18 Mr. M. M. P. Muir on Chemical Notation. 



ever (as is actually done in general practice), and the equation 

 represents the final atomic distribution. Yet, although the 

 equation does not express everything that occurs when zinc 

 and sulphuric acid are mixed together, it does express the 

 great fact that a defmit3 quantity of zinc is invariably asso- 

 ciated with definite quantities of sulphur and of oxygen in the 

 production of the substance called sulphate of zinc. (For a 

 further discussion of this subject, see the address of the Presi- 

 dent, Dr. Williamson, to the British Association at Bradford, 

 1873.) 



39. Further, such atomic interchanges will involve changes 

 in the form of the energy of the reacting molecules : potential 

 will be converted into kinetic energy, or vice versa. Such 

 changes will be marked by absorption or by evolution of heat. 

 These energy-changes will certainly bear a close relation to 

 the general character of the products of the reaction. Thus 

 evolution of heat (stoppage of molecular motion of some kind) 

 is accompanied with the production of a substance whose 

 " affinity-value " and whose boiling-point are higher than those 

 of the generating substance (Wright, Phil. Mag. Dec. 1874). 



40. Now this change in the form of energy will be depen- 

 dent upon a change in the relative position of the parts of the 

 material system, the energy of which is changed ; hence the 

 mere fact of " heat-disturbances " taking place in chemical re- 

 actions is in perfect keeping with a molecular theory of matter. 



41. But certain special cases of heat-disturbance have been 

 cited as being apparently inexplicable in terms of the mole- 

 cular theory. Thus Dr. Wright* brings forward the fact 

 that there is a change in the form of energy of the material 

 system accompanying the exchange of matter symbolized, by 

 the equation H 2 + Cl 2 = HCl + HClas opposed to, or at least as 

 being probably incapable of satisfactory explanation by, the 

 molecular theory. He holds that, " granting that the substi- 

 tution of chlorine for hydrogen is attended by a given change 

 of motion, the inverse substitution must be accompanied by the 

 opposite change." He supposes that in the reaction under 

 notice, an atom of chlorine in the chlorine molecule is replaced 

 by an atom of hydrogen, while an atom of hydrogen in the 

 hydrogen molecule is replaced by an atom of chlorine, and 

 that *" those complementary changes should cause, on the 

 whole, no difference as to the ultimate amount of motion in 

 the two original and the two resulting molecules." It appears 

 to me that Dr. Wright has here assumed that two equal and 

 opposite actions, called by him " substitution " and " inverse 

 substitution," are taking place. It would, I think, be more 



* Phil. Mag. [IV.] vol. xliii. p. 511. 



