394 Further Remarks upon Mr. Herapath's Theory. [Nov. 



for in those doctrines, no theory of the nature of heat is assumed ; 

 no supposition required but the mere facts, that equal degrees of 

 heat communicated are not equally effective in certain different 

 cases, in producing equal sensible heats. I allow that in the 

 common views of these doctrines, the facts are, in some in- 

 stances, perhaps injudiciously, but, at the same time, perhaps 

 unavoidably, classed together, and described by terms which 

 may sometimes mislead by their hypothetical appearance. 



I cannot conclude without remarking, that if I rightly under- 

 stand Newton's invaluable rules of philosophizing, on the nature 

 and object of physical researches, it appears to me that in order 

 to constitute a legitimate physical theory, two things are requi- 

 site : 1. That the causes assigned should be such as really exist; 

 and 2. That they be sufficient to explain all the phenomena. 

 With respect to the first of these qualifications, I contend that 

 Mr. H.'s theory is defective ; for his fundamental principle, the 

 intestine motion of particles, as far as I know, has not been, and, 

 I believe, cannot be proved to exist ; still less is it proved that 

 it uniformly accompanies expansion. With respect to the second 

 requisite, my opinion must be obvious ; and I would recapitu- 

 late it by saying, that, I think, he has not strictly deduced his 

 conclusions from his first principles (see Annals for Sept. p. 224, 

 at the top) ; and that his conclusions in general agree with expe- 

 riments ; but still that this is only within limits where the old 

 views of the subject agree with them too. 



I will take my leave of Mr. H. by first calling his attention to 

 an excellent remark of Dugald Stewart, that what chiefly misled 

 the ancients in their physical speculations was their inattention 

 to what were the proper objects of inquiry, and their confused 

 and wavering notions of that particular class of truths, which it 

 was their business to investigate. I am far from wishing to apply 

 this remark, in its full extent, to the present subject ; but, I think, 

 in general, it might be well to recommend it to the consideration 

 of many philosophers of the present day. It would tend to 

 confine phiiosophy to those subjects which it is its proper pro- 

 vince to examine, and prevent much fruitless labour in endea- 

 vours to penetrate where the utmost strength of our faculties is. 

 unable to arrive at certainty. I am, Sir, 



Your obedient servant, X» 



