Manchester Memoirs, Vol. /ii. {igoy). 3 



Another property of the chemical atom, valency, has 

 recently been shown by the brilliant work of Messrs. 

 Barlow and Pope to be connected in a most definite 

 manner with the volume of the region dominated by 

 the atom in crystalline structures. Following up this 

 discovery, Le Bas has shown that the " spheres of 

 influence " of carbon and hj'drogen have the same volume 

 throughout the whole series of solid normal paraffins — a 

 remarkable confirmation of the theory. Professor Pope 

 has brought before us a model of benzene, consisting of 

 columns built up symmetrically of close-packed assem- 

 blages of spheres of influence, the latter having volumes 

 proportional to the valencies of the atoms — carbon and 

 hydrogen. He cut a slice off the column and partitioned 

 off a piece of this in a hollow mould, and lo ! a new 

 "space" formula for the benzene molecule in which each 

 hydrogen atom snuggled against three carbons, and each 

 carbon atom touched three hydrogens. When we had 

 got over the initial difficulty caused by the absence of 

 bonds and our consequent wonder why the atoms stuck 

 together, and when we saw that the isomerism of the 

 benzene derivatives could be explained by this model as 

 well as by the ordinary plane hexagon or by the wedge — 

 we began to dimly recognise that we were in the presence 

 of a great idea. In watching the manipulation of these 

 close-packed assemblages 1 could not help recalling to 

 mind Dalton's explanation of the reason why water 

 expands when cooling from 4''C. to the freezing point. 

 He pictured the ultimate particles of water with their 

 ' atmospheres ' as spherical, and conceived them, at the 

 maximum density, to be piled up layer on layer like a 

 square pile of shot — where each sphere touches four others 

 in the same plane and rests upon four others in the plane 

 below. Then, says Dalton, if the square pile is distorted 



