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open veflel, be concentrated nearly fo much asby diftillation in a clofe apparatus.

It does not appear poffible to obtain the ful-phuric acid free from water, except it is com-bined with fome alkaline or earthy bafe^but asit is a matter of fome importance, both in che-mical experiments, and in feveral manufactur-ing procefles where this acid is employed, toafcertain the real ftrength of mixtures, con-taining different proportions of acid and water,feveral chemifts have engaged with variousfuccefs in this interefting enquiry. Upon thewhole the experiments of Mr. Kirwan, cor-rected as they have been by the obfervations ofMorveau and Berthollet , feem entitled to moltconfidence, and are fufficiently exact to anfwerevery practical purpofe. The method by whichMr. Kirwan has refolved this important pro-blem is the following. s He took fome purifiedcream of tartar, and calcined it in a filver cru-cible till the tartaric acid was entirely de-ftroyed; from the refidue by lixiviation withdiftilled water, he obtained a folution of purefubcarbonat of pot-afh, which, after evaporationto drynefs and fubfequent ignition, weighed1523. 5 grs. This fait was then diffolved inwater, and the whole weight of the folutionamounted to 4570 grs. Of this latter folution360 grs., containing 120 grs. of fubcarbonat ofpotafh, were faturated with 130 grs. of a ful-phuric acid, the fpecific gravity of which at 60“F. was = 1.565. During the combination ofthe fulphuric acid and the alkali, 34 grains ofcarbonic acid which had previoufly been com-bined with the alkali were expelled, thereforethe amount of pure potafh in the folution was— 120—34=86 grs. Water was now addedto the folution till it amounted to 3694 grs. ofwhich the fp. gr. at 6o° F. was = 1.013.Next, 45 grs. of fulphat of potafh were dif-folved in 1017 grs. of water, and the fp. gr. ofthis folution was exaftly equal to that of theone juft mentioned; but in this latter folutionthe proportion of fait to water was -.1:23.6, andthe fpecific gravities of both folutions being thefame, the fait in each muff alfo bear the fameratio to the water. Therefore in the formerfolution the amount of fait was r= -f-yfir =1 56-5 2 grs. now of this, 86 grs. were potafh,hence the remainder, namely 70.5 grs., wereacid in the ftate of concentration in which itexifts in fulphat of potafh; but fulphat of pot-afh lofes fcarcely any perceptible weight by ig-nition, from which it may be inferred that itcontains little or no water of cryftallization,

and that it is compofed of mere or real acidand potafh. Therefore 130 grs. of fulphuricacid at 1.565 contain 70.5 of real acid, and100 parts of the fame diluted acid contain 54.2of real acid.

By a fimilar method the proportion of realacid in fulphuric acid of the fpecific gravity of2-o or Jlandard acid was afcertained, and bydiluting this acid with various proportions ofwater, the quantity of flandard or real acid inacid of any fpecific gravity between 2.0 andr.0955 was found. But when fulphuric acidand water are mixed together, a mutual pene-tration takes place, in confequence of whichthere is- an increafe of fpecific gravity. Thisaugmentation was found to amount to about-'y of the whole denfity. The gravities asfound by experiment being corrected accord-ingly, a table was conftrurited fhewing thefpecific gravity of any mixture of fulphuricacid and water, from the molt concentrated oilof vitriol, down to one fo dilute as to containonly 1.78 per cent, of real acid. This tablewill be given in the Appendix .

The component parts of fulphuric acid are-well known to be fulphur and oxygen, as maybe demonflrated either by analyfis or fynthefis;thus if fulphur be digefled with nitric acid, ni-trous gas will be given out from the decompo-fition of the acid, while the oxygen, the otherelement, will combine with the fulphur, andform with it fulphuric acid; on the other hand,if fulphat of foda be mixed with charcoal, andexpofed to a red heat, the fulphuric acid is de-oxygenated by the fuperior affinity of the char-coal; carbonic acid and carbonous oxyd areproduced, and the fulphat of foda is found tobe converted into fulphuret of foda, fromwhich the fulphur may readily be procured byfolution in water, and the addition of an acid.

But though the elements of fulphuric acidare afcertained, much doubt ftill exifts withregard to their relative proportions, we fhalltherefore enter with fome minutenefs on thispart of the fubjedh. Three methods have beenfollowed in attempting to refolve this impor-tant queftion. The firft that we fhall mention,and apparently the inoft diredf, was employedby Lavoifier. He placed a given weight ofpurified fulphur in a receiver, with a little wa-ter to abforb the acid produced, then lettingfire to the fulphur, he fupplied it with oxygengas of known purity, till the combuftion ceafed,then by aicertaining the weight of fulphurburnt, and of oxygen gas confumed, he infer-

* Irlfti Phil. Tranf. vel. iv.