134 



SCIENCE 



[N. S. Vol. XLIV. No. 1126 



and the denial of such a possibility is a funda- 

 mental postulate of many writers on thermo- 

 dynamics. The following statement of the 

 postulate 2 may serve to bring out the signif- 

 icance of the differences referred to above: 



No engine of any kind can by any means be 

 made to maintain continuously or restore and main- 

 tain when changed, the state of the system which 

 initially set it in motion; and the difference in the 

 energy state which initially established the motion 

 will disappear the more quickly the greater the 

 activity of the engine. 



In reply to Professor Dadourian's objection 

 in Science, 3 I would call his attention to the 

 preceding remarks. In addition I would say 

 that he misinterprets my point of view if he 

 supposes that I am opposed to defining energy. 

 If I knew how I would define it myself. Else- 

 where 4 I have stated what I conceive consti- 

 tutes the laws of energy; and those three laws 

 are as near as I can come to a " definition of 

 energy." If he can produce a definition that 

 will convey the necessary information and not 

 conflict with known facts and laws the scien- 

 tific world will doubtless welcome it with open 

 arms. The field is open. But a definition that 

 claims to he general and leaves out, or even is 

 in opposition to, the most important character- 

 istic of the thing supposed to be defined is 

 worse than no definition at all. The absence 

 of a " definition " does not preclude the clari- 

 fying of our thought by diligent study of the 

 thing we wish to define. As an aid to study, a 

 provisional, or partial definition may often be 

 of great assistance as a working hypothesis 

 provided it is recognized as provisional and not 

 allowed to close our minds to evidence and 

 dominate our perceptive powers. 



M. M. Garver 

 The Pennsylvania State College 



"available energy" vs. "energy" 

 To the Editor of Science: The argument 

 between the scientist and the engineer over the 

 definition of energy is clearly saturated 



- Journal of Physical Chemistry, Vol. 15, p. 613 

 (1911). 

 s June 16, 1916. 

 * Loc. cit. 



enough to crystallize out the clean-cut defini- 

 tion of " available energy " and leave the in- 

 definite but exceeding rich mother-conception 

 of "energy" for those who shall see more 

 clearly or be able to unite our bewilderment 

 of facts and deductions to a concrete state- 

 ment. 



The communication of Professor Garver in 

 the April 21 issue is both a timely and an excel- 

 lent critique. Evidently he analyzed the diffi- 

 culty far better than he constructed a work- 

 ing presentation or Dr. Win. Kent would not 

 have been able to so well establish himself in 

 the reply of June 9. 



That the author of a leading engineers' hand- 

 book should express himself as Kent has done 

 may be considered as evidence to demonstrate 

 the narrow conceptions and limited field into 

 which practical men continually fall. From 

 the energy-to-sell point of view there certainly 

 is satisfaction in the Kent definition; but we 

 can not allow Dr. Kent to confine the use of 

 the term " energy " to engineering ; the engi- 

 neer clasps hands with the scientist in every 

 undertaking and acknowledges his past and 

 present effort as components of his own prac- 

 ticability. 



The men who have most carefully studied 

 thermodynamics and energy transformations 

 assert that one particular sort of energy mani- 

 festation can be designated as free energy, 

 available energy or by some factor indicating 

 potential or intensity variation. The " stored 

 work " is to be referred to this sort of energy, 

 but the converse is not true — that all the 

 energy in a given system which may thus be 

 described can be converted into work. With 

 Garver we have to say that a certain amount 

 of work may be done during the transfer or 

 adjustment of this sort of energy. Some 

 energy is always lost, as heat when the work 

 is done. We find, then, that Kent is careless 

 in using " energy " where he should say 

 " available energy " and he is inaccurate in 

 assuming that all such energy is transformable 

 into mechanical work. 



Recent writers often state the matter with 

 much conciseness : 



