282 SCIENCE AND THE HUMAN MIND 



is whether or no in its day it is a satisfactory " working 

 hypothesis." 



The history of the caloric theory of heat is a good 

 instance of the successive changes in value of a 

 scientific idea. This theory of a weightless fluid gave 

 a vivid conception of heat as a quantity to be 

 measured, and was indispensable to the earlier 

 workers in calorimetry. It failed to explain the un- 

 limited development of heat by friction, and was 

 accordingly laid on one side during the investigation 

 of the mechanical aspect of heat studied in the form 

 of energy. But now that further research has 

 analysed the concept of energy into the two factors 

 of intensity and quantity, there are signs that, as 

 applied to the latter, a modification of the old caloric 

 theory may once more find a place in the modern 

 structure. 



It is needless to repeat instances. Scientific theories 

 certainly " mount on stepping stones of their dead 

 selves to higher things." At our present stage, a 

 common basis of matter, energy as a quantity constant 

 in amount, evolution by means of natural selection, 

 are working hypotheses co-ordinating much of our 

 knowledge and underlying almost all our researches 

 into the unknown. They are suited to the age and to 

 the minds of present investigators, and we have not 

 yet exhausted their possibilities. But, in the light of 

 history, it would be rash to proclaim them as absolute, 

 eternal truth. We may hereafter discover new kinds 

 of corpuscles and again for a time have to give up the 

 idea of one basis of matter. Energy may prove to be 

 conserved only in the limited conditions hitherto 

 studied, and natural selection fail to explain all the 



