RECENT SCIENCE. 813 



Few scientific hypotheses have proved so productive in the de- 

 velopment of science altogether as the so-called " kinetic theory 

 of gases." A gas, according to this hypothesis, is an aggregate 

 of molecules which move very rapidly in all directions and 

 endeavor to disperse in space the rapidity of their movements 

 being increased by every increase of the temperature of the gas. 

 In their endeavors to escape in all directions the molecules of the 

 gases continually bombard the walls of the vessels which contain 

 them. They break them if they are weak enough, or else they 

 exercise upon them a pressure which is nothing but the sum of 

 all energies of the molecules which strike a unit of surface in 

 a unit of time. In our steam-engines the molecules (or rather 

 particles) of steam bombard the walls of the cylinder ; they push 

 the piston by their aggregate energies, and, setting it in motion, 

 make it move the huge masses it has to move. This is, of course, 

 but a hypothesis ; but since it so perfectly explains the pressure, 

 the elasticity, the diffusion, and the internal friction of gases, and 

 permits us to predict the consequences of the invisible bombard- 

 ment ; and since its consequences, mathematically deduced by 

 Maxwell, Clausius, Boltzmann, and many others, fully agree with 

 the reality of facts it can be considered no more as a mere guess : 

 it is a theory. 



Now, the Dutch chemist Van 't Hoff proved in 1886 that the 

 same theory holds good for weak solutions as well. If some sugar, 

 or some sulphuric acid, or any other liquid or solid, be dissolved 

 in some liquid, the bonds which keep together the particles of 

 sugar or of the acid are torn asunder by the solvent. The particles 

 spread among those of the solvent, and they take up the same 

 movements which they would perform if the sugar or the acid 

 were brought into a gaseous state in a free space. They bombard 

 the walls of the vessel, and exercise upon them a certain pressure 

 which will be increased if the bombardment is rendered more vio- 

 lent by either raising the temperature of the solution, or increasing 

 the number of bombarding particles by a limited increase of its 

 strength. Though there is not the slightest reason for supposing 

 that the dissolved solid or liquid may be in a gaseous state within 

 the solvent, the very fact of scattering its particles over a broad 

 space is sufficient to free them from their mutual bonds ; they be- 

 have exactly as if the sugar or the acid were brought into a gase- 

 ous state by evaporation and filled the space occupied by the solu- 

 tion. They obey all the physico-chemical laws (the laws of Boyle, 

 Marriotte, Gay-Lussac, and Avogadro) which hold good for gases. 



The kinetic theory of gases was thus extended to liquids, and 



fully recognizes the importance of the physical theories, and sums them up with his usual 

 clearness. 



