Inqutrif into the Laws af Affn\t^> -ig 



The fame happens when the fulphate of aluminc is decompofcd by ammonia : the preci- , 

 pitate always contains a portion of fulphuric acid. 



If the magnefia of the fulphate of alumine (qu. magnefia?) be precipitated by pot-alh, 

 the magnefia will alfo contain a portion of fulphuric acid ; for when the magnefia is fub- 

 mitted to Rrong heat, it is afterwards found to poflefs rather a ftrong tafte of fulphate. 



All thefe experiments prove, that in chemical analyfes we are liable to error, by taking 

 that for the real weight, of alumine or magnefia, which may have been contained in a 

 compound fubftance, and is merely the precipitate formed by eledlive affinity. 



From the preceding obfervations, and many others which cannot here be given, it ap- 

 pears that in operations of ele£livc affinity the fubje£t of the combination is divided 

 between the two fubftances which aiSl upon it, in confequence of the forces they are 

 capable of oppofing to each other. 



A circumftance worthy of attention, and which clearly proves that the chemical adlion 

 depends as much on the quantity as on the affinity of fubftances, is, that to produce oppo- 

 fite efFeiSis, it is only neceflary to make the requifite alteration in the quantities. 



ARTICLE, IV. 



On thofe Modifications of Chemical Aniotit -which arife from the Infoluhility of the Subjlances. 



The author fucceffively examines thofe affeftions of bodies which may difguife or modify 

 the applications of the principle eftabliftied in the preceding articles. He proves by a 

 number of experiments; i. The manner in which an infoluble fubftance afts when op- 

 pofed to a combination. 2. If a fubftance pofleffes fome degree of folubility, its a£lion is 

 compofed of that of the diffijlved part, and of that which remains folid. 3. If an infoluble 

 combination be attacked by a liquid fubftance, the inconveniencies of its infolubility fpee- 

 dily difappear, in the cafe when it is only neceflary that the infoluble fubftance ftiould lofe 

 a part of its conftituent principles, to become liquid. 



The author next proceeds to examine the difference of fpecific gravities of the infoluble 

 fubftance, and the liquid. According to him, this has an influence on their refpe(ftive 

 aftions, even when agitation and heat are employed ; becaufe it continually tends to fepa- 

 rate the infoluble fubftance, and to withdraw from the aftion oppofed to it. Hence there 

 is a difference on this account between the fulphatesof barytes and of aluminc. 



Laftly, as the infolubility prevents thofe proportions of divifion from taking place, which 

 might elfe have been eftablifhed by the oppofing forces, fo like wife it is found to retard 

 the operation of thofe which can take place ; fo that it may eafily produce deception by the 

 appearances which it affords at the commencement of an operation. For example, when 

 we mix fome concentrated fulphuric acid with a folution of fulphate of pot-afli, or any 

 other fait which requires a large proportion of water to dilTolve it, the acid foon combines 

 with the water ; and the fait wlMch lofes Its liquidity is precipitated ; but by continuing the 

 experiment, and multiplying the contaiSts, the fait becomes diflblved, and enters into com- 

 bination with the liquid. 



D 2 ARTICLE 



