542 SCIENCE PROGRESS 



As McLennan points out, there have been hitherto two 

 methods available for finding the ionisation potential of an 

 element, viz. determination of the minimum arcing potential 

 of its vapour, or using a specially designed electrical method. 

 We now possess a third possibility, viz. by determining 

 for the element the wave-lengths of its line spectrum and 

 selecting those which constitute its singlet principal series 

 #=(i'5, a) — {n, fi). Calculation of a and /3 from the series of 

 wave-numbers will give the convergence frequency, and the 

 quantum hypothesis will yield the required result. 



Of course, as is to be expected, this view has its own diffi- 

 culties to meet, and among others, the fact that under special 

 experimental conditions (for example, with high temperature 

 and considerable vapour density) arcs can be struck, or, when 

 struck, can be maintained by potentials lower than the ionisa- 

 tion voltages given above. The latter part of the address is 

 devoted to these topics and the discussion shows that they 

 are not insuperable difficulties. Under the experimental con- 

 ditions necessary for the striking or maintenance of the arc 

 with these abnormally low voltages, it is very probable that 

 the energy of the bombarding electrons, apparently too low 

 for ionisation, is supplemented by the photo-electrons ejected 

 from the cathodes by the incidence of the radiations from the 

 vapours, or by the thermionic electrons ejected from the 

 cathodes by reason of their high temperature. The work of 

 McLennan and his pupils is certainly providing extremely 

 valuable data in a very interesting field of research. 



PHYSICAL CHEMISTRY. By Prof. W. C. McC. Lewis, M.A., D.Sc, 

 University, Liverpool. , 



Fluorescence. — In connection with this phenomenon a very 

 interesting investigation has been carried out by J. Perrin 

 {Annates de Physique, 10, 133, 191 8). Perrin deals mainly 

 with the fluorescence of dissolved substances, such as uranine, 

 fluoresceine, phenosafronine, chlorophyll, esculine, anthracene 

 in different solvents. The fluorescence exhibited by these 

 substances in thin layers is observed by means of a microscope, 

 the lighting being so arranged that the fluorescence alone enters 

 the eyepiece. 



The most striking observation is the gradual fading of the 

 fluorescence which takes place on exposure, the fluorescence, 



