CONCEPTS OF PHYSICAL SCIENCE. 59 



cation and the time during which it is applied. The Newtonian con- 

 ception of force — the producer of motion — is adequate. All trouble- 

 some questions as to how force acts, of the mechanism by means of 

 which its effects are produced are held in abeyance. 



Speculative physics, to which the second set of concepts belong, 

 deals with those portions of the science for which the mechanical basis 

 has to be imagined. Heat, light, electricity and the science of the 

 nature and ultimate properties of matter belong to this domain. 



In the history of the theory of heat we find one of the earliest 

 manifestations of a tendency so common in speculative physics that it 

 may be considered characteristic; the assumption of a medium. The 

 medium in this case was the so-called imponderable caloric; and it was 

 one of a large class, of which the two electric fluids, the magnetic fluid, 

 etc., were important members. 



The theory of heat remained entirely speculative up to the time 

 of the establishment of the mechanical equivalent of heat by Joule. 

 The discovery that heat could be measured in terms of work, injected 

 into thermal theory the conception of energy and led to the develop- 

 ment of thermodynamics. 



Generalizations of the sort expressed by TyndalPs phrase, heat a 

 mode of motion, follow easily from the experimental evidence of the 

 part which energy plays in thermal phenomena, but the specification 

 of the precise mode of motion in question must always depend upon 

 our views concerning the nature of matter, and can emerge from the 

 speculative stage only, if ever, when our knowledge of the mechanics of 

 the constitution of matter becomes fixed. The problem of the mechan- 

 ism by which energy is stored or set free rests upon a similar specu- 

 lative basis. 



These are proper subjects for theoretical consideration, but the 

 dictum of Kowland* that we get out of mathematical formulae only 

 what we put into them should never be lost from sight. So long as 

 we put in only assumptions we shall take out hypotheses, and useful 

 as these may prove, they are to be regarded as belonging to the realm 

 of scientific speculation. They must be recognized as subject to modi- 

 fication indefinitely as we, in consequence of increasing knowledge, 

 are led to modify our assumptions. 



The conditions with which the physicist has to deal in his study 

 of optics are especially favorable to the development of the scientific 

 imagination, and it is in this field that some of the most remarkable 

 instances of successful speculative work are to be found. The emission 

 theory died hard and the early advocates of the undulatory theory of 

 light were forced to work up with a completeness, probably without 

 parallel in the history of science, the evidence, necessarily indirect, 

 that in optics we have to do with a wave-motion. The standpoint of 



Rowland, president's address to the American Physical Society, 1900. 



