632 SCIENCE PROGRESS 



attraction which tend to prevent it passing across the surface. 

 In the interior of the liquid, on the other hand, these forces, 

 though still as strong as before, mutually destroy one another 

 and the molecule is free to exert an enormous bombardment 

 pressure at an imaginary plane. Exactly the same thing may 

 be said of the molecules of the dissolved substance, though on 

 account of their smaller concentration in the interior they 

 exert there a correspondingly smaller pressure. At the free 

 surface of the solution the molecules of a solute such as sugar 

 are so effectually drawn back that even their vapour pressure 

 is immeasurably small. It is very reasonable to assume that 

 a similar state of things holds at the surface of a membrane 

 enclosing the solution, and if this be so, it is evident that the 

 bombardment pressure of the solute upon the membrane must 

 be quite negligible, and cannot therefore account for the 

 observed osmotic pressure. 



{To be continued) 



ORGANIC CHEMISTRY. By P. Haas, D.Sc, Ph.D., St. Mary's 

 Hospital Medical School. 



The origin and composition of coal forms the subject of a paper 

 by D. T. Jones and R. V. Wheeler (/. Chem. Soc. 191 6, 109, 

 707). The authors are of opinion that coal has been formed 

 by the action of pressure and heat not exceeding 300 C. on 

 decaying vegetable matter. By extracting with various 

 solvents coal may be shown to be composed of " resinic " and 

 " cellulosic " constituents. Thus pyridine extracts from coal 

 a mixture of cellulosic and resinic compounds, which may be 

 further separated by means of chloroform in which the resinic 

 constituents are soluble. The cellulosic constituents on de- 

 structive distillation yield phenols which constitute the chief 

 difference between petroleum distilled from coal and natural 

 petroleum, and this fact is construed as evidence that natural 

 petroleum is not of vegetable origin. The cellulosic constituents 

 also contain compounds whose molecular structure resembles 

 that of the carbon molecule, but there is no evidence of the 

 presence of free carbon. The resinic compounds contain free 

 hydrocarbons, but only a small quantity of paraffins, and they 

 also contain alkyl naphthene and unsaturated hydroaromatic 

 residues in a highly polymerised condition. The oxygenated 

 compounds are most probably cyclic oxides. 



