MOLECULAR PHYSICS. 81 



a vacuum the rate of molecular motion is deduced. From the 

 rate of diffusion of two gases the frequency of impact is derived. 

 Thereon is based the attempt to determine within limits the size 

 of atoms. 



The hypothetical medium ether is supposed generally to be 

 continuous through otherwise empty space, and through matter, 

 or, at all events, through intermolecular space. Imponderable in 

 the ordinary sense, it must, if it be continuous, be elastic and com- 

 petent to convey the enormous mechanical force involved in 

 stellar radiation. If elasticity of continuous matter be inadmissible, 

 then ether itself must be atomic and convey force by its own 

 interatomic impacts. The thrust which radiation through the 

 most perfect artificial vacua is said to give to matter may, if it be 

 due directly to radiation, be attributed to such blows. 



(3.) Holding a place somewhat between the energetic, abrupt, 

 and character-changing chemical phenomena and those of heat, 

 light and electricity are the innumerable and ill-classified pheno- 

 mena of what is more restrictedly called molecular physics ; such 

 are condition of structure or texture, cohesion and adhesion, mix 

 ture, and the more palpable phases of intermolecular motion. 



Soon following on the acceptance of the atomic hypothesis was 

 the endeavour to deduce the outward form of crystals from the 

 .supposed arrangements of their stationary atoms and molecules. 

 The doctrine of isomorphism taught what elements and com- 

 pounds could replace one another in crystalline bodies without 

 affecting their geometric character. The conception of the 

 internal motion of the atoms appears for a time to have consider- 

 ably turned aside the current of thought from this direction, per- 

 haps to be resumed should a clue be got to the shape of the poly- 

 gonal atomic paths. The study of solids as distinguished from 

 liquids, as well as that of liquids as apart from gases, has been 

 checked by the discovery of the continuity which exists between 

 liquids and their vapours under great pressure and the alteration 

 of the temperature at which solidification ensues when a liquid is 



G 



