of Electricity through Gases. 31 ( J 



field round the latter maybe sufficient to cause condensation, 

 that round the molecule may not. 



Now, suppose that molecules which enter into chemical 

 combination come together and form a new compound, re- 

 quiring a rearrangement of the atoms, then while the chemical 

 combination is going on an atom in one of the original mole- 

 cules must leave its old partner and find a new one; there will 

 therefore be an interval in which it is comparatively free to 

 move : that is, the forces on it must be small compared with 

 what they were previously. In other words (assuming that the 

 forces on it are electrical) this atom is in a very weak electric 

 field : there is thus in the region just round the atom nothing 

 to counterbalance the effect of the charge on the atom, so 

 that in this region we shall get an electric field almost as 

 strong as if the atom were dissociated. In this way the che- 

 mical combination may produce very strong local electric fields 

 and thus promote the condensation of the steam. 



If we apply considerations similar to those previously used 

 for the steam-jet to the case where only a small quantity of 

 aqueous vapour is present, we arrive at results which seem to 

 throw light on the effect of water- vapour in promoting che- 

 mical action, the enormous influence of which has been shown 

 in the most striking way by the experiments of Dixon, 

 Pringsheim, and Baker. R. von Helmholtz, in the paper 

 already referred to, found that no white fumes were produced 

 when dry ammonia and hydrochloric acid were mixed to- 

 gether ; he attributed this to the sal-ammoniac being unable 

 to condense in the absence of nuclei. Baker, who has recently 

 observed the same phenomenon, attributes the absence of the 

 white fumes to the absence of sal-ammoniac in any form. 



In the case of the steam-jet the region in which condensation 

 took place was supersaturated with steam, so that when once 

 the drops were formed they would continue even though the 

 chemical action had ceased to take place. If the region were 

 not supersaturated, then we might have sufficient tendency to 

 produce condensation whilst the chemical combination was 

 proceeding, but the drops would evaporate as soon as the 

 chemical action was over. If, however, drops of water were 

 formed during the chemical action, they would react on this 

 chemical action. For suppose we have two substances A and 

 B in the gaseous state present along with a little vapour, let 

 us suppose that the molecules of A and B act on each other so 

 as to tend to alter the arrangement of the atoms in the mole- 

 cules (it is not necessary for the success of the explanation 

 that a new compound should be produced by this direct action 

 of the molecules of A and B) ; then in consequence of this 



