A F F 



A F F 



fiilpiiat of potath by cither nitrous acid or mercurial oxyd, 

 adtiiig fcparately : if, Iiowevcr, their aftion be combined, 

 fo as that the mcta'hc o>:yd fliall exert its affinity on the 

 fulphuric acid, whilo the nitrous acid is doing the fame 

 with refpfdl to the potaft, tlien, as 9 -j- 8 is greater than 

 12, fo will the afliiiities of nitrat of potafh and fulphat of 

 niercuiy be fuperior to tliat of fulphat of potadi : hut ul» 

 trous acid and mercury, the two agents in this decompo- 

 fition, have alfo a llrong attraftion for each other, tending 

 to unite them into the compound fait nitrat of Mercury, 

 let this force be =: 4 ; it is evident then, tliat, upon the 

 addition of nitrated mercury to fulphat of potafli, th<.re 

 are four dillinft affinities aAing two by two againll each 

 other. The qmefcent affinities, or thole which rehft de- 

 compofition, are the attraftion between fulphuric acid and 

 potafh = 12, and that between nitrous acid and mercury 

 =: 4, the fum of which is = 16. The d'lvclknt affinities, 

 or thofe which tend to break the original combinations, 

 are thofe of nitrous acid and potafh = 9, and of fulphunc 

 acid and mercury = 8, which together are =17. Now 

 as 17 is to 16, fo is the fum of the divellent, to that of the 

 quiefeent affinities. On the addition, therefore, of nitrat 

 of mercury to fulphat of potalli, there will be a decompo- 

 fitiou of both falts, and the formation of two new ones, nitrat 

 of potafh and fulphat of mercuiy. This is an example of 

 compound ekSive ajiiiiiy ; wliich may therefore be delincd 



a^ the refohition of a compound by rrreans of the u-.'t'd 

 affinities of its elements for thofe of another comp.)Ui;d. 

 It is obvioufly inipoffiblc, by means of double eleclivc at- 

 traiflio!), to obtaii\ either of the elementary parts of a bcdy 

 in a frparate uncombiiicd (lite; but although in this rcfpeOl 

 it is inferior to fingle eleftive affinity, it is neverthtlefs in- 

 finitely fuperior in the vail variety of its application, and 

 in its rendering not m?rely poffible but even ealy, a number 

 of deconipoHlions, which are abfohit .'.y impractic 'hlc by 

 fingle eleflive attraftion. Almoll all the fubllances in na- 

 ture are compounds, and the changes that we fee conti- 

 nually operating around us, are brought about by very 

 complicuLed affinities; fo it is in nearly every proccfs of art 

 in- which chemillry is concerned, a thorough knowledge, 

 therefore, of the general principles of compound affinity is 

 abfolutcly ncccfliiry to be acquired at the very threfhold of 

 the fcicnce. 



From what has been already faid, it is obvious that every 

 chemical fatt arranges itfelf under one or other of the three 

 fpecies of chemical affinity ; and hence may be conceived 

 the obhgation that fcience is under to Geoffroy, for his in- 

 genious method of arranging cafes of fingle eleftive at- 

 traction, fo as to enable the enquirer to dileover in an in- 

 ftant any particular fadt that he is looking for, or to com- 

 pare, at a fingle glance, the refults of numerous and com- 

 plicated experiments. 



