368 Dr. A. Ransome on some of the 



a feeble cohesive attraction for one another, but tbey are the 

 more capable on this account of exerting their molecular affini- 

 ties for other bodies ; a crystalloid therefore diffuses into a gela- 

 tinous mass as readily as into water. During dialysis the par- 

 ticles of the colloid cling powerfully to the substance of the sep- 

 tum, and thus are unable to pass off into the opposite fluid ; but 

 the crystalloid readily traverses the colloid lining the pores of the 

 membrane, and is taken up by the molecular affinities of the so- 

 lution on the opposite side. 



Molecular Attraction of Colloids and heated metals for Gases and 

 Vapours. — Dr. Mitchell of New York (in 1830) first noticed the 

 varying rates at which different vapours and gases will diffuse 

 through films of india-rubber and other substances; and Dr. 

 Draper, in his work 'On Human Physiology ' (p. 152), makes 

 further observations on the subject, and notices its bearing on 

 the physiology of respiration. 



The same phenomena have recently been more fully investi- 

 gated by Dr. Graham ; and his results were given in an admirable 

 memoir to the Royal Society, and are now published in the Phi- 

 losophical Magazine. It will not be necessary therefore to review 

 this work ; it will be sufficient to state that his experiments show 

 that different substances possess very different powers of attract- 

 ing vapours and gases, that the attraction exerted seems to be 

 independent of mass, that the power of attraction which can be 

 exerted by the particles of red-hot or molten metals seems to be 

 sufficiently great in many instances to overcome the dissipating 

 influence of heat, and that certain gases will pass at various rates 

 into and through the substance of these metals when their mo- 

 lecules are so far separated as to allow the passage of liquid or 

 gaseous particles. All these phenomena lead to one general ob- 

 servation, which is most important so far as our present subject 

 is concerned — that in all the actions which have been mentioned 

 powerful molecular influences are at work, and that there is a 

 great diversity in the degree of molecular affinity put forth by 

 different substances. 



2. Molecular conditions favourable to Catalysis. 



It might be anticipated that molecular actions would take 

 place most readily when the substances involved in them are 

 either in a fluid or gaseous condition ; but in most so-called ca- 

 talytic actions a powerful influence upon the result is generally 

 exerted by solid substances. 



The researches of Messrs. Jamin and Bertrand* and Professor 

 Magnus f have proved that all solid bodies condense upon their 



* Phil. Mag. S. 4. vol. vi. p. 156. t Ibid. p. 337. 



