•Inquiry into the Laws of Afinit^. " ' -y^ 



tlie property of cryftallifing f it is this tartrite alfo which is neceflarily obtained, when aa 

 cxcefs of the bafe is prcfent in the experiment ; but when an excefs of acid is required, the 

 degree of faturation in which the acidulous tartrite exifts, may be determined by the quan- 

 tity of pot-a(h which is neceffary to efFeft its neutralifation. 



3. One confideration which is immediately feen to deferve much attention, is, that on 

 comparing the affinities, it is neceflary to employ in all the experiments, the fame propor- 

 tions of all the fubftances which might be fucceffively put in oppofition ; becaufe, if the 

 proportions vary, the refult of the adlion will be no longer the fame, nor can the affinity be 

 reprefented by the fame number. I fhall proceed to illuftrate this obfervation by an 

 example : 



Let the number loo reprefent the pot-afh, which is to be faturated by 100 parts of ful- 

 phuric acid ; I oppofe to it 100 parts of foda ; I fuppofe that after the adtion the pot-afli 

 will be found to have abforbed 60 parts of the acid, and the foda 40 ; I fhall then conclude, 

 that the affinities of thofe two bafes for the fulphuric acid are in the proportion of do to 40. 

 But 40 parts of -pot-afh remain uncombined ; thefe ftill continue to a£l, and contribute by 

 their aftion to the divifion of the acid ; fo that if this quantity were to be varied, the refult 

 would no longer be the fame. If inftead of ico parts of pot-a(h and 100 parts of foda, I 

 take only 80 parts of each, the uncombined remainder will afford 20 parts of pot-afh, and 

 another quantity of foda, fo that the forces exerted by thofe two parts, are no longer in the 

 above-mentioned proportion j whence it appears that the two faturations cannot be in the 

 proportion of 60 to 40. 



4. But in order toafcertain the degree of faturation acquired by each of thefe fubftances, 

 it is necelTary to efFe£l: a feparation j and this can only be obtained by elafticity, by cryflal- 

 lifation, by precipitation, or by the aftion of a folvent. But we have feen that thefe dif- 

 ferent means ought to be confidered as foreign powers which alter the refults, and determine 

 the combinations that are formed, without our being_ able to meafure their efFedl:, fo as to 

 difengage that of the eleftive affinity ; and confequently, that thofe feparations which, ia 

 the articles I, II, III, have only been confidered as the proportional cfFeft of elective affinity, 

 are really the eifedl of feveral joint forces, as is proved by the obfervations fubfequent to 

 thofe articles. 



When it is necefTary, for inftancc, to afcertain the quantity of fulphate of pot-afh and 

 foda that might be formed, the force of cohefion not differing confiderably in thefe two 

 fulphates, ft is probable that it would not effedt any great change in the proportion of the 

 two falts which would cryftallife } but it would be necefTary to feparate the excefs of alkali 

 by alcohol, in order to obtain the whole cryftallifation. Now, the adtion of the alcohol 

 being unequal upon pot-afli and foda, would again produce a change. To thefe conlide- 

 rations we may add, that a change of proportions would not only caufe the force of the 

 -foda and the pot-afh (No. i) to vary, but alfo that of the alcohol, without mentioning the 

 affinity of the water, which ferves as the folvent. ' 



1 2 But 



