CHEMISTRY. 



presented to each other in succession, we 

 may conceive a variety of results, all 

 which appear to take place in nature. 

 Thus (a) they may not perceptibly unite, 

 or (6) two may unite, acl the third have 

 no action,either upon them singly or when 

 in their combination ; or (c) all three may 

 unite, from attractions exerted between 

 each singly upon the others, and form a 

 triple compound ; or (</) two of them may 

 have no attraction for each other, but be- 

 ing both capable of combining with a 

 third, this last may be the instrument of 

 union between the two, and a triple com- 

 pound will be thus formed. In this case, 

 the effect is said to be performed by in- 

 termediate attraction, and the attracting 

 body is called a medium. Thus oil and 

 water will not unite, but either of them 

 will unite with an alkali ; and if this last 

 be uni.ed \sith oil, it forms soap, which 

 can be united with water. The alkali is 

 the medinm ; or (e) two principles which 

 attract each other may neither of them be 

 capable of direct or ready union with a 

 third ; but when the two former shall be 

 actually combined together.the compound 

 shall attract and combine with the third 

 body, and form a triple compound. This 

 new power is culled resulting attraction. 

 Thus neither sulphur nor potash have 

 any sensible action upon gold; but when 

 they are fused together, they combine 

 with that metal. Most of the effects of 

 resulting attraction are consequences of 

 the change of state of bodies, particularly 

 to that of fluidity ; and the effects of this 

 attraction and that by a medium often ex- 

 ist in the same case ; or (/) if we sup- 

 pose three principles to be in such cir- 

 cumstances of aggregation or tempera- 

 ture, as to have no perceptible disposition 

 to unite in pairs, but that the resulting at- 

 traction of a compound of two of them, 

 if united, would then act upon the third, 

 and produce a triple combination, it may 

 happen that this resulting attraction, 

 which seems to be only in prospect, shall 

 have power to complete the triple com- 

 pound; and the modification is called 

 disposing attraction. Thus vinegar has 

 no perceptible action upon copper, but it 

 can dissolve the compound of copper and 

 oxygcMi, called the oxide of copper : nei- 

 ther vinegar nor copper have any dispo- 

 sition to take oxygen from its elastic state 

 in the atmosphere, so that copper and vi- 

 negar may be kept together without solu- 

 tion in a closed vessel : but if the air be 

 admitted, the presence of the vinegar will 

 dispose the copper to take oxygen and 

 form au oxide, and with tiiis combination 



the vinegar will unite. There is much 

 convenience in the term, disposing attrac- 

 tion, as used to express this phenomenon, 

 though it must be confessed that tlm- 

 prospective disposition, ascribed to un- 

 conscious beings, seems to produce some 

 confusion in the mind. It may therefore 

 be proper to notice-, that the case seems to 

 belong to disposing 1 attraction, and may 

 be thus hypothetically explained. Copper, 

 and several other metals, which attract 

 oxygen from the air, become covered with 

 a thin oxide or rust, which prevents any 

 farther access of that fluid,and consequent - 

 ly it rusts no farther, unless the thin coat 

 of oxide be scraped off and a new surface 

 exposed; and if this were continued to be 

 done, all the copper would be gradually 

 oxided. Now the vinegar, by the condition 

 of our case, does this, and the copper is 

 gradually and totally dissolved ; not, as it 

 appears, because the copper and oxygen 

 are disposed to unite by a third power, 

 which, as it were, waits for them, but 

 because this power removes an impedi- 

 ment, which would impede their progres- 

 sive union. 



(_) The case of attraction, which has 

 most particularly engaged the attention of 

 chemistry, is that, where two principles 

 being combined are separated from each 

 other by the addition of a third, which 

 combines with one of them. This has, 

 perhaps improperly, been called simple 

 elective attraction, and by others precipi- 

 tating attraction: its principal effects or 

 distinguishing character would, it seems, 

 be better designated by the term de- 

 composing attraction. Thus, if sulphuric 

 acid and magnesia be combined in the salt 

 called sulphate of magnesia (dissolved in 

 water) and potash be added, the acid will 

 unite with this last, and the magnesia will 

 be separated and fall down. It was for 

 a long time thought that these combi- 

 nations and separations were complete 

 and entire ; but they appear in every in- 

 stance to form cases of the intermedi- 

 ate or resulting attractions, wherein the 

 proportions of the soluble and insoluble 

 parts are extremely different, and the de- 

 grees of saturation often modify the re- 

 sults. For the body separated has al- 

 ways a small proportion of the two 

 others, and the new compound usually 

 suspended is not binary, but triple at 

 least; and the proportions and effects 

 are more or less altered by the quantity 

 of solvent present, and the aptitude of 

 the new Combinations to take the solid, 

 fluid, or elastic states. 



Tables of separation or decomposi 



