246 PHYSICAL SCIENCE 



absorbed or emitted in a given case. With quite 

 reasonable assumptions, this statistical method 

 accounts for the facts it was framed to meet. 



But most theories can do as much as that. 

 The real test comes when a theory is extended 

 to cover other facts which were not in mind 

 during its inception. Hence the evidence for 

 the quantum theory was much strengthened when 

 Einstein, in 1907, applied it successfully to explain 

 the fact that the specific heats of certain solid 

 elements like carbon, and other elements at low 

 temperatures, were not constant, as classical 

 physics required, but varied with temperature. 

 If energy be absorbed not in infinitely small 

 quantities but by finite units, we can explain this 

 result, for when temperature is low and heat 

 units scarce, some atoms will possess no units 

 at all, and thus the total content of energy, and 

 therefore the specific heat, is small. A mathe- 

 matical investigation shows that the expected 

 change of specific heat with rising temperature 

 is in accurate accordance with observation. 



To fit in with the numerical results, the 

 unit of energy e must be equal to /iv, where v is 

 the frequency of vibration and /i a constant called 

 Planck's constant, which has the value 6.5 x io~^^ 

 erg-seconds. It will be seen that the size of the 

 units of energy depends on the frequency of 

 vibration, and is larger when that frequency is 

 great, as in violet and ultra-violet light, than 

 when the frequency is small and the wave-length 

 large, as in red light or invisible radiant heat. 



The real constant is Planck's quantity /i = -, 



V 



which is not energy, but energy divided by 



