ESSENTIAL AND PECULIAR. 61 



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 modifications of some etherealised and in- 

 corporeal substrate, combining- itself with the material mass, and exciting 

 many of its most extraordinary phenomena. It is at present, however, very 

 much the habit to generalise 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 " Ele- 

 ments of Chemical Philosophy :" " Electrical effects are exhibited by the 

 same bodies when acting as masses, which produce chemical phenomena 

 when acting by their particles ; it is not improbable, therefore, that the pri- 

 mary cause of both may be the same."* And in like manner, in an adjoin- 

 ing 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 incompe- 

 tent to determine upon the subject. "With regard to the great speculative 

 questions, whether the electrical phenomena 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 pro- 

 ducing heat and light, or whether they may be particular exertions of the ge- 

 neral attractive power of matter, it is, perhaps, impossible to decide, in the pre- 

 sent imperfect state of our knowledge."! 



And hence, heat, in the view of Sir Humphry Davy, Count Rumford, and 

 various other justly celebrated chemists and philosophers of the present day, 

 coincidently with the doctrine of the Peripatetic school, is a mere property of 

 matter, and not a substance sui generis, as was contended for by the Epicu- 

 reans, in opposition to the disciples of Aristotle, and is contended for by the 

 disciples of Boerhaave, Black, Crawford, and most of the chemists of our own 

 times. The cause of heat, among those who deny it a substantive existence, 

 consists in a vibrating motion of the constituent particles of the heated body, 

 too rapid to be traced by the eye. And as it is known to every one that bodies 

 in general, as they become heated, occupy a larger space, and have their parti- 

 cles more widely repelled and separated from each other than in a colder 

 temperature, it has of late become a favourite doctrine that the repulsive 

 power, which in our last lecture we noticed to exist throughout matter, de- 

 pends altogether upon the property of heat; in consequence of which Sir 

 Humphry Davy uses heat and calorific repulsion as synonymous terms, and 

 hence regards heat and gravitation, or general attraction, as antagonist 

 powers. 



There is much plausible reasoning to be urged in favour of this hypothesis. 

 It will as readily account for many, perhaps most, of the phenomena which ac- 

 company bodies in their change from one temperature to another, as the posi* 

 tion of the substantive form of heat, and has some advantage in point of sim- 

 plicity ; but it is opposed by a variety of facts of so stubborn and intractable 

 a nature, that no efforts of ingenuity have hitherto been capable of bending 

 them into the service of the new doctrine. I observed, for instance, in our 

 last lecture, that when two plates of glass are within a ten thousandth part 

 of an inch of each other, they cannot be made to approach nearer without a 

 strong additional pressure. I observed, farther, that Professor Robison has 

 calculated the extent of this pressure from actual experiment, and finds it 

 amount to not less than a thousand pounds weight for every square inch of 

 the glass. Now this resistance or repulsive power between the two plates 

 of glass takes place equally under an air-pump and in the fullest exposure to 

 the air of the atmosphere, but it appears to cease under water. By what 

 cause the repulsion is excited in the two former instances, or disappears 

 in the latter, we know not; but it does not seem possible for any ingenuity 

 of argument to connect this repulsive power with heat, whether regarded as a 

 substance or a mere property. 



* Elm. p. 164,105 t W p. 176 



