OF NATURAL PHILOSOPHY. 195 



animal, and he would not be without plausible 

 analogies in the warmth, the supply of fuel and 

 water, the breathing noises, the smoke, and above 

 all, the mechanical power exerted. He would say 

 (not without a show of reason), that where there is 

 a positive and wonderful effect, and many strong 

 analogies, such as materials consumed, and all the 

 usual signs of life maintained, we are not to deny 

 the existence of animal life because we know no 

 animal that consumes such food. Nay, he might 

 observe with truth, that the fuel actually consists 

 of the chemical ingredients which constitute the 

 chief food of all animals, &c. ; while, on the other 

 hand, his brother theorist, who caught a glimpse 

 of the fire, and detected the peculiar sounds of 

 ebullition, might acquire a better notion of the 

 case, and form a theory more in consonance with 

 fact. 



(207.) Now, nothing is more common in phy- 

 sics than to find two, or even many, theories main- 

 tained as to the origin of a natural phenomenon. 

 For instance, in the case of heat itself, one con- 

 siders it as a really existing material fluid, of such 

 exceeding subtlety as to penetrate all bodies, and 

 even to be capable of combining with them che- 

 mically; while another regards it as nothing but 

 a rapid vibratory or rotatory motion in the ulti- 

 mate particles of the bodies heated ; and produces 

 a singularly ingenious train of mechanical reason- 

 ing to show, that there is nothing contradictory to 

 sound dynamical principles in such a doctrine. 

 Thus, again, with light: one considers it as con- 

 sisting in actual particles darted forth from lumin- 

 o 2 



