FUNDAMENTAL CONCEPTS OF PHYSICAL SCIENCE 21 



We have, then, two distinct sets of physical concepts. The first of 

 these deals with that positive portion of physics, the mechanical basis 

 of which, being established upon direct observation, is fixed and defi- 

 nite, and in which the relations are as absolute and certain as those of 

 mathematics itself. Here speculation is excluded. Matter is simply 

 one of the three factors, which enters, by virtue of its mass, into our 

 formulae for energy, momentum, etc. Force is simply a quantity of 

 which we need to know only its magnitude, direction, point of appli- 

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

 ception of force the producer of motion is adequate. All 

 troublesome 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 belongs, 

 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 mag- 

 netic 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 in- 

 jected into thermal theory the conception of energy, and led to the 

 development 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 

 mechanism by which energy is stored or set free rests upon a similar 

 speculative basis. 



These are proper subjects for theoretical consideration, but the 

 dictum of Rowland * 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. 



1 Rowland, President's Address to the American Physical Society, 1900. 



