EXAMPLES OF ISOMERISM. 97 



in changing the chemical properties of bodies. 

 For example, if phosphoric acid be obtained dry 

 by evaporating its solution, and then fused, it 

 becomes what is called pyro-phosphoric acid. In 

 these two states, although composed of phosphorus 

 and oxygen in the same proportions, it produces 

 totally different salts, on uniting with equal pro- 

 portions of the same base. It has also been found, 

 that crystals of tartaric and raceinic acids, in their 

 ordinary class of salts, are both composed of oxy- 

 gen, hydrogen, and carbon, in the same propor- 

 tions, and are so modified by heat as to form dif- 

 ferent salts on uniting with the same proportions 

 of the same base : and so of many other acids. 



Among the most civilized nations of antiquity, 

 it was supposed that everything in nature is 

 composed of four primitive elements, which they 

 termed earth, water, air and fire, to which 

 Aristotle added a fifth. But the tendency of 

 modern science has been to increase the num- 

 ber, until it has extended to fifty-four, all of 

 which are regarded as simple, because chemists 

 have not yet been able to resolve them into fewer 

 elements. There is reason, however, to believe 

 that none of our chemical atoms and elements 

 are perfectly simple, if we except caloric, which 

 also exists in a great variety of states, as will be 

 shown hereafter ; and that it is only in the sub- 

 tilized form of light that we can hope to discover 

 the ultimate constitution of ponderable matter.* 



* For example, if it be true that all the reputed elements are 



H 



