ESSENTIAL AND PECULIAK. 



51 



of chemistry, but of too vduminous a nature to touch farther upon at 

 present. 



IV. V. The two remaining kinds of attraction to which I have adverted, 

 those of ELECTRICITY and of magnetism, are still more select, and per- 

 haps still more powerful than even the preceding : but the phsBnomena to 

 which they give rise cannot, I think, be attributed to any modification of 

 a gravitating ethereal medium, we call the medium in both these cases a 

 fluid, but we know little or nothmg of the laws by which they are regu- 

 lated ; whether they be different substances, or, according to M. Ampore, 

 the same substance under different modifications, or whether, in reahty, 

 they be material substances at all. They are certainly deficient in tho 

 most obvious properties of common matter, and may be another Bubstrate 

 of being united to it. 



There are also two other substances, or which are generally conceived 

 to be substances, in nature, of a very attenuate texture, which largely 

 contribute to the changes of material bodies. I mean light and heat, 

 of the general nature of which we are still also in a considerable degree of 

 ignorance. Like the powers of magnetism and electricity, we only know 

 them, and can only reason concerning them, by their effects. These 

 effects, indeed, , are of a most curious and interesting character, but spread 

 too widely to be followed up in the course of the present lecture, though 

 we may endeavour to pursue them, and, as far as we are able, todevelope 

 them hereafter. 



All these four powers or essences, for we know not which to call them, 

 concur in exhibiting none of the common prt^perties of matter ; their re- 

 spective particles repel each other at least as powerfully as they attract, and 

 in the cases of light and heat repel alone, and without attracting. They 

 may, possibly, be ponderable ; but if so, we have no instruments fine 

 enough to detect their relative weights ; and we are hence incapable of 

 determining, as I took leave to observe on a former occasion, whether they 

 be matter at all, whether mere properties of matter, or whether modifica- 

 tions of some etherealized and incorporeal substrate, combining itself with 

 the material mass, and exciting many of its most extraordinary phaenomena. 

 It is at present, however, very much the habit to generalize them into one 

 common origin ; and to conceive the whole as modified results of matter, 

 or of the gravitating property of matter. Thus, the attractive powers of 

 chemical affinity and of electricity are identified in the following passage 

 of Sir Humphry Davy's valuable " Elements of Chemical Philosophy :"— - 

 " Electrical effects are exhibited by the same bodies when acting as masses^ 

 which produce chemical phaenomena when acting by their particles ; iti^ 

 not improbable, therefore, that the primary cause of both may be the 

 same."* And in like manner, in an adjoining passage, he suggests that 

 all the various properties or essences that have thus far passed in survey 

 before us may be nothing more than the general attractive power of matter, 

 though he admits that at present we are incompetent to determine upon 

 the subject. With regard to the great speculative questions, whether 

 the electrical phaenomena depend upon one fluid in excess in the bodies 

 positively electrified, and in deficiency in the bodies negatively electrified, 

 or upon two different fluids capable by their combination of producing 

 beat and light, or whether they may be particular exertions of the general 



* E!em. p. 164, 165, 



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