112 WRITINGS OF JOSEPH HENRY. [1855- 



and as we may consider these spheres as constituting the sire 

 of the atomsj in reference to the space which they occupy in 

 combination, their magnitudes may be calculated with a 

 view to ascertain whether any similarity can be found in 

 the properties and action of bodies having equal atomic vol- 

 umes. To explain how this may be done, let us suppose we 

 wish to know the number of atoms in a given volume of 

 matter of which the whole weight is known, and also the 

 weight of a single atom; we shall then evidently have the 

 required number of atoms by dividing the weight of the one 

 atom into the weight of the whole. Now if we know the 

 number of atoms in a body of given size, we can find the 

 size of each atom by dividing the bulk of the whole by the 

 number of atoms; but since we can only ascertain relative 

 atomic weights and volumes, we suppose the volume of the 

 mass to be unity, and the weight of the same to be the spe- 

 cific gravity, or weight relatively to that of water. If we then 

 divide the atomic Aveight into the specific gravity, we shall 

 have the relative number of atoms; and if we divide this 

 number into 1, or what is the same thing, invert the frac- 

 tion and divide the atomic weight by the specific gravity, 

 we shall have the relative atomic volume. We find in this 

 way that there are groups of simple bodies having nearly 

 the same atomic volume, and that, when crystallized in the 

 same form, one may be substituted for the other, giving rise 

 to compounds of similar forms, and in some cases of similar 

 properties, though of different chemical constitution; and 

 on the other hand, by the differences in the grouping of the 

 same atoms bodies may be formed having entirely different 

 properties. 



It frequently happens that in the union of different bodies 

 in the gaseous state a condensation takes place, and the 

 volume of the compound molecule is not equal to the sum 

 of the volumes of atoms of which it is composed; and in 

 other cases the reverse effect has place, and an expansion is 

 the result. 



The following table, from Faraday's lectures,* exhibits the 



* [The subject matter of a course of Six Lectures on the non-metallic 

 Elements. Lect. iv. — Nitrogen; p. 206. 16mo. London. 1853.] 



