206 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



character of the heat, (art. 14). This result is the more probable from the size of 

 the som'ce of heat necessarily used in the crucible experiments (art. 70), which 

 tends to render the passage from partial to total reflection more gradual, and thus 

 to flatten the curve. To the same cause may also probably be attributed the some- 

 what greater index of mean refraction obtained for heat from this source than 

 that of dark heat of higher temperatm-e. 



82. The following method might perhaps be used with success for obtaining 

 more exact data respecting the refrangibility, and especially the dispersion, of heat, 

 than that just described pretends to give. It must insure a beam of parallel rays 

 of heat of sufficient intensity and uniform in every part of its section. A small 

 point of heat placed behind a lens (or two or three lenses to diminish aberration) 

 is the most obvious plan. But the intensity would be inadequate. I would, there- 

 fore, propose a platinum- wire, heated by one of Mr Daniell's constant voltaic bat- 

 teries, placed behind a refracting semi-cylinder of rock-salt.* The central rays 

 should be alone employed, and the prism for total reflection should be high and 

 narrow as well as the aperture of the pile. It is possible that in this case the tran- 

 sition from partial to total reflection would be so rapid as to make the error aris- 

 ing from the varying intensity of partial reflection (art. 76) inconsiderable. By 

 changing the force of the battery, heat of all temperatures might be employed in 

 succession. The numerical analysis of the heat spectrum would then take place 

 as described in art. 79. 



Conclusion. 



83. My object in these, as in former researches, has not been to group expe- 

 riments of mere curiosity indiscriminately selected, but to present a basis for a 

 proper theory of heat. Without some such end in view I should have thought the 

 time and labour spent on these experiments in some degree misapphed. Mere 

 numerical results, though ultimately of the highest consequence to science, should 

 never form the exclusive object of the philosopher. I ti-ust to have shewn that 

 though many of the conclusions in this paper are based upon quantitative results, 

 these have not been the ultimate aim of the inquiry. 



84. The mutual bearing of the three sections of this paper, and of aU upon 

 what (from analogy to physical optics) we may call physical thermotics, is now 

 evident. (1.) In the First Section we have minutely discussed a point apparently 

 perhaps of minor importance, namely, the unequally polarizable natm*e of the rays 

 of heat. The importance of the doctrine lies in this : that the common theory of 

 undulation recognises no such variation, nor perhaps does it exist in the case of 

 light (I know, however, of no decisive experiments on this point), with the excep- 



* Such a one I have had executed. 



