144 Analyjis of the Arfe'niales 



mentioned by Mr. Prouft *, was produced. To obviate both 

 inconveniences, alcohol was added, immediately before the 

 liquor was quite evaporated, and long after the precipitate 

 had begun to appear; in a few minutes, all the arfeniate of 

 lead fell to the bottom, while the nitrate of copper was held 

 in folution. Thefe new produfts being feparated by filtra- 

 tion, the fpirituous liquor was diftilled ; and, from the nitrate 

 of copper, the quantity of that metal contained in the ore 

 •was obtained by boiling the folution with potalli or fodaf. 



To 



* Annales de Chimie, vol. xxxii. p. 26. 



\ By potafii and foda, I mean thofe alkalies pure, obtained according 

 to the method prefcribcd by Berthollet. I know of no other. It is not 

 that I have any predilcftion for thofe identical terms ; yet, whatever me- 

 lioration fubfequent improvement may introduce in particular cafes, if 

 principles are to be adopted, they fliould, in general, be ftriflly adhered 

 to. But it mull be a violation of them to apply a word, appropriated by 

 common confcnt to dcfign a pure, and as yet a fimple fubftance, to fuch 

 heterogeneous mixtures as lapis ca\ifticus, carbonates of potafli and foda, 

 &c. It is indeed much to be defired, that the epithets caujlic, pure, fa- 

 iuraie, &c. fhould be regarded as tautology, whicii they really are. There 

 is no potafli purer than potafh. When it is not pure, we fliould fay, in- 

 fiead of " I took fo much potafli,'' " I took fo much of a mixture of 

 pota.fli, and whatever other fubftance is mixed with it." Thus, inftead of 

 calling lapis caufticus, cauftic potafii, or potafli, as is often done, we 

 fhould fay, " I took fo much of the mixture of potafli, fulphate, muriate, 

 carbonate, aTld fulphuret of potafli ; filiceous and aluminous earths ; iron 

 and manganefe ;" for fuch I find, by analyfis, lapis Caudicus to be. To 

 all this is added, by apothecaries, a little lime. Yet this is the fubftance 

 fometimes called potaih. 



M. Lowitz's manner does not give potafli pure enough for delicate ana- 

 lyfes of ftciies. I have never feen any prepared by his method, in which 

 I could not difcover iron, fiiica, alumina, and carbonic acid. To the proofs 

 given by Dr. Kennedy, (in his paper intituled " A Chemical Analyfis of 

 three Species of Whinftone and two of Lava," in the Edinburgh Tranf- 

 afliions for 1799,) of the efficacy of his method, I propoic the following 

 objeftic.'ns : That chemift fuperfaturates by nitric acid, and examines by 

 nitrates of barytes and of filver. This will be a fufficient teft for fulphuric 

 and muriatic acids; but carbonic acid may have been preftnt before fatu- 

 ration. He then evaporates, and, if all is rtdiffolvable, concludes there is 

 no fiiica nr alumina ; but, after faturation by an acid, ammonia is a more 

 Jclicaie teft than evaporation for fmall portions of thofe earths. 



By treating Dantzic potafli, or, ftill better, pearlafli, with lime, and 

 evaporating in a well plated copper veflel, a white mafs is left. This 

 mafs, diflToIved as far as it can be in alcohol, and the liquor diftilled to 

 dryncfs in a plated alembic, gives an alkali of a perfeft whitenefs. Iti 

 this ftatc it is dangerous to touch it, its aclion on animal matter is fo fud- 

 den and fo violent. It attacks all ftones with the grtateft cafe and rapidity. 

 DiflTiilvcd in water, it makes not the leaft cloud in bavytes water, or in a 

 folution of nitrate or muriate of that earth ; and may be ufed as a very 

 delicate and fcnlible reagent, to diftinguifli it from ftroiitian. By fatu- 

 rating with an acid, and then feeking fiiica or alumina, by ammonia, no 

 trace of them can be found, nor indeed of any thing elfe. I do not fay, 

 however, that the potafli is perfcdtly free from every other fubftance ; I 



believe 



