28 On Some Processes of Scientific Reasoning. 



convenient to introduce the Metempirical conception of Heat 

 as an unknown something whose presence produced these 

 phenomena, just as Force denotes the unknown something 

 which produces the phenomena of motion and pressure. We 

 have, however, no such subjective knowledge of Heat as we 

 have of one species of Force, viz., Effort. I must here notice 

 an unfortunate ambiguity of language which employs the 

 same word to denote the subjective sensation, heat, and the 

 objective cause of that sensation. " Hot " has very different 

 meanings in the sentences, " I am hot," and " this stone is 

 hot." There is a similar ambiguity in the use of the words 

 light, sound, &c. 



Heat was by some conceived as a substance possessing all 

 the qualities of a fluid except the quality of weight. This 

 conception, was, however, inadequate to explain all the 

 phenomena, and it and other ways of conceiving heat have 

 now given place to the Empirical conception of Heat as a 

 vibratory motion of particles.* 



What has been said about Heat is, mutatis mutandis, 

 applicable to the sciences of Light and Sound. As an 

 example of Ideal Construction in Light, I may instance that 

 of a body homogeneous in refractive power, or of a body 

 heterogeneous in respect to that quality according to a 

 simple law. How inaccurately deductions from this concep- 

 tion represent some physical phenomena is exemplified by 

 the impossibility of determining with a close approximation 

 to accuracy the effect of refraction on a heavenly body near 

 the horizon. On the other hand, the results of mathematical 

 calculation represent with practically perfect accuracy its 

 effect on a body near the zenith. 



The old metempirical conception of Light resembled very 

 closely the metempirical conception of Heat ; and the modern 

 empirical conception of Light, as a vibratory motion of 

 particles, resembles the empirical conception of Heat ; indeed, 

 it is now generally believed that Light and Heat are identical 

 — that the same vibrations which, under certain conditions, 

 produce the phenomena of heat, under other conditions 

 produce the phenomena of light. There is not perfect 

 agreement amongst physicists as to what it is, the 



* In calling this conception Empirical, of course I do not mean that the 

 vibration of molecules is a phenomenon which could be perceived by the 

 senses, but that it differs from sensible pbenomena in degree, and not in 

 kind. It is empirical in the same sense as a million miles or the millionth 

 part of an inch is empirical. 



