708 Miss Agnes M. ClerJce 



ture of liquid hydrogen, phosphorescent action is still further intensi- 

 fied ; it may, although exceptionally, even at —250° C. be set on foot 

 by ligbt deprived of its ultra-violet rays. 



The electric stimulation of crystals by cooling brings about actual 

 discharges between the molecules. In some platino-cyanides and in 

 nitrate of uranium, the temperature of liquid air suffices to develop 

 marked electrical and luminous phenomena, which are intensified and 

 extended through the agency of liquid hydrogen. The importance 

 of a systematic study of pyro-electricity under such conditions was 

 pointed out by Professor Dewar in the Bakerian Lecture delivered 

 June 13, 1901'. 



Chemical affinity is almost completely abolished by cold. Phos- 

 phorus, sodium, potassium, remain inert in liquid oxygen ; and voltaic 

 combinations, brought down to its temperature, cease to give electric 

 currents. Photographic films, however, retain about one-fifth of their 

 ordinary sensitiveness to light, nor does it wholly disappear even 

 through the agency of liquid hydrogen. Possibly the decomposing 

 force which comes into play under these circumstances is not chemical, 

 but mechanical ; if so, no trace of photographic action would be 

 apparent were it possible to carry out development under the frigid 

 conditions of exposure. 



An elaborate course of experiments on thermal transparency, 

 carried out in 1897—8, completely negatived Pictet's conclusion that, 

 at a given degree of cold, non-conducting substances lose their 

 faculty of insulation. They were proved to retain it unimpaired at 

 the boiling-point of air, the abnormal transferences of heat observed 

 at Geneva having been due, not so much to the materials themselves, 

 as to the air contained in their interstices. The utility was thus 

 rendered apparent of investigating problems of heat-transmission 

 with the aid of liquid air. The comparative absorption of Rontgen- 

 rays by various frigid bodies formed about the same time a subject of 

 inquiry ; and the view that the atomic weight of argon is double its 

 density relative to hydrogen, obtained confirmation from the approxi- 

 mately equal opacities found for that substance in the liquid state, 

 for liquid chlorine, and for potassium. This was the first use of the 

 Rontgen radiation for the purpose of defining atomic weight. 



Liquefaction of Fluorine. 



The liquefaction of fluorine preceded by one year the liquefaction 

 of hydrogen. That this peculiar substance would prove highly 

 recalcitrant to condensation had long been known by sure indications 

 derived from the character of its compounds. Thus, while chloride 

 of ethyl boils at + 12° C, fluoride of ethyl boils only at —32° ; and 

 similarly, the boiling points of chloride and fluoride of propyl are 

 respectively + 45° and —2°. Analogous values for the same con- 

 stant are given by the various inorganic haloid compounds. The 

 obstinate gaseity of fluorine was at length overcome through a com- 



