t 311 3 



XI.V. On the. Alkalks of Commerce, and on the least ex- 

 pensive Process for ascertabnns. their commercial Falue ly 

 Means 'f the Instrument called the ^IkaU-meter. Read 

 to the Academy of Rouen, 5 ThermidoTy An. 13, hj 

 M. Descroizilles senior. 



[Concluded from p. 252.] 



Necessifj/ of a sufficient Proportion of IFater^ ivith supersa- 

 turated Carbonate of Potash, in order that the Potash may 

 be completely disengaged ly Means of Lime. 



JLt must have been observed that, in the first article quoted 

 in the preceding section, it has been prescribed to mix the 

 potash with double its weight of lime, while in the second 

 article no more than one-half is d'^manded ; it is true, that 

 less i-! requ'red for the truly neutral carbonate of potash, be- 

 ca:!&e it contains a less quantity oF alkali, and because the 

 water of crys'illization is here in greater proportion than in 

 the supersaturated carbonate of potash ; but if we consider 

 thai the richeit in alkali (being that which is obtained from 

 cream of tartar) absorbs only 0-72 of its weight in sulphuric 

 acid, while the quantity absorbed by the carbonate is 0*50, 

 we muit admit that the proportions indicated in the two 

 cases are no longer relative to each other. 



On the other hand, numerous experiments, made on a 

 large scale, liavc constantly proved to me that four-tenth,'* 

 of lime are sufficient to take off the carbonic acid Irom the 

 best potash of commerce ; How happens it, therefore, that 

 twenty tenths have been prescribed, and still fears enter- 

 tained that the potash was not perfectly caustic? I shall 

 now show that this fear appears well founded, so long as 

 ihfc following observation of mine is unknown : I have ascer- 

 tained that, whatever be the proportion of the lime to the 

 supersaturated carbonate of potash, if the proportion of water 

 to this salt is not as seven to one, there will be a quantity 

 of carbonate not decomposed, and proportionate to tlie de- 

 ficit of water, in the seven parts whi' h are strictly necessary. 

 Besides, if we consult the tables of Bergman as to the re- 

 U 4 spective 



