ATTRACTION, CnEmicAL, usually called AF FINITY, an attraction which exists between the mi. nute particles of matter, which urges theih together, and which keeps them united. It acts only at insen. sible distances, and becomes imperceptible when the distances between bodies is sensible. Hence we have no means of knowing the rate according to which it varies. Some philosophers have endeavoured to prove that it is merely a case of gravitation, and of course that it is inversely as the square of the distance; but the most cautious and best informed philosophers have inclined to the opinion, that it follows a different law, varying as —, or even according to some higher power. This in particular was the opinion of Sir Isaac Newton.
The opinion at present entertained by chemists is, that the affinity between bodies varies in intensity according to the body ; for example, that the affinity between sulphuric acid and barytes, is not the same in point of force as the affinity between sulphuric acid and potash. This opinion is founded upon a well known fact, that if sulphuric acid he in combi nation with potash, if we mix barytes with the com pound, the acid leaves the potash and unites with the barytes. Hence it was inferred, that sulphuric acid has a stronger affinity for barytes than it has for pot ash. On similar experiments the proportional strength of the affinity of various bodies for each other was founded, and the results were drawn op into tables, which were considered as denoting the strength of the affinity of different bodies for each other. Ber thollet has lately shewn, that these decompositions are much more complicated than had been suspected, that they are never complete, and that they may be explained upon other principles. He has endea voured to show, that affinity in all cases produces combinations, and never decompositions; and that the decompositions which take place are owing to other circumstances, many of which he has enumerated. If this notion, which is at least plausible, be well found ed, it destroys the whole doctrine of elective attrac tions. Berthollet has pointed out another method of
determining the strength of affinity of various bodies for each other. According to him, that body, the least weight of which is capable of neutralizing an acid, has the greatest affinity for that acid. Thus, of all the bases capable of combining with sulphuric acid, the least weight of ammonia is capable of neu tralizing a given quantity of the acid, while the greatest weight of barytes • is required. According to this doctrine, ammonia has the strongest affinity, and barytes the weakest affinity of all the bases for sulphuric acid. This opinion seems at first sight plausible, but its plausibility depends upon the inde finite meaning attached to the word neutralize. The truth is, that at present we have no means whatever of determining either the intensity or the variation of the force called affinity, and know only that it exists, and that it is very strong.
Mr Davy has lately added a new and very curious fact respecting compo'unds. He has shewn, that when two particles are united, they are in different states of electricity, the one positive and the other negative, and that the difficulty of decomposing them depends upon the intensity of these states. Oxygen and acids arc always negative ; hydrogen, and alkalies, and earths, always positive. If, by means of electri city, we bring them into the same electrical state, as by making them both positive or both negative, then" they instantly separate front each other, and the compound is decomposed. By this contrivance, he decomposed the alkalies and earths, and several of the acids and metallic oxides. Hence it is not un likely that chemical affinity and electrical attractions may in reality be one and the same force. The sub ject is still involved in obscurity ; but we may ex pect much elucidation from theiikill and industry of the philosophers at present engaged in the investiga tion.