THE SPACE RELATIONS OF ATOMS. 455 



way indicated by Bischoff. In one case at least, however, 

 there is a limitation of motion which seems due to a directive 

 force, and which tells in favour of the tetrahedron hypothesis. 

 The existence of the stereomeric inactive trioxyglutaric acids 

 corresponding to the types R" R" 



R'CR'" R'CR" 

 XCY and YCX 

 R'CR" R'CR" 

 R" R" proves that the 



groups attached to the carbon atoms do not rotate about one 

 common axis ; i.e., as the tetrahedron hypothesis demands, 

 the lines joining the carbon centres are not in a straight 

 line. 



Before leaving the stereochemistry of carbon it is worth 

 while to notice the calculations of O. E. Meyer in his Kine- 

 tische Theoi'ie der Gase, 1 concerning- the arrangement of the 

 atoms in various molecules. According to his results mole- 

 cules may be divided into four classes: (1) the volume of the 

 molecule is equal to the sum of the molecules of the atoms ; 

 (2) the area of the molecule is equal to the sum of the areas 

 of the atoms ; (3) the diameter of the molecule is equal to 

 the sum of the diameters of the atoms ; (4) for some mole- 

 cules none of these relations hold. In molecules of the first 

 class we must suppose that the atoms are arranged sym- 

 metrically in every direction, so that they occupy as nearly 

 as possible a sphere. Such a molecule according to Meyer 

 is CH 4 , which may be represented with the four hydrogens 

 symmetrically arranged on the surface of a sphere, and 

 rotating about the carbon atom at the centre — a confirma- 

 tion of the tetrahedral hypothesis. In molecules of the 

 second class the atoms must be and move in one plane, 

 and the figures obtained for NH, show this to be such a 

 molecule — another confirmation of current theory. An ex- 

 ample of the third class is H 2 0, which we must therefore 

 suppose to have the atoms arranged in a straight line. HCl 

 satisfies all three of the previous conditions almost equally ; 

 this indicates that the two atoms lie so close together that 



& 



1 Breslau, 1877, Maruschke and Berendt. 



