A F F 



And it 13 impofTi'jle hence to conclude whether or not any 

 chemical change will take place : but if we find the affinity 

 of nitrous acid for potafh = iz, of the fame for lime 

 = 7 ; of acetous acid for potafli =: 14, of the fame for 

 lime =i 6, 



Nitrat 



itrat fNit. a. 6 I.ime T Acetitc 

 of -J 12 -{-7 = 19 >■ of 

 italh. (.Potafh 1+ Acct. a. J Lime. 



?.o 



rtiall then have 



Q^uiefccnt affinities = 12 -|- 7 = 19 

 Divellent affinities = 6 + 14 = 20 



and hence we may with certainty infer that a double de- 

 compofition will take place, with a force equal to the ex- 

 cefs of 20 over 19. 



Chemical philofophers have not, however, by any means 

 agreed on the method to be followed for the attainment of 

 this defirable object ; fome with M. Wenzel, confider the 

 time requifite for cfTei^ling folution as the expreffion of 

 the force of the afTiiiily bvjtween a fubftance and its mcn- 

 rtruum ; others with Fourcroy, believe the intenfity of this 

 to be more accurately mcafured by its reii (lance to decom- 

 pofition, than by its rapidity in uniting bodies : according 

 to Macquer the force of affinity is exprelfed by the facility 

 with which bodies unite, compounded with the force by 

 which they continue united. Kirwan, on the other hand, 

 has eltimatcd the affinity of acids for their bafes by the 

 different proportions of them that they require for fatura- 

 tion. And from this verj- diverfity of opinions may be in- 

 ferred the great difficulty of the fubjetl. 



According to M. Wenzel, the difpofition to chemical 

 union between bodies varies with the figure of their conlH- 

 tuent parts ; and, regarding the adlion of menilrua upon 

 them as a mere mechanical impulfe, fubjeft to calculation 

 upon the principles of Italics, he concludes, that the rapi- 

 dity of folution is an exponent of the force of affinity ; 

 and therefore, that the aJjinUy of different bodies tvith any 

 common menjlruum, is in an inver/e ratio of the time required 

 for their folution. To prove this, he procured equal cylin- 

 ders of fdver, copper, lead, and other metals in a ilate of 

 purity, having weighed them, he covered them with varnifli 

 fo as to leave only one end expofed to the aftion of the fol- 

 vent : they were then feparately fufpended in equal quanti- 

 ties of nitric acid, and left to its aftion for an hour ; being 

 then taken out and freed from their varnith, they were 

 weighed, and the quantity diflblved of each was foimd to 

 correfpond with the order of their refpeftive affinities for 

 the acid. This large conclufion of M. Wenztl's from fo 

 partial an experiment, is however, mofl ftrikingly contraditled 

 by fafts : the cyfinders of filver and lead will be fcarce per- 

 ceptibly afted on in muriatic and fulphuric acids, whereas 

 thefe will precipitate a nitrous folution of either metal with 

 the greatefl eafe ; a multitude of fimilar obvious contradic- 

 tions to the rule might be mentioned, if need were, befides 

 that M. Wenzel's method, if ever fo accurate, is capable 

 only of being applied in cafes where one of the fubftances 

 i« a folid. 



The refiftanc^ which the parts of a compound offer to 

 any force that is employed to feparate them, offers a pro- 

 bable method of eftimating the degree of their mutual af- 

 finity : fmce however the union ii chemical, it is plain that 

 mechanical means of feparation cannot be ufed for this pur- 

 poXcj and there are very few chemical agents that can be 



A F F 



mad* the fuhjccl of calculation. The only method pro- 

 pofed by Fourcroy, and one that he hlmfilf allows, is inap- 

 plicable to the greatell number of c:'fes, is to afcertain the 

 thcrmometrical temperature neceffiiry to cITed a feparation. 

 It is certain that the application of high degrees of^ heat in 

 many cafes oppofcs the aiftion of chemical affinity : the 

 force of affinity for caloric (heat) is alfo probably different 

 in every natural fubllance, we know in many cafes, as in 

 the burning of lime, the redudion of mercurial oxyds, &c. 

 that mere caloric is capable of fepaniting carbonic acid from 

 hme, oxygen from mercun-, &c. it is not therefore a priori 

 improbable, that the affinity of any acid for the difl'erent 

 alkaline, caithy, and metallic bafes fliould be truly expreffed 

 by the various quantities of caloric, as indicated by the de- 

 grees of the thermometer and pyrometer, required for the 

 decompofuion of the different falts. But although this rule 

 certainly holds good in fome cafes, yet the greater number 

 of compound falts are incapable of being decompofed by 

 the greatefl quantity of mere caloric that we are able to 

 apply, and almoft all the cafes of chemical affinity through 

 the medium of water, are alfo incommenfuratc with the fcale 

 propofed. 



In confequence of thefe difficulties both Fourcroy and 

 Morveau have agreed in propofmg an arbitrary number as 

 the bafis of their refpeftive numerical feries, all the other 

 terms of which are brought by approximation and compa- 

 rifon with the refults of known experiments, to bear certain 

 proportions to the (landard term, and to each other. 



It is plain, therefore, that a table, conflrufted on thefe 

 principles, can never be any thing more than an approach 

 towards the truth ; and even this can only be effeAcd by 

 repeated correttions according to the refults of a vaft mul- 

 titude of experiments ; and in all untried cafe,-, it is reduced 

 merely to an argument from analogy. In order to verify 

 the numbers in the following (hort table of Morveau, no 

 lefs than 490 experiments would be necefTary, and probably 

 more than half thefe would require feparate adjuilments of 

 all the numbers in the table ; it may be conceived therefore 

 how many centuries of incelfant labour would be required 

 to tabulate in this manner with any accuracy, even the 

 binary combinations of the prefent chemical iubilances. 



How far this Table, which has already undergone repeated 



corrections by its a- 

 ble author is to be 

 depended upon, a 

 few examples will 

 (liow. 



It acetite of ba- 

 r)'tes be added to 

 fulphat of foda, a 

 detompofition will 

 take place, and there 

 will be produced ful- 

 phat of barytes and 

 acetite of foda : now 

 according to the Ta- 

 ble, the ium of the 

 Qiiiefcent affini- 



ties = 28+78 = 86 

 DiveUcnt affinities 



= 66 -f 25 = 91. 

 this cafe, therefore, 

 is refolved truly by 

 the Table. 



Nitrat of Potafh 



and acelite of lime 



mutuaUy 



, Barytes j 66 ' 62 ( 36 i 28 



n 



H 



