A F F 



A F F 



Bergman arc expunged on tlic iuitliority of later inveftiga- 

 tions, and the iur.nl)cr ot coliiinns is incrcaltd to 62. 



It is not, however, to the eonihuttion ot" tables, ini- 

 ^xu'tant a.-i thev are, that the rel'earclies of ehemilU on tlie 

 inbicct of afHnitv liave hcen tonfnied. Since the dlfcovery 

 of the great law of attractioji, hy Newton, it lias been the 

 uniform endeavour ot t'le ahlelt philolophcrs to iltow tiiat 

 the canfe of chemical phenomena is only a branch or modi- 

 fication of tliis uiuverlid property ot matter, and the nan\es 

 of liullon, Macqiier, IJmbourg, and Morvcan, Hand C(m- 

 fpicuo\is tor then' endeavours in this department : it is to 

 Kirwan that we owe the able attempt to reduce the force of 

 contending alhnltics to numerical calculation ; and the 

 fagacious lierthoUet, m his ^'liviherchesfiir la his ilr I'nJJimIe" 

 has juft now opened a new held of enquiry on thib moll im- 

 portant fubjeCh 



^ II. Caiifi- (if Clyemical yljfmhy. 



There have been only two ways of accounting for che- 

 inical affinity ; the one is by having recourfe to a gratuitous 

 ;ind inexplicable princi])Ie ot lympathy, and whieii there- 

 fore is merely the I'ubllitution of one metaphor for another ; 

 and the other is an endeavour, by the help of experiment 

 and calculation, to lliew the identity of affinity and the 

 Newtonian attracHon. The liril ot tliefe, as it does not 

 profcls to be fupported by any extenial evidence, may be 

 palTcd by ; the other requires a particular examination. 



It was the opinion of Newton, and a very' natural one 

 in his lituation, that the force of attracTrion which he liad 

 demonftrated to be the efficient caufe of the planetaiy mo- 

 tions, of the alternation of the tides, of the defcent of heavy 

 bodies, and of the ofcillation of the pendulum, was an eden- 

 tial property of matter, and, as iuch, the caufe of chemi- 

 cal phenomena : perceiving acids to be feme of the moll 

 powerlul agents in the production ot tliefe effecfls, he hence 

 dehned them as bodies that attract ftrongly, ami are Ib'ongly 

 attracted ( " acu'um didnius quod muJtum atlnthit et altrahi- 

 tur" J. This however is to be conlidered merely as a con- 

 jeclure of that great man, fmce no attempt was made by 

 him to fubmit to calculation any cafes of affinity, or even 

 to obviate the weighty objeftions that might be brought 

 againft the theoiy. The effcntlal foundations of the New- 

 tonian attraclion are, that the force of gravitation is in a 

 direct ratio to the mais or quantity of ponderable matter ; 

 and that the increide of the force is in an inverfe ratio to 

 jlie fquare of the dillance, or, to make this plainer by an 

 example : If the lead of a phunb-line is fufpended two yards 

 from the fide of a moimtain, the attractive force exercifcd 

 upon it will be four times kfs than if the dillance between 

 the lead and the mountain \vas only one yard ; fo-.' 



2 X 2 : I X I : : 4 : I . 

 Although, however, the iuftnefs of this law be rigoroufly 

 demontlrated in all cafes where the dillance is capable of 

 being mealured, how does it apply to thofe inllances in 

 wiiich bodies are fuppofed to touch each other ? How can 

 the apparent uniformity of a.ttraction be made .to explain 

 the infinite variety of chemical affinity ? To this funda- 

 mental and obvious objection I5uffon has gi>'en the following 

 reply. Tiie dillances between the feveral heavenly bodie-^s 

 are fo coullderable, that they may be looked upon with re- 

 gard to their aiition on each other as fo many gravitating 

 points, the flight differences in their figure being of little 

 or no account. If the moon and the earth, iiiltead of beiii"- 

 fpherical, were each in the form of a (hoit cyhnder, whofe 

 tranfverfc axis fliould be equal to their prefent diameters, 



the law of tlieir reciprocal adlion would not be mattriallv 

 altered by fuch a change, becaufe the relative dillance of 

 eacli particle of tlie moon from the earth would, notwith- 

 fhinding, be nearly the fame as before ; but if tliefe globes 

 were drawn out into very hmg cylinders, and brought w llh- 

 in a Ihort dillance of each otlier, the law of their recijirocal 

 aclion would feem very different, on account of the prodi- 

 gious change in the fituation of their i)artieles relatively 

 to each otiier, and to the whole ; tiius in propi>rtion as 

 figure enters as an clement into tlie calculation of dillance, 

 the law would aijpear to varv, thoui'-h reinaininir fuiidainen- 



y the fame. 



\Vhatloevcr ilrels be laid upon this propofition (which ap- 

 pears to have been acquiefcid in by IVrgman and Macquer), 

 tiiat in attractions between bodies tliat are nearly in Contact 

 with each other, tiie force is modllieri by the figure of the 

 niolecul-.c, it mull be confefied that not a fingle cafe of 

 affinity has yet been refolved by tlie application of the law 

 of t)\c fquare of the dillance, modified l)y the figure ; and 

 feveral eminent mathematicians, at the fame time that they 

 admit cliemical affinity to be only an effert of attradlion, 

 maintain it to follow in tlitfe cafes a different law from 

 tliat which Newton demonllrated, which vet remains to be 

 inveiligated. 



Morveau, in his elaborate treatile of affinity in the D'u- 

 lioiKiiir Blitljodirjtic, Ims endeavoured to lupport the theory 

 ot Piutlon, by certain analogical argiiiv.cnts, the fcopc of 

 which is, that in the iiltractions of adhefion and cohcfion,. 

 in capillary attraction and cryflalli/.atloii, all of w hich are 

 generally admitted to depend upon the fame law as th;; 

 attraction of gravitation, there are cafes equally difficult to be 

 reconciled with the rule of the fquare of the dillance, as 

 thole in chemical affinity : he alfo brings to his aid an in- 

 genious, argument of Macquer, to this effect ;_ Since we 

 are ign-araiit of the deiifity of the elementary particles of 

 bodies, it is impoliible to: afcertaln the denfity of the ag- 

 gregates formed by their union ; it may therefore happen, 

 that a body, whofe primitive [)articles have little denlitv, 

 fliould, notwithllanding, be an aggregate of great denlity, 

 provided thele particles are of fuch a figure as to adiiere in- 

 timately to each other by all their furfaces : for the fame 

 rcafon, a compound may have but little denfity, though its 

 conllitucivt particles have individually a great deal, if their 

 form is fuch as to allow of but fc\y points of contact. 

 Thus, although copper in mafs has lefs denfity than filvcr, 

 it is poffible that its ultimate particles fhould be fuperior 

 in this refpecl to thofe of lilvcr; or, allowing it to be of 

 inferior denfity, it may flill be capable, on account of the 

 figure of its comjKfuent particles, to enter into fuch intimate 

 contacl; with thofe of a third body, as Ihall more than make 

 up for its inferior denfity : hence the fuperior affinity whick 

 copper has tor nitric acid, over that Avhich filver jjoilelies, 

 may be owing to a fuperior attraiftion, on account of the 

 greater denfity of its primitive particles, or their better ap- 

 titude for contad.. 



It Is obvious, however, that all thefe arguments are merely 

 hypothetical, and at bell, only enable us to conceive the pof- 

 fibility of the phenomena of chemical attraction being eq\ially 

 recoiicil.ible to the laws ot general attraction as thofe cafes 

 of adhefion, cajujlary attrac'tion, &.c. wiiich liave not yet, 

 by the ablell mathematicians, been reduced to calcnliitlon. 

 If a finglii cafe of affinity had been demonllrated by the 

 rule of the fquare of the dillance, modified by even the 

 fuppofed figure of the molccuhi', it might be admitted as a 

 (Irong prclumption, that affinity depended on the fame laws 

 as gravitation ; but as long as this rcinains a dcfideratum^ 



wc. 



