AFFINITY. 



happen, when the fubftances are mixed, that the infoluble 

 fait precipitates, on account of its infolubility. It is the 

 infolubility of lime that caufes it to precipitate when potafh 

 is dropped into nitrate of lime. Accordingly, when potafh 

 is dropped into nitrate of foda, no precipitation whatever 

 takes place, becaufe both the potafh and the foda are verj' 

 loluble in water ; but if we concentrate the folution fuffi- 

 cicntly by evaporation, cryftals of nitrate of potafh will be 

 depofited, becaufe that fait is much lefs foluble in water 

 than nitrate of foda. So alfo, when nitric acid is poured 

 upon carbonate of lime, the carbonic acid is difengaged, 

 and flies off, becaufe its elafticity induces it to feparate from 

 the folid, and affume the gafeous ftate as foon as the nitric 

 acid weakens the attraftion, by means of which it was 

 attached to the lime. 



" Thus," fays Dr. Thomfon, to whom we have been par- 

 ticularly indebted in the compilation of the prefent article, 

 " we have two dodtrines refpefting affinity oppofite to 

 each other. According to Bergman, affinity is eleSive. 

 The body which has the ftronger affinity difplaces that 

 which has a weaker, and the ftrength of affinity may be 

 meafured by decompofition. According to BerthoUet, 

 affinity is not ele3i-ve. It never produces decompofitions, 

 but only combinations, and the decompofitions vvhich take 

 place are owing to the agency of other caufes. The 

 ftrength of affinity is not an abfolute quantity, but 

 jncreafes with the mafs of the attraftiag body. Berthol- 

 let's doftrines lead to the opinion, that bodies are capable 

 of uniting together indefinitely in any proportion what- 

 ever : Bergman's, that they unite only in determinate pro- 

 portions, and that thefe proportions are independent of the 

 relative qujmtities of the combining fubftances which are 

 prefent." See Atomic Theory 



earths, and metals, are accumulated round the negative Pole. 

 From this general law Berzelius deduced the tonfequence 

 tliat the decompofitions in fuch inftanc * 



,. „• n- , 'ces were owing to the 



attraa.ons exifting between the bodies and the refpeftive 

 eleftncities. This opinion was afterwards extended bv 

 IJavy, and the opinion in its extended form fubfequentlv 

 adopted by Berzelius himfelf. According to thefe cele- 

 brated chemifts, chemical affinity is identical with eledrical 

 attradion, and bodies which unite chemically poffefs differ- 

 ent kinds of eledrical attraaions. Every body, in their 

 opinion, poffeffes a permanent eleftive ftate, either refinous 

 or vitreous. Two bodies in the fame ftate of eledricity have 

 no affinity for each other. Thofe in oppofite ftates ha>-e an 

 affinity, and the ftrength of the affinity is proportional to 

 the degree of intenfity of the different eleAricities in the 

 two bodies ; and m order to make bodies feparate from each 

 other, we have only to bring them into the fame eledrical 

 Itate, by making them both vitreous or both refinous. 

 See Electricity and Galvanism. 



Such is a fummary account of the revolutions in opinion 

 vvhich have taken place refpeding tie nature, of chemical 

 affinity, and the principal difcoveries which have given on- 

 gin to thefe changes fince the time of Bergman. We 

 ffiall conclude this article with a few general remarks upon 

 the fubjeft. '^ 



la the firft place, the queftion whether the affinities 

 of fubftances for one another be definite quantities ca- 

 pable of being reprefented by numbers, cannot, in the 

 prefent ftate of chemical fcience, be fatisfaftorily deter- 

 mined. For though fome fubftances always appear ca- 

 pable of feparating others, as, for example, barj-tes, 

 potafh ; yet the reafon may be, that the falts of barytes 

 are lefs foluble than the falts of potafh. Again, iron. 



A moft important fact refpefting the combination of as is well known, feparates oxygen from water at all tem- 



bodies was afcertained by Richter. This was illuftrated in 

 an elaborate work, publifhed at different times between 

 1792 and 1802, which contains the refult of his refearches 

 on the decompofitions and combinations of chemical bodies. 

 He obferved, that when two neutral falts, which mutually 

 decompofe each other, are mixed together, the two newly 

 formed falts ftill retain the fame neutral ftate as the two 

 original ones, from which they were formed. He likewife 



peratures ; but, on the other hand, it has been equally 

 well afcertained, that the oxyd of iron is reduced when 

 heated in hydrogen gas: " hence," fays Dr. Thomfon, 

 " we have no data for determining whether iron or hydro- 

 gen have the greateft affinity for oxygen ; each feeming 

 capable of depriving the other of oxygen in the very fame 

 circumftances." 



In certain cafes, alfo, of double decompofitions, it is 



obferved, that the fame proportions of bafes that faturate a often equally difficult to diftinguifh on which fide the 



given weight of one acid, faturate all the other acids ; and ftrongeft affinities lie. Thus, as is well known, carbonate 



the fame proportion of acids that faturate one bafe, fatn- of barytes and fulphate of potafh, when digefted together, 



rate all the other bafes ; which law enabled him to explain decompofe each other, and are converted into fulphate of 



why two neutral falts form, as above-mentioned, two new barytes and carbonate of potafh ; but on the other hand, it 



falts, likewife neutral. Thefe experiments and obfervations has been equally fatisfaftorily fhewn by Mr. Philips, that 



of Richter likewife enabled Fifcher to attach a fet of carbonate of potafh is capable of decompofing the fulphate 



numbers to the acids and bafes, indicating the weight of of barytes 



each which will faturate the numbers attached to aU the 

 other acids and bafes. 



Mr. Dalton, without being aware of the law already dif- 

 covered by Richter, turned his attention to the fubjeft 

 about two years afterwards, and was ftruck with the fmall 

 number of proportions in which fimple fubftances are capa- 

 ble of combining, and the conftancy of thefe proportions. 

 This led him to form the doftrine of definite proportions, 

 or atoms, as it is ufually termed, and which, as well as 

 Gay Luffac's modification of it, our readers will find fully 

 explained in the article Definite Proportions. 



About the fame period, that is, in the year 1803, the 



Pfaff, however, has fhewn, that the tartrate of lime and 

 the oxalate of lead are completely decompofed by the addi- 

 tion of no more fulphuric acid than is neceffary to form 

 fulphate of lima and fulphate of lead ; and hence he infers, 

 that the affinity of fulphuric acid for lime and lead is 

 aiilually fupen'or to the affinities of tartaric and oxalic acids 

 for the fame bafes refpeftively. 



BerthoUet has attempted to account for the firft of the 

 above experiments by the effecl of mafs ; and the experi- 

 ments of Pfaff he endeavours to explain by the folubility of 

 tartrate of lime and oxalate of lead, and the infolubility of 

 the fulphate of lime and fulphate of lead in acids. And this 



grand law refpefting the agency of the galvanic battery in brings us, 

 the decompofition of bodies was difcovered by Berzelius In the fecond place, to make a few remarks upon 



and Hifinger. This law is, that oxygen and acids are accu- the fuppofed effefts of mafs, and the modes of exijenee of 



mulated round iht pofitive pole ; while hydrogen, alkalies, bodies in modifying chemical decompofitions, t-.t- —r—a. 

 Vol. XXXIX. M m 



With refpea 

 to 



