366 Meeting of June 20, 1901. and List of Payers read. 



about by the use of liquid air. A body like sulphide of zinc cooled 

 to 21° absolute and exposed to light shows brilliant phosphorescence 

 on the temperature being allowed to rise. Bodies like radium that 

 exhibit self-luminosity in the dark, cooled in liquid hydrogen maintain 

 their luminosity unimpaired. Photographic action is still active 

 although it is reduced to about half the intensity it bears at the 

 temperature of liquid air. Some crystals when placed in liquid hydro- 

 gen become for a time self-luminous, on account of the high electric- 

 stimulation brought about by the cooling causing actual electric dis- 

 charges between the crystal molecules. This is very marked with 

 some platino-cyanides and nitrate of uranium. Even cooling such 

 crystals to the temperature of liquid air is sufficient to develop marked 

 electrical and luminous effects. 



Considering that both liquid hydrogen and air are highly insu- 

 lating liquids, the fact of electric discharges taking place under such 

 conditions proves that the electric potential generated by the cooling 

 must be very high. When the cooled crystal is taken out of either 

 liquid and allowed to increase in temperature, the luminosity and 

 -electric discharges take place again during the return to the normal 

 temperature. A crystal of nitrate of uranium gets so highly charged 

 •electrically that, although its density is 2*8 and that of liquid air 

 about 1, it refuses to sink, sticking to the side of the vacuum vessel 

 and requiring a marked pull on a silk thread, to which it is attached, 

 to displace it. Such a crystal rapidly removes cloudiness from liquid 

 air by attracting all the suspended particles on to its surface. The 

 study of pyro-electricity at low temperatures will solve some very 

 important problems. 



During this inquiry I have had the hearty co-operation of Mr. 

 Robert Lennox, to whom my thanks are due, and Mr. J. W. Heath 

 has also given valuable assistance. 



June 20, 1901. 



Sir WILLIAM HUGGINS, K.C.B., D.C.L., President, in the Chair. 



Professor William Schlich and Professor Arthur Smithells were 

 admitted into the Society. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them. 



The following Papers were read : — 



