indicate the quantity of ful-phuric acid belonging Amply to the fulphurettedhydrogen.
Mr. Kirwan b gives the following method ofdecompofing the fulphuretted hydrogen con-tained in any water and colle&ing the fulphur.Introduce two pounds of the hepatic water intoa bottle, fo as to fill about £ of its capacity,and place it in a tub filled with common water.Into this bottle introduce a few cubic inches ofnitrous air, which will be immediately deccm-pofed, and after fome time the water will be-come turbid. Continue letting up nitrous airas long as the rednefs of its mixture with com-mon air {hews its decompofition; this done,