318 Prof. J. J. Thomson on the Discharge 



more than the red, so that the jet when seen by transmitted 

 light will appear reddish. 



The preceding explanation does not require dust to be 

 present originally in the air, so that it would apply to cases 

 where, if it w r ere not for the electrification, there would be no 

 condensation ; when, however, dust is present, it is probable 

 that the effects due to electrification arise from several causes. 

 Thus if the air contained excessively fine dust, so fine that 

 the drops of water deposited on it were too small to be per- 

 manent, the electricity, by causing these fine particles to 

 strike together, might produce particles large enough to act 

 as nuclei for the w T ater-drops. Again, as Aitken has pointed 

 out (Proc. Roy. Soc. li. p. 408, 1892), the electricity would 

 tend to prevent fine drops from coalescing to form larger ones, 

 it would thus tend to keep the number of individual drops 

 very large, and so promote dense condensation. Even when 

 the drops could be formed without the agency of electricity, 

 the effect of electrification will be to increase the number of 

 the drops, since, as we have seen, it diminishes their tendency 

 to evaporate. 



Another very remarkable phenomenon discovered by R. v. 

 Helmholtz, the laws of which are described in two papers, one 

 by himself (Wied. Ann. xxxii. p. 1) and another published 

 after his death, describing further results which he had ob- 

 tained in conjunction with Richarz (Wied. Ann. xl. p. 161), 

 is the effect on the steam-jet of chemical action going on in 

 its neighbourhood. They found that chemical action in the 

 neighbourhood of the jet affected it in much the same way as 

 a discharge of electricity, i. e., it produced dense condensation. 

 The chemical actions they investigated were of the most varied 

 character, they included the combination of NO and 0, the 

 splitting up of N 2 4 into N0 2 , the combination of hydro- 

 chloric acid and ammonia to form sal-ammoniac, — these all 

 affected jets not only of steam, but also of the vapours of 

 alcohol and of formic and acetic acids. 



If we suppose that the forces which hold the atom together 

 in the molecule are electrical in their origin, and that in a 

 diatomic molecule one atom has a positive, the other an equal 

 negative charge, the preceding explanation for the effect of 

 electrification on the jet will also apply in this case. When the 

 molecule of a gas is in its ordinary state, it contains one atom 

 positively electrified, another one negatively electrified. In the 

 region outside the molecule, the equal and opposite charges will 

 produce forces which tend to neutralize each other, so that 

 the electric field round the molecule will be much less intense 

 than that round a single charged atom : hence, though the 



