142 Professor Sir J. J. Thomson [April 7, 



nitrogen, and might thus have suspected the presence of another 

 element, but spectrum analysis could not tell us anything about the 

 nature of this element, whereas the positive ray spectrum at once gives 

 us its atomic weight. 



The positive-ray method is even more delicate than that of spec- 

 trum analysis, for by it we can detect the presence of quantities of a 

 foreign gas too minute to produce any indication in the spectro- 

 scope. I have, for example, often been able to detect the presence 

 of helium by this method, when no indication of its presence could 

 be detected by a spectroscope. 



Again, when a line in the positive-ray spectrum can be seen, the 

 atomic weight of the carrier which produces it can be determined 

 with great accuracy. Though the method is only a few months old, 

 it is even now sufficiently develo^Ded to determine with an accuracy 

 of 1 per cent, the atomic weight of a gaseous substance, without re- 

 quiring more than ^^^ milligramme of the substance. Another very 

 important advantage of this method is that it is not dependent 

 upon the purity of the material ; if the material is impure the impurities 

 merely appear as additional lines in the spectrum, and do not affect the 

 parabola due to the substance under examination, and therefore produce 

 no error in the determination of the atomic weight. The method 

 would seem to be peculiarly suitable for the determination of the 

 atomic weights not merely of the emanation from radio-active sub- 

 stances, but also those of the products into which they disintegrate. 



The rays, too, are registered within less than a millionth of a second 

 after their formation, so that when chemical combination or decom- 

 position is occurring in the tube, the method may disclose the existence 

 of intermediate forms which have only a transient existence, as well as 

 of the final product, and may thus enable us to gain a clearer insight 

 into the process of chemical combination. 



I will now show a few slides prepared from photographs we 

 have taken of the positive-ray spectra. The first is that of nitrogen 

 prepared from air ; the measurements of the photograph showed that 

 the atomic weights of the carrier producing these curves were as 

 follows : — 



Positive. Negative. 



1 H+ 1 H_ 



1-99 H2+ 11-20 C_ 



6-80 N++ 15-2 0_ 



11-40 0+ 



13-95 N+ 



28-1 Na^ 



39 Arg+ 

 100 Hg++ 

 198 Hg+ 



The symbol H+ denotes that the carrier is an atom of hydrogen 



