64 SCIENCE PROGRESS 



and oxygen under conditions of fairly high pressure in the 

 camera, has been included in order to give the reader some 

 idea of the extreme complexity of the secondary rays, which 

 in this particular instance form a perfect network of lines, 

 straight and curved. Out of five apparently distinct lines 

 on the negative side, only two, the H x _ and O x _ lines, are 

 genuine. For a detailed discussion as to the origin and be- 

 haviour of secondary rays, the reader is referred to Prof. 

 Thomson's original papers on the subject which have been 

 published from time to time in the Philosophical Magazine. 



From these few illustrations and brief descriptions, ideas of the 

 possibilities and limitations of the method will doubtless have 

 already been formed. As to the latter, some are obvious, such 

 as the fact that in order to apply the method to the determination 

 of atomic weights the substance analysed must exist in a state 

 of vapour and be able to support an electric discharge. There is, 

 however, another more subtle disability which is also known to 

 affect ordinary spectroscopic analyses of gases : this is that a 

 substance may be present in quite large quantities and yet its 

 characteristic lines may not be apparent. When it was stated 

 that mercury was the only metal so far clearly identified, it 

 must not be understood that it is from any lack of trying others. 

 As soon as the method was found to afford results of reason- 

 able accuracy, Sir J. J. Thomson endeavoured to apply it to 

 settle the much vexed question of the atomic weight of nickel, 

 the value generally accepted by chemists appearing incompatible 

 with the results obtained by physicists on studying the char- 

 acteristic radiation of the metal. But although nickel carbonyl 

 was passed through the tube and nickel chloride was vaporised 

 inside it, the plates obstinately refused to vouchsafe the least 

 indication of a nickel parabola and results with potassium were 

 very nearly as negative. It seems almost inconceivable that 

 these elements cannot exist as ions in the discharge tube but 

 it is quite possible that they are incapable of retaining their 

 charge after reaching the cathode and so are not analysable by 

 the method. Another less likely possibility which may shortly 

 be tested is that the parabolas are there but are incapable of 

 affecting the screen or the plate. From the point of view of 

 accuracy, the limitations of the method are almost entirely those 

 of apparatus, design and technique ; it is therefore to be 

 supposed that they will be removed as experience grows. 



