j^a Aliin! fi.'umtal "wkh lis Earth, tfi- 



may be of fome utilHy ; for Bcrghiart pretends, I don't knOw-on what foundation, that urine 

 communicates to alum properiics whicK art hhrtful in dyeing : it is not probable, however, 

 that this chemill aflcftcd the fa£l without proof. In all the worts where putrid urine is 

 employed for the treatment of ahirii waters, the ^lani contains the fulphates of pot-afli 

 and of ammoniac, becaufc tlic conibuftibles which firve to roaft the ores depofit a certain 

 <|uantity of alkali, which unites with the fulphuric aciJ, and contributes, according to its 

 proportion, to tlie formatidii of a grtater or lefs quantity *. 



It is knowii, frorri the eipcrinicnts of Bergman and fcvcral other chcniifts, that by boiling 

 a folution of ordinary aliim wit!-, pure ahimine in a very divided ftate, this lad combines 

 with the alum, and rlnders it iiifoluble in water; that is to fay, it converts it into the 

 Hate of neutral fulphate of alumiiie, or faturated with its earth I have repeated this ex- 

 periment with the dcfign to afcertain whether the fulphates of pot-a(h aid ammoniac are 

 precipitated with tlie alum ; and I immediately obferved, tliat the combination does not 

 take place but by means of heat, though I ufed alumine recently precipitated from a folu- 

 tion, and ftill humid; and that at the end of a certain time the alum was entirely pre- 

 cipitated, and fcarccly any figns of it left in the water. I re-diffblved the alum thus pre- 

 cipitated, in diluted fulphuric acid, and this folution afforded very fine cryftals of octahedral 

 alum by cooling ; whence it follows that the pot-afli and ammoniac did fall down with the 

 fulphate of alumine, and joined in the formation of this quadruple, earthy, taftelefs fait. I 

 at fufl imagined that this fad might ferveto explain how it fomctiiries happens that alu- 

 minous waters, parted over materials. lefs rich than thofe froin which they were obtained, 

 diminifhcd in denfity by the lofs of a portion of the acid neceffary to the folution of the 

 alum ; but as the combination is not made without heat, and it requires a great divilion in 

 die alumine, I prefume that this efFeft is owing to another caufe : neverthelefs, it might 

 be poDible, in tha procefs of time, efpecially in hot weather, that fonvething of this nature 

 might happen. It was interedijig to determine what happens with regard to the fulphates 

 of pot-afli and ammoniac in the precipitation of alum by its own bafe. I therefore 

 boiled a folution of pure fulphate of alumine, that is to fay, which contained no alkali, 

 and was not cryftallizable, with a certain quantity of the earth in a very divided (late. It 

 diffolved a fmall quantity, loft its flight acidity, but did not become infoluble. Having 

 afterwards dropped a very fmall quantity of the folution of pot-afli into this liquor, a 

 precipitate was formed in a fliort time, which was of the fame nature as that of the 

 foregoing experiment; that is to fay, what is called alum faturated with its earth. It is 

 therefore proved that the fulphates of pot-afti or* ammoniac are neceflary to render the 

 alum capable of being precipitated by its earth, or to caufe it to pafs, as it were, to the 

 earthy (late. It is alfo proved, that the aluminous waters which do not contain pot-aflv 

 may remain as long as may be defired on their materials, without becoming faturated 

 with too great a quantity of earth, or fuffering alum to precipitate. 



From the whole of what wc have thus far explained, it will be cafy to draw a number 

 of confequences of importance to the arts, chemiftry, and natural hiilory. 



1. It is not, at leaft in the grcateft number of circumftanccs, the excefs of acid which Im- 



" Wbtn alums contain at the Tame timt tlie fulphate of ammoniac ami of pct-a(h, the timntity of the latter 

 is Icfs; and in this refpefl there muft be great varieties in the proportions, according to the dofe of urine or of 

 fot-a(h which is added. V' 



pedes 



