78 



ATTRACTION. 



Attraction distances between bodies is sensible. Hence we have 

 Chem ical. no means of knowing the rate according to which it 

 v ~"""' 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 -ry, or even according to some higher 



?ower. This in particular was the opinion of Sir 

 saac 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 be 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 up 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 shew, 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 compounds. 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 acidi are 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 from each other, and the Attraction 

 compound is decomposed. By this contrivance, he Cbwnical. 

 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 the "skill and industry of 

 the philosophers at present engaged in the investiga- 

 tion. 



Mr Dalton has lately thrown out a very ingenious 

 idea respecting chemical affinity, which deaerres t 

 be mentioned. According to him, bodies unite ei- 

 ther atom to atom, or two or three atoms ei one to 

 one atom of another. Thus water is co f an 



atom of oxygen and an atom of hydrogen united to- 

 gether ; ammonia of an atom of hydrogen and an 

 atom of azote united together ; carbonic acid of two 

 atoms of oxygen united to one of carbon. Sulphate 

 of potash consists of a particle of sulphuric acid 

 united to a particle of potash ; supersulphate of pot- 

 ash of a particle of potash united to two particles of 

 sulphuric acid. If we admit that matter is com- 

 posed of atoms, as is at least probable, it is difficult 

 to refuse admission to this hypothesis, though it is 

 probable that more complicated cases may exist. For 

 example, two atoms of one body may combine with 

 three of another, and so on. It is even possible that 

 the proportion in which "bodies unite cannot always 

 be represented by numbers. But this hypothesis of 

 Dalton is much more probable, and corresponds much 

 better with the phenomena, than the opposite one of 

 Berthollet, that bodies combine in all proportions 

 whatever. Dalton's hypothesis is very useful, be- 

 cause it facilitates the knowledge of the composition 

 of bodies. For Example, if water be composed of 

 an atom of hydrogen and an atom of oxygen, it fol- 

 lows, from the known analysis of water, that the 

 weight of an atom of hydrogen is to that of an atom 

 of oxygen as 1 to 6. Hence 'we know, that when 

 oxygen enters into a combination, it will always en- 

 ter as 6, or as some multiple of 6. This is very ob- 

 servable in the metallic oxides. The quantity of 

 oxygen in the second oxide is usually double that in 

 the first oxide, and that in the third triple that in the 

 first. The same thing is equally remarkable in the 

 salts; the supersulphate of potash contains just double 

 the quantity of sulphuric acid that exists in the sul- 

 phate, and all the supersalts contain twice as much 

 acid as the neutral salts, with the same acid and base. 

 This atomic theory of Mr Dalton, plausible as it is, 

 will be overturned, if Mr Davy succeeds in proving 

 that azote is a compound of oxygen and hydrogen, 

 as his experiments already published give us some 

 reason to suspect, (c) 



ATTRACTION, Physical, comprehends the 

 attraction of gravity, electricity, and magnetism, and 

 that which is exerted upon light in its transmission 

 through diaphanous bodies, or in its passage near those 

 which are opaque. The theory of gravitation has 

 already been fully discussed under the head of Phy- 

 sical Astronomy, and the other kinds of attraction 

 shall be treated of at full length under the articles to 

 which they more particularly belong. It may be 

 proper, however, to state in general, that the attrac- 



