April 21, 1916] 



SCIENCE 



567 



Garrett Eyland, Ph.D. (Jolins Hopkins, 

 '98), has been made professor of chemistry at 

 Richmond College, Richmond, Virginia. 



Mr. a. V. Hill, Humphrey Owen Jones lec- 

 turer in physical chemistry at the University 

 of Cambridge, has been elected a fellow of 

 King's College. 



Mr. F. p. White, St. John's College, has 

 been elected to an Isaac Newton studentship 

 at the University of Cambridge. 



Professor Siegfried Garten, of Giessen, has 

 been called to the chair of physiology at Leip- 

 zig as successor to Professor E. Hering. 



DISCUSSION AND CORRESPONDENCE 



THE CURRENT "DEFINITION" OF ENERGY 



To THE Editor of Science: In a book re- 

 view by Professor Millikan^ the reviewer inci- 

 dentally mentions the existing confusion in 

 the use of the word " energy." In my judg- 

 ment, Professor Millikan's remark is fully 

 justified; for it is not only the writers of text- 

 books, but scientific writers of the first rank 

 who find themselves more or less entangled 

 with the current definition of energy and the 

 terminology to which the definition leads be- 

 cause the terminology is inconsistent with a 

 logical use of the facts. Recent and present 

 writers are not wholly to blame for this state 

 of affairs for they have inherited a " defini- 

 tion " and a terminology from the pioneers in 

 the science of thermodynamics that conflict 

 with facts whose full significance was dis- 

 covered only after the terms were introduced 

 and their use established. Under such cir- 

 cumstances confusion is inevitable until the 

 terminology is revised to fit the facts. 



Many of our text -books on physics " define " 

 energy as the " capacity of doing work " (Max- 

 well), as the "ability to do work," or, even as 

 the " power of doing work." This last is par- 

 ticularly reprehensible, because " power," as 

 used in physics, is the rate of doing work. Aa 

 a matter of fact, even if work were a form of 

 energy, none of these definitions would be an 



1 Science, October 2, 1914, p. 486. 



adequate " definition " of energy any more 

 than a quart measure would be a definition of 

 " space." Because heat is a form of energy it 

 does not follow that " energy is heat," or, be- 

 cause our standard of mass is a piece of 

 platinum that " matter is platinum." But the 

 above definitions of energy are worse even than 

 the above logical absurdities would indicate, 

 for work, as may easily be seen, is not even a 

 form of energy, like heat, but is in reality 

 merely a phenomenon that accompanies its 

 transfer or transformation. The reason why 

 our unit of work is also our unit of energy is 

 that all of our measurements of work are 

 energy-changes involving transfers which may 

 be measured by the work done on or hy a body 

 or system. The actual doing of work is al- 

 ways found to depend upon the existence of 

 energy differences ; and these differences are 

 just as essential to the doing of work and the 

 transfer of energy as the presence of energy 

 itself. This fact, which is ignored in the above 

 definitions, is expressed in a variety of ways 

 by the second law of thermodynamics. " The 

 capacity of doing work," if the words are to 

 mean anything definite should be taken as 

 referring to the " availability of energy " ; and 

 the availihility of a thing is not the thing 

 available. In explaining work and energy. 

 Professor Millikan states :2 



... it is obvious that they are not synonymous 

 terms, for a body may possess energy and yet never 

 apply it to the production of work. Work is done 

 only when energy is expended. 



If he had here used the word " transferred " 

 instead of " expended " his statement would 

 confirm what I have been endeavoring to 

 present. 



There is no more necessity for a " defini- 

 tion " of energy than there is for a definition 

 of "matter." Both are known only by their 

 characteristic phenomena; and these char- 

 acteristics must serve to identify them and to 

 differentiate them from each other. With the 

 " units " of each, however, the ease is quite 

 different. They may be defined in terms of 



2 "Mechanics, Molecular Physics and Heat," p. 



