Thomson's System of Chemistry. 129 



case of combustion." Has Dr. T. never heard of invisible com- 

 bustion ? We refer him to the Philosophical Transactions for 

 1817, Part I. The Doctor's enumeration of the sources of 

 light is very defective. Why does he not mention among them 

 gaseous expansion, as shewn in the interesting experiment of 

 bursting in vacuo a spherule of thin glass, containing air ? 



The commencement of caloric is a fair specimen of Dr. 

 Thomson's style, when he writes from his own resources. 

 " Nothing is more familiar to us than heat; to attempt, there- 

 fore, to define it is unnecessary. When we say, that a person 

 feels heat, that a stone is hot, the expressions are understood 

 without difficulty; yet in each of these propositions, the 

 word heat has a distinct meaning. In the one it signifies the 

 sensation of heat ; in the other, the cause of that sensation. This 

 ambiguity, though of little consequence in common life, may 

 lead in philosophical discussions to confusion and perplexity. 

 It was to prevent this, that the word caloric has been chosen to 

 signify the cause of heat. When I put my hand on a hot stone, 

 I experience a certain sensation, which I call the sensation of 

 heat; the cause of this sensation is caloric*.'^ By the aid of 

 many Italics, the Doctor tries, but in vain, to give emphasis to 

 his favourite mode of writing, which from its extreme rare- 

 faction of ideas, might be called the vacuous. Conscious 

 that his long dissertation on heat, was, in most respects, 

 identical with that in the former edition, he has distorted its 

 arrangement to make it pass for a new article. In the fifth 

 edition, the subjects were discussed with some shew of method. 

 " I shall divide this chapter (on heat) into six sections : the first 

 will be occupied with the nature of caloric ; in the second, I 

 shall consider its propagation through bodies; in the third, its 

 distribution ; in the fourth, the effects which it produces on 

 bodies ; in the fifth, the quantity of it which exists in bodies ; 

 and in the sixth, the different sources from which it is ob- 

 tained -f." " I shall divide this chapter into eight sections. In 

 the first, I shall consider the phenomena of expansion, because 

 they have furnished us with an instrument to which we are 

 indebted for all our accurate notions respecting heat; I mean 

 the thermometer ; in the second section, I shall consider the 

 changes of state induced in bodies by heat, and endeavour to 

 deduce the laws upon which these changes depend ; in the third 

 section, I shall treat of the radiation of heat; in the fourth, of 

 lis conduction through bodies; in the fifth, of specific heat; in 

 the sixth, of the laws of cooling ; in the seventh, of the nature 

 of heat ; and, in the eighth, of the sources of heat|." 



A more involved distribution of a scientific subject was never 

 before presented to the public. Radiation, conduction, laws of 



* System, I. p. 26. f '<J Edition, i. p. i(,. J 6'tli Eililioii, I. p. 26. 



Vol. XI. K 



