252 Prof. J. J. Thomson. [June 20, 



From these experiments it would appear that the chlorine atoms in 

 the chlorine derivatives of methane are charged with electricity of 

 the same sign as the hydrogen atoms they displace. 



When we can determine the signs of the electrical charges carried 

 by the atoms in a molecule of a compound, we can ascertain whether 

 any given chemical reaction does or does not imply interchange 

 between the electric charges on the atoms taking part in the reaction. 

 Thus take the reaction 



If we represent the sign of the charge of electricity carried by an 

 atom by + or placed below the symbol representing that atom, 

 we may write the last reaction as 



CH 4 + C1C1 = C 



-- h + - -- h+ + 



so that this reaction could be prpduced by a rearrangement of the 

 atoms without any alterations of their electrical charges. 

 If, however, we take the reaction 



HH + C1C1 = 2HC1, 



+ - + - + - 



we see that in addition to a rearrangement of the atoms there must 

 in this case be an interchange of electric charges between the atoms ; 

 for before combination half the hydrogen atoms had a negative 

 charge, and half the chlorine atoms a positive one, whereas after com- 

 bination no hydrogen atom has a negative charge, and no chlorine 

 atom a positive one. We may thus distinguish between two classes of 

 chemical reactions, (1) those which do not necessarily require any 

 interchange of the electrical charges carried by the atom, and (2) 

 those which do. It might, perhaps, repay investigation to see 

 whether the occurrence of chemical change is affected by the presence 

 of a third substance in the same way in these classes of chemical 

 combination. 



Another point to be considered is the effect of this difference 

 between the chemical actions on the amount of heat developed during 

 chemical combination. When hydrogen and chlorine combine the 

 heat produced may be regarded as the joint effect of three pro- 

 cesses : 



1. The splitting up of the moleculesj (H H) and (Cl Cl) into the 



+ - + - 



atoms H, H, Cl, Cl. 

 + - + - 



2. A transference of electricity by which the negative charge on 

 one atom of hydrogen is replaced by an equal positive charge, while 

 the positive charge on an atom of chlorine is replaced by an equal 

 negative charge. 



