306 DIRECT PROPERTIES OF MOLECULES 112 



chemists, viz. that the three atoms of hydrogen are so 

 arranged that their common centroid is always within the 

 atom of nitrogen, and that they circle about this atom 

 in plane orbits. 



There is, however, no agreement in general between 

 calculation and experiment for molecules which contain 

 five or more atoms, though in many cases the agreement 

 appears still to hold. This, indeed, is not recognisable from 

 the few cases investigated by Graham, but from the 

 numerous determinations of molecular sections which have 

 been deduced from other observations on viscosity, diffu- 

 sion, or conduction of heat. I here subjoin some examples 

 which I have taken from a very compendious collection 

 of such numbers, arranged by Landolt and Bornstein. 1 



For marsh gas (or methane), in the case of which the 

 number 22200 for Q deduced from Graham's obser- 

 vations does not agree with that calculated theoretically 

 (30100) , we find from Stefan's 2 experiments on diffusion the 

 value 30000, which agrees very exactly with that calculated 

 from the formula CH 4 . So for chloroform, the same tables 

 assign a value 73700, deduced from one of Puluj's 3 experi- 

 ments on viscosity, which agrees exactly with that (73400) 

 theoretically calculated from the formula CHC1 3 . And 

 Graham's number 40100 for methyl chloride CH 3 C1 

 agrees, at least approximately, with the calculated number 

 44600. 



From these examples we may look upon such molecules 

 also as are made up of one tetravalent atom of carbon and 

 four univalent atoms as possessing a plane or, at least, a 

 flat shape. This differs from the usual idea of the grouping 

 in which the four univalent atoms are put at the apices of 

 a regular tetrahedron, at whose centre is placed the carbon 

 atom. If we rely on the assumption that each of the four 

 univalent atoms is bound by the carbon atom in the same 

 way, we can scarcely make any other supposition so long 



1 Phys.-chem. Tabellen, 2. Aufl. 1894, tab. 126. But the corrigenda 

 published later must be consulted ; from these the numbers quoted are 

 taken. 



2 Wien. Sitzungsber. 1872, Ixv. Abth. 2, p. 323. 



3 Ibid. 1878, Ixxviii. Abth. 2. p. 279. 



