568 



SCIENCE 



[N. S. Vol. XLUI. No. 1112 



any constant, suitable, measurable, character- 

 istic, phenomena. We do not have to " de- 

 fine " space because we have units of volume, 

 or extension because we make use of meters, 

 yards and feet. IsText to an ignorance of facts, 

 the principal source of confusion in the case of 

 energy arises from using one characteristic 

 attribute, and that not a universal one, as a 

 " definition " of energy. Through supposing 

 the indefinable to he defined, even the most 

 careful writers are led into inconsistencies and 

 mis-statements. The result, to the alert and 

 critical student, is " confusion worse con- 

 founded." It does not follow that because 

 our unit of work furnishes a very convenient 

 and definite unit of energy that it is possible 

 to " define " energy, or that work is a kind of 

 energy. There is only one fundamental and 

 universal characteristic of energy which we 

 can be sure holds true for all of its various 

 forms and that is its conservation. Energy is 

 conserved; and this, if merely regarded as a 

 postulate, necessitates our recognizing that 

 when one form of it disappears another form 

 takes its place. Equivalents of both can not 

 exist at the same time. Hence, if work is a 

 kind, or form, of energy it must possess and 

 exhibit this characteristic, that while it exists 

 in the form of work some other form must 

 cease to exist, and vice versa. It can not be 

 too strongly insisted upon that the property, or 

 attribute, of conservation necessarily excludes 

 all processes not included tmder transference 

 and transformation. Again, although energy 

 changes may be m.easured in terms of work 

 the principle of conservation applies only to 

 the energy; and it becomes possible to prove 

 this principle only through the existence of 

 some one universal form of energy into which 

 all other kinds raay be transformed. For it is 

 evident that if there is no universal form there 

 must be for each form, or kind, some special 

 means by which it may be identified as energy 

 and its equivalent value measured; otherwise 

 the " principle of conservation " is a mere 

 delusion, or purely imaginary. But so far as 

 is now known all forms of energy without ex- 

 ception are susceptible to transformation into 

 heat, either directly or indirectly through work, 



and their energy values determined in terms 

 of heat. Hence for the present, at least, heat 

 may be regarded as the imiversal form of 

 energy. 



In order to establish, definitely, the relation 

 between heat and energy let us consider for 

 a moment Joule's classical experiments for 

 the determination of the mechanical equiv- 

 alent of heat. The potential energy of the 

 elevated weights disappeared during their de- 

 scent and produced a quantity of heat which 

 was measured. Now, by the principle of con- 

 servation, potential energy could be imparted 

 to the weights only by the disappearance some- 

 where of an equivalent, either of heat, or of 

 some potential form of energy. In either case, 

 the elevated weights represented energy that 

 has been accounted for without coimting work 

 as energy; hence the work done in elevating 

 them can not have been energy. Nor is it in 

 the case of the descending weights ; for the po- 

 tential energy of the descending weights dis- 

 appears as potential energy and reappears as 

 heat. Worh is then, it can be seen, a hind of 

 process hy means of which energy is transferred 

 and transformed. 



Doubtless many will find it difficult to 

 understand how the unit of work can be a cor- 

 rect and convenient unit of energy and yet 

 not he energy. A parallel case is found in the 

 measurement of temperature. The indications 

 of the thermometric substance are due to heat 

 yet are not heat; they must be interpreted as 

 ratios, and merely show the relation of the 

 temperature measured to some temperature 

 assumed as a standard. Likewise a standard 

 energy state is assumed and the change in the 

 energy of the system may be measured by the 

 work done on or hy the system, an inverse 

 corresponding change taking place in some 

 other body or system. From the fact that the 

 ratio of the work unit to the heat unit 

 (energy) is known, the energy change is 

 readily obtained by applying the ratio. 



Since in teaching, concise, definite state- 

 ments are desirable whenever possible, the cur- 

 rent, defective and misleading " definitions " 

 might be replaced by short statements like the 

 following : 



