180 CORRELATION OF PHYSICAL FORCES. 



or resolve them definitely into motion ; still we could never 

 avoid the use of different conventional terms for the different 

 modes of action of this one pervading force. 



Reviewing the series of relations between the various 

 forces which we have been considering, it would appear that 

 in many cases where one of these is excited or exists, all the 

 others are also set in action : thus, when a substance, such as 

 sulphuret of antimony, is electrified, at the instant of electri- 

 sation it becomes magnetic in directions at right angles to the 

 lines of electric force ; at the same time it becomes heated to 

 an extent greater or less according to the intensity of the 

 electric force. If this intensity be exalted to a certain point 

 the sulphuret becomes luminous, or light is produced : it ex- 

 pands, consequently motion is produced ; and it is decomposed, 

 therefore chemical action is produced. Jf we take another 

 substance, say a metal, all these forces except the last are 

 developed ; and although we can scarcely apply the term 

 mechanical action to a substance hitherto undecomposed, and 

 which, under the circumstances we are considering, enters 

 into no new combination, yet it undergoes that species of 

 polarisation which, as far as we can judge, is the first step 

 towards chemical action, and which, if the substance were 

 decomposable, would resolve it into its elements. Perhaps, 

 indeed, some hitherto undiscovered chemical action is pro- 

 duced in substances which we regard as undecomposable : 

 there are experiments to show that metals which have been 

 electrised are permanently changed in their molecular consti- 

 tution. Oxygen, we have seen, is changed by the electric 

 spark into ozone, and phosphorus into allotropic phosphorus, 

 both which changes were for a long time unknown to those 

 familiar with electrical science. 



Thus, with some substances, when one mode of force is 

 produced all the others are simultaneously developed. With 

 other substances, probably with all matter, some of the other 

 forces are developed, whenever one is excited, and all may be 



