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grotto in Mount St. Amiato in Tufcany; italfo occurs-in the crevices of volcanic moun-tains, and difl’olved in a few mineral waters.It is not however from any of thefe fourcesthat the fulphuric acid of commerce is obtained,the whole of this being procured either fromthe diftillation of fulphat of iron, or from thecombuftion of fulphur.

Sulphat of iron (or green vitriol) as we haveelfewhere fhown, conlifts of fulphuric acid,water, and oxyd of iron; by proper methodsthe acid may be feparated from the other in-gredients of the fait; and this continued to bethe only origin of fulphuric acid in the greatway, till the difcovety, by the manufacturingEnglifli chemifts, of the art of preparing it bythe combuftion of fulphur. As this latter dif-covery has not however as yet entirely fuper-feded the former, we fhail give an account ofboth, beginning with the molt ancient.

Sulphuric acid is thus prepared at Bleyl, inBohemia. 3 A long horizontal furnace or gal-lery of brick-work is conftructed capable of re-ceiving a number of retorts; the retorts them-felves are pear-fhaped veflels, with a flightlycurved neck, by which they fit into earthen re-ceivers nearly of the form of common retorts."The whole apparatus being prepared, each re-tort is charged with 3 lbs. of fulphat of iron,previoufly calcined at a full red heat, and thefire is lighted. The firft effeCft of the heat isto drive off the moiftnre abforbed by the vitriolin the interval between its calcination and dif-tillation; this phlegm being only very flightlyacidulous is allowed to efcape, and when itceafes to come over, the receiver with a littlewater in it is luted on to the retort; the fire isnow raifed and kept up brisk for 32 hours,during which time the acid rifes in the form ofdenfe white vapours, which fill the receiver,and are there abforbed by the water. Thefevapours being at a high temperature foon ren-der the receiver very hot, hence the workmenjudge of the termination of the procefs by thereceiver becoming cool in confequence of thevapour ceafing to rife. The red oxyd of ironor colcothar, is now taken out of the retort andits place is fupplied with a frefli charge of cal-cined vitriol; the diftillation then takes place asalready deferibed, except that the former pro-duce of acid is not emptied out of the receiver,and therefore there is no occafion to add anywater. If the retort is well made and carefullyluted all over, it will laft for three fucceftivediftillations, and the quantify of acid obtained

• Fragofo dc Sequeira in Joi

is nearly equal to half the weight of the calcinedfulphat.

If the acid be examined at different periodsof the diftillation, it will be found to be moreand more denfe according to the violence of thefire required for its extrication; the latter por-tion if received in a feparate veffel will gene-rally congeal upon cooling, hence it is calledglacial fulphuric acid ; this property however isnot entirely owing to its denfity, as we fhailprefently fhow.

The lulphats of copper and zinc have occafi-'onally been employed, inftead of the fulphat ofiron, but with a manifeft difadvantage, bothbecaufe they are dearer than the latter fait, andbecaufe they require a higher and longer con-tinued heat to drive off the whole of the acid.

The following is the ufual method of manu-facturing fulphuric acid from the combuftionof fulphur. A chamber is conftruCled of framework, and lined with ftrong fheet lead; theonly aperture is a fmall door, made to fhutvery clofe, the bottom of which is a littlehigher than the floor of the chamber. Wateris poured into this chamber till it rifes to theheighth of an inch or two upon the floor, anda ftand is introduced on which is placed anearthen pot containing a few pounds of fulphurand nitre, in the proportion of from eight to tenof the former to one of the latter; this mixtureis fet fire to by means of a red hot iron, andthe door is immediately clofed; at the expira-tion of about fix hours a fecond charge of ful-phur and nitre is introduced, which after afimilar interval is replaced by a third, and fo onwithout intermiffion for a fortnight or threeweeks. At the end of this period the water inthe chamber is fufficiently acidulated; it is ac-cordingly transferred to a leaden boiler, wherethe greater part of the water is evaporated; inproportion however as the acid becomes moreconcentrated, it is more difpofed to corrodeand difi’olve the lead of the boiler; thereforebefore this degree of concentration takes place,the liquor is transferred into large green glafsretorts, where a degree of heat is applied fuf-ficient to drive off almoft the whole of thewater. As the acid becomes ftronger it alfobecomes clearer and lefs coloured in confe-quence of a portion of acid re-a£ling on theimpurities with which it is tinged, and thus de-ftroying them. When the acid is thus broughtto the required denfity and clearnefs, it ispoured out ol the retorts into large globularglafs bottles, furrounded with wicker workrn. de Phj-f. vol. l i . p. 40.