114 WRITINGS OF JOSEPH HENRY, [1855- 



reference to this point have been classed under the head of 

 electro-chemistry ; and in this case, as in every other sub- 

 division of our general subject, we have merely indicated a 

 group of phenomena, each of which has occupied the atten- 

 tion of a number of scientists, and in some cases during a 

 long term of years. 



Until recently it was supposed that the physical qualities 

 of bodies must depend on the nature of their elements, or 

 in other words upon their chemical composition ; but a great 

 many substances have been discovered composed of the same 

 elements in the same relative proportion and yet exhibiting 

 physical and chemical properties entirely distinct one from the 

 other. For example, according to Liebig, the oil of turpen- 

 tine, the essence of lemon, oil of balsam of copaiba, oil of rose- 

 mary, oil of juniper, and many others differing widely from 

 each other in their odor, in their medicinal effects, in their 

 boiling points, in their specific gravities, all contain the same 

 elements, carbon and hydrogen, and in precisely the same 

 proportion. The crystallized part of the oil of roses, a vol- 

 atile solid, of which the delicious fragrance is so highly 

 esteemed, is a compound bod}'' containing exactly the same 

 elements and in the same proportions as the gas employed 

 in lighting our streets. 



Such bodies are called isomeric (literally, of equal parts), and 

 the phenomena are classed under the head of isomerism. 

 These remarkable facts can only be accounted for by the 

 different groupings of the atoms. They exhibit as it were 

 the economy of Nature in producing the most multiform 

 effects from combinations of the simplest principles, and 

 almost revive in us the dreams of the alchemists relative to 

 the transmutation of matter. 



Combinations of this kind are generally of a very unstable 

 character and the atoms can sometimes be made to change 

 their positions by an impulse from without, or by the addi- 

 tion of heat, and to combine again, forming other substances 

 having entirely different properties. 



The changes we have mentioned are those of bodies which 

 are formed of groups of many chemical atoms; but a fact of 



