A F F 



we muft he content to atqiiicfte in our tut:J ignorance of 

 the primary caufc of chemiciJ pliciiomena. 



§ III, 0/ (Ijfmnt I'inJs of AJfimly, and the Conp-uBion 

 of Tithht ami Saiemcs, 



Whether the attraclions of gravitation, of adbcfion, of 

 colidJoii, and of compofition,- be or be not coiilidered as 

 clU-iitially the fame, there is yet difference enough between 

 them to allow of a very accurate definition of each, and 

 this is the more neccffary to be done, as tlicre are certain 

 con-.plicattd cafes of chemical afrinity, in wliich the agency 

 of all thcfe forces may be dillincUy perceived. 



Gravitation tlien is an attraclion between two bodies at 

 an afcci tainable dilhnce fram eacli other, whofe force is 

 dlicdly as the mafs, and inverfely as the fquare of the 

 dillaiice. 



Adhedon is an attraftion that takes place at the plane 

 of eontad, whofe force is peculiar for each fubftance in 

 nature, and in a dired ratio to the furface of contad. 



Cohefion, or aggregation, is an attradion between mo- 

 lecula; of tlie lame nature, whole force is peculiar for each 

 fublV>iiee, and in an inverle ratio to the quantity of caloric, 

 inttrpofcd between the particles. 



Affinity, or the attradion of compofition, is that which, 

 uniting together different homogeneous fubllances, whe- 

 ther (imple or compound, produces an uniform whole, in- 

 capable of being refolved by meclianical force, and whofe 

 charaderillie properties are often different, and fometimes 

 contrary to thofe of its conilitucnt parts. Tims, if run- 

 ning mercury is added to melted fulphur, a compound is 

 produced, which has neither the colour, the fplendour, the 

 inflammability, the volatility, nor the fpecific giavity, of ei- 

 ther of itsconflituent parts. 



It is this affmity of compofition which is the great agent 

 in all the operations of nature and art, that are referable to 

 the fcience of chcnuilry ; not only as an inftiument of 

 fynthcfis, as might be fuppofed from tlie primary meaning 

 of the term, but alfo as the fole means of analyfis ; there 

 being no Avay of refolvlng a chemical compound, but by 

 expofmg its elements to the adion of ftronger af&nities 

 than thofe which retain them in union. 



All the known inftances of affinity may be arranged un- 

 der three claffes, according to the number of elementaiy 

 fubllances, ading on each other at the fame time, and the 

 number of new compounds thus produced. — \Vhere only 

 two are concerned, it may be called a cafe of concurrent 

 affinity, or affinity of compofition, in which, if the force of 

 their mutual affinity is ever fo httle fuperior to the fum of 

 their refpcdive degrees of cohefion, combination will take 

 place. Thus, if a piece of quicklime is put into muriatic 

 acid, the fiun of their cohefions being lefs than the force 

 of their mutual affinity, the two fubllances will unite toge- 

 ther, and there will rtfult a homogeneous compound mn- 

 riat of lime, poffeffing the properties neither of the earth 

 nor of the acid. Thofe inftances alfo, in which more than 

 two bodies unite together into one compound, come equally 

 under this rule ; as, when fulphuric acid, aluniine and pot- 

 a(h a e imxed together ; the refult is common alum, a fait 

 poffeffing peciihar properties, which could never have been 

 inferred from thofe of its elements. All the cafes belong- 

 ing to this firft clafs are thofe of concurrent affinities, 

 where two or more fubftances by virtue of their attradion 

 -.'>r each other, unite i.ito one homogeneous body. Hence 

 u appears, that thoui'H every fubftan' ■_• has different degrees 

 oT affi lity for other fubftai ces, yet the ftrongtfl; does not 

 neccffarily ad to the exclulion of the reft. 



A F F 



It is not, however, always, nor indeed generally, tlie cafe, 

 where more than two fubllances are concerned, that their 

 refpecllve affimties concur to produce one new fubllanee ; 

 for it ufually happens that their difference of force pro- 

 duces one bi.iai-y compound, to the exclulion of the third 

 element, on account of the weakncfs of its attradion for 

 the nev/ compound. Thus, if muriatic and fulphuric acid 

 are mixed togetiier, and an aqueous folution of pure ba- 

 rytes is then added, an inllant combination will take place 

 between the fulphuric acid and the baiytes, to the exclu- 

 iion of the muriatic acid ; and tlr.s lail, having alfo no affi- 

 nity with the fulphat of barytes thus formed, or at leall 

 not fufficiently lliong to overcome the fum of their feveral 

 forces of cohefion, remains permanently excluded. In 

 tliis inftance, therefore, we fee how two bodies, whofe mu- 

 tual affinity is very confiderable, may unite together to the 

 total exclulion of a third fubftance. To make this plainer, 

 let A B, be two fubllances, whofe feveral affinity for C, 

 is equal refpedively to 4 and 7 : it is obvious then, that 

 B, will unite to C, with a force =7 — 4=:;3 ; the firft effcd, 

 therefore, of mixing thefe fubllances will be the produc- 

 tion of B C, to the exclufion of A : and if the affinity of 

 A, for B C, ffiould be inferior to the cohefive attradion 

 of the feveral particles of B C, for each other, it is clear 

 that A, mull be permanently excluded, notv.4thllanding its 

 original affinity for C. This exclufion of the weaker by 

 the ftronger affinity takes place, not only when the two 

 forces commence tlieir adion at the fame time, but even 

 when the weaker affinity has been previoufiy allowed to 

 exert its whole adion on the bafe ; thus, if muriatic acid 

 and baiytes are brought into contad, they com.bine and 

 form muriat of bai7tes, which compound is held together 

 by the force of affinity between the two ; but when to 

 this compound we prefent fulphuric acid, whofe affinity 

 for the earthy bafe is ftronger than that of muriatic acid, 

 an immediate change takes place, the whole of the mu- 

 riatic acid is diflodged, and the fulphuric acid combines 

 with the baiytes with a force equal to their mutual affinity, 

 minus that of the muriatic acid : or, to recur to our for- 

 mer illuftiation ; if A C, arc held together by a force = 4, 

 upon the addition of B, whofe force is = 7, the attrac- 

 tion of A, to C, will be counterbalanced by ^ of B's 

 affinity for A, and the remainder of B's force will pro- 

 duce the combination B C, = f of the original attradion 

 between B and C. This, and fimilar cafes are naturally 

 illuftratcd by luppofing C, to have a difpnfitlon to unite 

 with A and B, A, being at firft the only one prefent, the 

 combination A C, is produced ; afterwards, when B, offers 

 itfelf, C, having a preferable aliachmcnt to B, quits A, 

 and fonns the com.bination or paytnerfl.np B, C. It is this 

 metaphorical explanation which induced Bergman to call 

 all thofe inftances where a compound already formed is 

 feparated by the adion of fuperior af&nities, cafes of 

 elecli-ve attradion ; and becaufe in the above example only 

 three fubftances are concerned, one new compound being 

 foi-med, and the element of weakcft affinity being excluded, 

 it is properly diftinguifhed as a cafe oi finale elciiive affinity, 

 which forms the fccond clafs, ranking immediately after 

 that of concurrent affinity. 



From the confideration of fingle eledive affinity, the 

 progrefs is eafy to that of double, or, more properly fpeak- 

 ing, compound eledive affinity. Suppofe the affinity be- 

 tween fulpharic acid and potafli, the conftituent parts of 

 fulphat of potalh, to be =12, and the affinity of nitrous 

 acid for potaffi = 9, and that of oxyd of mercury for ful- 

 phuric acid = 8, it is evidently impoffible to decompofc 



fulphat 



