1906.] Studies on Charcoal and Liquid Air. 488 



WEEKLY EVENING MEETING, 

 Friday, June 8, 1906. 



His Grace the Duke of Northumberland, K.G. P.O. D.C.L. 

 F.R.S., President, in the Chair. 



Professor Sir James Dewar, M.A. LL.D. D.Sc. F.R.S. M.R.L, 



Fulleriun Professor of Cliemistry R.I. 



Studies on Charcoal and Liquid Air. 



The object of the lecture was to demonstrate experimentally a few 

 novel applications of liquid air observed in the course of laboratory 

 experiments. Some of them may be said to take the character of 

 lecture illustrations, while others deal with an extension of the scien- 

 tific uses of charcoal at low temperatures, a subject which was first 

 discussed in a Friday Evening Discourse delivered by the Professor 

 in the year 1896. 



Electrical Stimidation at Low Temperatures. 



Bodies cooled to the temperature of liquid air have the surface 

 film of water, which is more or less deposited on them, frozen excep- 

 tionally hard, and therefore not in a good condition for the dissi- 

 pation of any electric charge that may be given to them. The 

 result is that a substance like glass cooled in liquid air is exceedingly 

 easily electrified by friction, and retains its charge for a long time. 

 This property of glass was shown in the following manner : — 



A glass tube shaped like a two-pronged fork was arranged so that 

 a magnified image of it could be projected on a screen. (See 

 Fig. 1.) The prongs were cooled in liquid air, and one prong, A, 

 while it was being removed from the liquid air, was gently flicked 

 with flannel or silk to electrify it. On being exposed to the air the 

 moisture in the atmosphere was deposited as ice on both prongs, 

 but the character of the deposit was different on the unelectrified 

 and the electrified prong, crystals of ice beginning to form on the 

 latter, speedily causing it to assume the appearance of a miniature 

 forest of growing and moving crystals shown on A in Fig. 1, while 

 B shows simply a dead coating of ice. The electrified prong by 

 induction attracts the moisture in the air, and as fresh moisture is 

 drawn in repulsion between like electricities causes the new crystals 

 to be deposited as far out as possible, thus presenting the appearance 

 in the diagram. With good projection one can see some of the 

 crystals on A repelled and shot across to \\. A similar experiment 



Vol. XVIII. (No. 100.) 2 f 



