178 Mr. H. Moore on the Ionization in Carhon- 



" atomic" ionization or other "atomic" property (if such 

 there be) can be determined from observations on two of 

 the compounds (the effect of hydrogen being inappreciable) 

 and these values tested by substitution in the other com- 

 pounds. These calculated results can then be compared 

 with the experimental observations, agreement or disagree- 

 ment (within experimental error) indicating the existence of 

 the law, or not. 



To still further simplify matters, secondary homogeneous 

 beams of X-radiation of definite hardness were used, the 

 type of radiation used throughout the ionization experiments 

 being the secondary radiation from copper. 



A long cylindrical chamber was used, lined with aluminium 

 and with aluminium ends, but it was found alter a number 

 of preliminary experiments, that the ionization due to radia- 

 tion from the sides and ends (chiefly corpuscular) formed an 

 appreciable proportion of the total ionization produced. This 

 could have been allowed for with sufficient accuracy as it 

 proved later, but the correction would have been unduly 

 large in one or two cases. For this reason the preliminary 

 results were discarded, and for the final experiments a lining 

 of pure filter-paper was used inside the aluminium lining 

 already referred to, the correction being thereby reduced to 

 less than one- third of its previous value. 



The observations given below were obtained with this 

 paper-lined chamber ; they differ in each case by an almost 

 constant quantity from the preliminary results obtained y 

 which shows that the correction, even without the paper 

 lining, could have been made with reasonable accuracy, and 

 also indicates the accuracy with which the various observa- 

 tions could be made. 



After each experiment, the paper lining was renew r ed, as 

 there w r as some difficulty in completely removing the last 

 traces of the organic vapours used. On account of this diffi- 

 culty, the observations on hydrogen were always carried out 

 immediately after re-lining the chamber, the experiments on 

 the vapour-mixture being carried out subsequently. 



An electroscope attached to an axial electrode inside the 

 chamber served to detect and measure the leak produced by 

 ionization inside the chamber. 



The method of experiment was to pass a mixture of 

 hydrogen and the vapour employed into the chamber, the 

 pressure of the vapour being its saturation pressure at 0°C, 

 while the total pressure was atmospheric. This mixture was 

 then exposed to the homogeneous secondary radiation from 

 copper, and the leak from the electroscope corresponding to 

 the ionization produced was compared with that in a standard 



