228 



INDUCTION. 



other words, between different pro- 

 perties of the same natural agent. 

 This portion of the Laws of Nature 

 will be treated of in the latter part of 

 the present Book, under the name of 

 the Specific Properties of Kinds. 



§ lO. Since the first publication of 

 the present treatise, the sciences of 

 physical nature have made a great 

 advance in generalisation through 

 the doctrine known as the Conserva- 

 tion or Persistence of Force. This 

 imposing edifice of theory, the build- 

 ing and laying out of which has for 

 some time been the principal occupa- 

 tion of the most systematic minds 

 among physical inquirers, consists of 

 two stages : one, of ascertained fact, 

 the other containing a large element 

 of hypothesis. 



To begin with the first. It is 

 proved by numerous facts, both natu- 

 ral and of artificial production, that 

 agencies which had been regarded as 

 distinct and independent sources of 

 force — heat, electricity, chemical ac- 

 tion, nervous and muscular action, 

 momentum of moving bodies — are 

 interchangeable, in definite and fixed 

 quantities, with one another. It had 

 long been known that these dissimilar 

 phenomena had the power, luider 

 certain conditions, of producing one 

 another : what is new in the theory 

 is a more accurate estimation of what 

 this production consists in. What 

 happens is, that the whole or part of 

 the one kind of phenomena disappears, 

 and is replaced by phenomena of one 

 of the other descriptions, and that 

 there is an equivalence in quantity 

 between the phenomena that have 

 disappeared and those which have 

 been produced, insomuch that if the 

 process be reversed, the very same 

 quantity which had disappeared will 

 reappear, without increase or diminu- 

 tion. Thus, the amount of heat 

 which will raise the temperature of a 

 pound of water one degree of the 

 thermometer, will, if expended, say 

 in the expansion of steam, lift a 

 "height of 772 pounds one foot, or a 



weight of one pound 772 feet : and 

 the same exact quantity of heat can, 

 by certain means, be recovered, 

 through the expenditure of exactly 

 that amount of mechanical motion. 



The establishment of this compre- 

 hensive law has led to a change in the 

 language in which the scientific world 

 had been accustomed to speak of what 

 are called the Forces of Nature. Be- 

 fore this correlation between pheno- 

 mena most unlike one another had 

 been ascertained, their unlikeness had 

 caused them to be referred to so many 

 distinct forces. Now that they are 

 known to be convertible into one 

 another without loss, they are spoken 

 of as all of them results of one and 

 the same force, manifesting itself in 

 different modes. This force (it is said) 

 can only produce a limited and defi- 

 nite quantity of effect, but always 

 does produce that definite quantity ; 

 and produces it, according to circum- 

 stances, in one or another of the 

 forms, or divides it among several, 

 but so as (according to a scale of 

 numerical equivalents established by 

 experiment) always to make up the 

 same sum : and no one of the mani- 

 festations can be produced save by 

 the disappearance of the equivalent 

 quantity of another, which in its turn, 

 in appropriate circumstances, will re- 

 appear undiminished. This mutual 

 interchangeability of the forces of 

 nature, according to fixed numerical 

 equivalents, is the part of the new 

 doctrine which rests on irrefragable 

 fact. 



To make the statement true, how- 

 ever, it is necessary to add, that an 

 indefinite and perhaps immense inter- 

 val of time may elapse between the 

 disappearance of the force in one form 

 and its reappearance in another. A 

 stone thrown up into the air with a 

 given force, and falling back im- 

 mediately, will, by the time it reaches 

 the earth, recover the exact amount 

 of mechanical momentum which was 

 expended in throwing it up, deduction 

 being made of a small portion of 

 motion which has beeii communicate<J 



