at the Royal Institution, 1908-1916 753 



The alteration of the magnetic moment of magnets on cooling 

 was another subject considered. 



A year later (January 18, 1895), the subject considered was that 

 of " Phosphorescence and Photographic Action at the Temperature 

 of Liquid Air." Those present on this occasion will remember the 

 remarkable character of the demonstrations given and the beauty of 

 the phenomena brought under notice. A great variety of substances 

 were shown to become temporarily phosphorescent when they were 

 exposed during a few seconds to the full light of an electric arc after 

 being thoroughly cooled by immersion in liquid air. Among the 

 most remarkable of these demonstrations were those given with 

 substances such as eggshell and feathers ; the eggshell shone brilliantly 

 «.s a globule of blue light and the feather was equally brilliant, its 

 -outlines being clearly seen in the darkened room. 



Water, when pure, is at most only faintly phosphorescent but 

 remarkably so when impure. It is clear that the character of the 

 phenomena exhibited are in close correlation with constitutional 

 pecuHarities. The two most remarkable classes of compound exhi- 

 biting phosphorescence at low temperatures are the platinocyanides 

 among inorganic compounds and ketonic compounds such as aceto- 

 phenone among organic. In this connexion, the observ^ations made 

 by Emerson Reynolds in the Davy Faraday Laboratory, on the 

 •changes in composition undergone by certain platinocyanides at low 

 temperatures, are of interest as an indication that "constitutional 

 changes " may well be involved in the manifestation of phos- 

 phorescence at low temperatures (comp. R. Soc. Proc, 1909, pp. 82, 

 S80). 



Attention was drawn in this lecture to the reduction of photo- 

 graphic activity by at least 80 per cent, at - 180°. This matter has 

 been referred to in my previous essay (p. 37). 



In a lecture on March 27, 1896, on " New Researches in Liquid 

 Air," Prof. Dewar gave a summary of the advances made in engineer- 

 ing plant in the interval since Pictet made his momentous discovery 

 in 1878. He expressed the opinion that, apart from important 

 mechanical details and the development of laboratory technique, 

 nothing new had been added by any investigator to the principles 

 involved in the construction and use of low-temperature apparatus 

 since the time of Pictet's discovery. Reference was made to the 

 fantastic claim put forward by Prof. Olzewski that because, in 1890, 

 he used a steel tube combined with a stop-cock to draw off hquid 

 oxygen, he had taught the world " the method of getting large quan- 

 tities of liquid gases." To this Prof. Dewar very properly remarks, 

 " As to such claims, one can only wonder at the meagre additions to 

 knowledge that in our time are unhesitatingly put forward as original 

 Vol. XXL (No. 110) 3d 



