Conditions of Chemical Change* 16? 



The periods may be defined as follows : — (i.) of Commence- 

 ment; (ii.) of Inertness* or Reluctance followed by Accele- 

 ration ; (iii.) of Constancy ; (iv.) of Diminution of Velocity f. 

 The first three periods will be considered seriatim more fully. 



(i.) Period of Commencement. 



Writers on the kinetic theory of gases have supposed that 

 the velocity of movement of translation of gaseous molecules 

 is, from the nature and degree of violence of their mutual 

 impacts, an average of velocities of widely different values ; 

 tbeir kinetic energy is therefore expressible by the factor 

 ^ mu 2 j wherein u is the average of velocities Y 1} V 2 , . . . V n « 

 It is supposed that certain molecules will be moving with a 

 velocity which would thereby render them more liable to 

 atomic disruption when they meet molecules of a different 

 kind of matter, so that the free constituent atoms which 

 would otherwise pair with atoms of a like kind pair with 

 atoms of an unlike kind. 



Clausius further assumes that an E.M.F. (and probably 

 changes induced by light are to be included under the same 

 category) tends to give a "set" or direction to the atomic 

 constituents, causing those of an electro-positive nature to 

 pass in one, and those of an electro-negative nature to pass 

 in another way. The same course of reasoning will apply to 

 solutions, except that the problem is rendered more com- 

 plicated by the internal viscosity of the liquid and the thereby 

 diminished probability of molecular encounter. The theory 

 of the free and paired atoms has been developed on mathe- 

 matical lines by J. J. Thomson t ; and the results calculated 

 from the equations set forth are in agreement with those 

 obtained in experiments on the dissociation of iodine (Crafts 

 and Meier), of the compound of methyl oxide and hydro- 

 chloric acid (Friedel), and on the dissociation of phosphoric 

 chloride (Wurtz). 



In the particular case of hydrogen and chlorine (discussed 

 more fully below), let us postulate the condition that certain 

 molecules of each gas not only are moving faster, but also by 

 the agency of light have a tendency to move in one direction 



* It seems preferable to use the term "Inertness" or "Reluctance" 

 rather than "Inertia," as the cause is chemical rather than dynamical, and 

 the implied analogy would therefore be misleading. 



t Hell and Urech (Ber. deut. chem. Ges. xiii. p. 531) divide the course 

 of chemical change into the three last periods, while Pringsheim (Wied. 

 Ann. xxxii. p. 400) gives the first three, but the method of observation 

 adopted precluded any observations of the fourth period. 



% Phil. Mag. [5] xviii. pp. 233-268. 



N2 



