1 04 SUBSTITUTION. PART i. 



what it formerly had. Chemical equivalency between 

 two or more substances consists in their capacity for 

 being exchanged one for the other. Direct or indirect 

 substitution forms the basis of the modern doctrine of 

 chemical equivalents. 



Substances which are capable of replacing one another 

 in compounds, and which are endowed with qualities 

 mutually analogous, are said to be isomeric. Many 

 isomeric compounds are formed of the same materials, 

 in the same proportions, and yet differ essentially both 

 in their physical and chemical properties ; whence M. 

 Daniel observes, that a specific and definite arrange- 

 ment of the constituent molecules in space appears to 

 be no less essential to the individual constitution of 

 bodies than a certain proportion between their hetero- 

 geneous ingredients. 



Successive substitution in isomeric bodies does not 

 alter the character of the chemical formulee of these 

 bodies ; thus chlorine, bromine and iodine, are chemi- 

 cally equivalent with an atom of hydrogen, for they 

 may be put for one or more atoms of hydrogen in 

 various compounds without changing the character of 

 the chemical formulse of these compounds. The per- 

 oxide of hydrogen consists of one atom of hydrogen 

 and two of oxygen; hence if 32-5 parts of zinc, 28 of 

 manganese, and 29 of copper be successively put for the 

 atom of hydrogen, the result will be the peroxides of 

 zinc, manganese and copper respectively. Here the 

 character of the chemical formula of the original com- 

 pound remains the same, and the three metals are 

 chemically equivalent to one another, and to the atom 

 of hydrogen. In many compounds organic and inor- 

 ganic, one or more atoms of hydrogen may be replaced 

 by an equal number of atoms of sodium, potassium, 

 zinc, &c., without altering the character of the chemical 

 formula of the compound. 



Olefiant gas, olefiant oil and paraffin, form an iso- 



