230 Sir J. J. Thomson : Further 



number of particles which produce the lines, prevents ns 

 from drawing conclusions from the inspection of the photo- 

 graphs as to numbers of the various types of carriers of 

 positive electricity present in the positive rays, and indeed 

 would very often convey a wrong impression. Thus, for 

 example, in a number of photographs I possess of the rays 

 passing through mixtures of hydrogen and oxygen in 

 varying proportions, the relative brightness of the lines of 

 hydrogen and oxygen shows but little relation to the relative 

 quantities of these gases present in the mixture. When, 

 however, the numbers of particles in these lines were mea- 

 sured by directing them in succession on to the slit of a 

 Faraday cylinder and measuring the charge received in a 

 given time by the cylinder, it was found that the relative 

 number of particles in the hydrogen and oxygen lines varied 

 directly as the relative number of hydrogen and oxygen 

 molecules present in the mixture. 



The relative number of the different kinds of particles 

 present in the positive rays is a quantity of fundamental 

 importance for the consideration of the chemical changes 

 which go on in the gas in the discharge-tube ; this quantity 

 is being measured in the Cavendish Laboratory by receiving 

 the particles separately on a Faraday cylinder and measuring 

 the charge received by the cylinder. The results of these 

 measurements will be given in a separate paper. 



Though the photographs do not give accurate quanti- 

 tative estimates of the numbers of the different particles, 

 they do give some very interesting results as to the pro- 

 cesses at work in the ionization of the gases. Thus on 

 some photographs the line due to an atom of, say, hydrogen 

 was stronger than that due to the molecule, while on others 

 the line due to the molecule was the stronger. Another 

 interesting and in my opinion very suggestive result, comes 

 to light from an inspection of the plates taken when the 

 discharge passed through mixtures of hydrogen and oxygen. 

 It is illustrated in figs. 17 & 18 (PI. IX.) and also in 

 figs. 21 a, 21 b. & 21c. In the plates from which these figures 

 were made there were very pronounced differences between 

 the appearance of the lines due to the hydrogen atoms 

 and those due to the hydrogen molecule. These differences 

 may be of various kinds : for example, in fig. 17 the line 

 due to the hydrogen atom is of uniform intensity through- 

 out, while that due to the molecule is very weak at the head 

 but intense elsewhere ; in others the line due to the atom is 

 uniform, but that due to the molecule is much more intense 



