RECENT ADVANCES IN SCIENCE 631 



barding the semipermeable membrane, has long been held, 

 though not without considerable scepticism on the part of 

 those who rightly maintained that a solution was after all a 

 liquid in the ordinary sense of the term, and that in a system 

 of this nature we have to deal with forces the magnitude of 

 which is extremely large. Undoubtedly the simple bombard- 

 ment view owes its popularity to the very simple and obvious 

 explanation which it offers of the fact that the gas law holds 

 good for the dissolved substance in dilute solution, a fact 

 first discovered by the eminent Dutch physical chemist van 't 

 Hoff. On the other hand attempts have not been wanting- to 

 show that van 't Hoff's law is the limiting expression of a 

 more complicated law which can be derived by regarding the 

 phenomenon of osmotic pressure as primarily connected with 

 the solvent and only in a secondary sense with the dissolved 

 substance. One of the most recent attempts in this direction, 

 and one which is at the same time eminently successful, is that 

 of Tinker, briefly outlined in the Phil. Mag. vol. xxxii. p. 295, 

 1916, and the British Assoc. Rep., 191 6. Tinker bases his 

 considerations on the Dieterici equation of fluids. According 

 to Dieterici a pure liquid possesses what may be called a liquid 

 pressure — when the liquid is acting as a solvent we may call 

 this pressure the solvent pressure — denoted by the symbol tr, 

 which is connected with the free or unoccupied space in the 

 liquid and with the temperature by the ordinary gas law. 

 Thus if we denote the volume of the liquid by V and the 

 occupied space by b, then the free space is V — b and the pressure 

 7r is related to this free space by the equation ir (V— b) = RT. 

 This solvent or liquid pressure is itself a bombardment 

 pressure due to the kinetic energy of the molecules of the 

 liquid, the pressure being exerted on any imaginary unit area 

 in the interior of the liquid. It is a quantity which cannot 

 be measured directly, for any direct measurement involves a 

 contact surface at which the observed bombardment pressure 

 is very much less than ir. As a matter of fact ir may be several 

 hundreds or thousands of atmospheres, whilst the bombard- 

 ment pressure actually observed is exceedingly small. The 

 vapour pressure of the liquid, in fact, gives us a measure of 

 this latter quantity, The reason why the observed bombard- 

 ment pressure is so small is, that when a molecule approaches 

 the surface it is drawn back by strong unbalanced forces of 



