674 3fr. Liidwig Mond [June 3, 



show you, the visible part of the spectrum appearing continuous, but 

 beyond the visible part, the photograph shows a large number — 

 over fifty — of well-defined lines in the ultra-violet. I will show you 

 these lines in another photograph taken with greater dispersion, and 

 on which have also been photographed the spark spectrum of nickel. 

 You will see that all these lines correspond absolutely to lines ap- 

 pertaining to the spark spectrum ; in fact the greater part of the 

 lines in the spark spectrum are also shown in this flame spectrum. 

 We have here another and very striking example of the fact dis- 

 covered on the same day by Professors Dewar and Liveing and by 

 Dr. Huggins, that the spectrum of luminous flames is not always con- 

 tinuous throughout its whole range, a fact which has at one time been 

 much debated and discussed. 



One of the most remarkable discoveries made within the precincts 

 of this Institution by that illustrious man whose centenary we cele- 

 brated last year was that of the connection between magnetism and light, 

 which manifests itself when a beam of polarised light is sent through a 

 substance while it is subjected to a strong magnetic field, under whose 

 influence the beam of light is rotated through a certain angle. Dr. 

 W. H. Perkin has prosecuted this discovery of Faraday's by a long 

 series of most elaborate researches, and has established the fact that 

 this power of magnetic rotation of various bodies has a definite relation 

 to their chemical constitution, and enables us to gain a better insight 

 into the structure of chemical compounds. Dr. Perkin has been good 

 enough to investigate the power of magnetic rotation of the nickel 

 carbonyl, and has found it quite as unusual as its chemical properties, 

 and to be, with the sole exception of phosphorus, greater than that of 

 any other substance he has yet examined. 



The power of difi'erent bodies of refracting and dispersing a ray 

 of light has been shown by the beautiful and elaborate researches 

 undertaken many years ago by Dr. Gladstone — who has given an 

 account of them in this theatre in 1875, and who has since continued 

 them with indefatigable zeal — to thi'ow a considerable light upon the 

 constitution of chemical compounds. 



I have investigated the refractive and dispersive powers of nickel 

 carbonyl in Eome, in conjunction with Prof. Nasini. We found that 

 the atomic refraction of nickel in the substance is nearly two and a 

 half times as large as it is in any other nickel compound — a difference 

 very much greater than had ever before been observed in the atomic 

 refraction of any element. To give you some idea how these figures 

 are obtained, Mr. Lennox will now throw on to the screen a beam of 

 light through a double prism, filled partly with nickel carbonyl, and 

 partly with alcohol. You will notice that the top sj^ectrum is turned 

 much further to the left, showing the nickel carbonyl to possess a 

 much greater power of retraction, and you will also notice that it is 

 much wider than the bottom spectrum, which shows the greater dis- 

 persive power of the nickel carbonyl. 



It is now generally supposed that if one element shows different 



