5^ 



ON THE PROPERTIES OF MATTER, 



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 pre-- 

 sent day, coincidently with the doctrine of the Peripatetic school, is a 

 mere property of matter, and not a substance sui generis, as was con- 

 tended for by the Epicureans, 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 l>ecome 

 heated, occupy a larger space, and have their particles more widely re- 

 pelled 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, depends 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 hypothe- 

 sis. It will as readily account for many, perhaps most, of the phenomena 

 which accompany bodies in their change from one temperature to another, 

 as the position of the substantive form of heat, and has some advantage in 

 point of simplicity ; 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 ap- 

 proach 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 ap- 

 pears 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 repul- 

 sive power with heat, whether regarded as a substance or a mere property. 



Heat, again, which undoubtedly makes the particles of iron repel each 

 other, so that given weights of them occupy a larger space — makes the 

 particles of a ball of clay on the contrary attract each other into a closer 

 approximation, so as very considerably to lessen its dimensions ; and it 

 was on account of this pecuhar property that Mr. Wedgewood selected this 

 last material for the purpose of forming his celebrated pyrometer, or instru- 

 ment for measuring intense heats, the increase of the heat being indicated 

 by the decrease of the mass of clay. 



So water, at about 42^ of Fahrenheit, which forms its medium of den- 

 sity, begins to expand upon exposure to heat, and continues to expand in 

 proportion as additional heat is applied ; but below 42° it begins to ex- 

 pand also, upon exposure to cold, and continues to expand in the very 

 game ratio upon the application of additional cold, till at 32° it freezes and, 



* Elem. p. 176. 



