164 GENERAL VIEW AND BASIS OF SCIENTIFIC RESEARCH. 



more or less in properties with each change of pressure and 

 temperature, and may be regarded under each such change 

 as a different substance. 1 



X. The principle of transformation of energy r , or con- 

 vertibility of forces, is proved by the fact that wherever one 

 force (or form of energy) disappears, it is either stored up, or 

 another force appears in its stead. For instance, when 

 one kind of motion disappears, another often appears in 

 its place ; thus rectilinear motion of masses may be con- 

 verted into circular movement, or vice versa ; also the 

 molecular motion we term heat may be resolved into that 

 of electricity, and vice versa. 



The heat of the sun, absorbed by and disappearing in 

 the leaves of plants during growth, is converted into 

 stored-up chemical power, by decomposing the carbonic 

 acid of the atmosphere, converting the carbon and 

 hydrogen of those compounds into combustible bodies and 

 retaining them in the plant, whilst simultaneously setting 

 the oxygen of them free into the atmosphere, and enabling 

 it to support combustion. As a wound-up top retains in 

 a stored-up condition, ready to be released on any future 

 occasion, the mechanical power imparted to it, so the 

 carbon and hydrogen of plants retains, in a latent or 

 potential state, the chemical energy, ready to be again 

 liberated and converted into heat at any future time, 

 either when burned as wood or coal in the atmosphere, or 

 when eaten by animals, and oxidised by slow combustion 

 in their tissues. 



XI. The principle of equivalency of forces (or of cause 

 and effect) has been firmly established by the indefatigable 

 labours of many eminent investigators, Dr. Joule in par- 

 ticular, and has been proved by the fact that in the con- 

 version of a definite quantity of one force into another, a 



1 For example, see page 34. 



