473 



WEEKLY EVENING MEETING. 



Friday, April 6, 1900. 



Sir Frederick Bramwell, Bart., D.C.L. LL.D. F.B.S., 

 Honorary Secretary and Vice-President, in the Chair. 



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



Solid Hydrogen. 



Before proceeding to discuss the immediate subject of this lecture, 

 it will be advisable to contrast experimentally some of the pro- 

 perties of hydrogen, nitrogen and oxygen in the liquid condition. 

 The two vacuum cups (Figs. 1 and 2) are charged half full 

 respectively with liquid hydrogen and liquid air. When the cup 

 containing the liquid air is placed in front of the electric lamp, the 

 image thrown on the screen reveals the continual overflow of a 

 dense vapour round the outer walls of the vessel. The saturated 

 vapour coming from the steady ebullition of liquid air is three times 

 denser than the free air of the room, and the result is it falls 

 through that air just as if it were a dense gas like carbonic acid or 

 ether vapour. To observe this phenomenon, the vacuum cup must 

 be shallow, otherwise the vapour gets heated up before reaching the 

 mouth of the vessel, and no difference of density in the air coming off 

 is observed. We will now project the image of the cup containing 

 liquid hydrogen, covered loosely in this caso with a glass plate, 

 upon the screen ; here, no heavy vapour escaping round the sides is 

 visible. The vapour of the boiling liquid hydrogen has a density 

 nearly equal to the air of the room, but as it gets very rapidly 

 heated up by the glass cover the gas that is escaping is seen to rise 

 in air like any light gas. On now removing the glass plate, a very 

 different phenomenon is observed, which contrasts markedly with the 

 behaviour of the liquid air in the former vessel. The cup and the 

 air above is filled with a dense surging snowstorm of solid air ; the 

 air coming in contact with the excessively cold hydrogen vapour is 

 suddenly solidified, and a part of it falls into the liquid hydrogen, 

 causing more rapid evaporation, thereby intensifying the cloud con- 

 densation. After the mist has disappeared and all the liquid hydrogen 

 gone, the cup contains a white deposit of solid air. This shortly 

 melts, and on allowing the nitrogen to boil off, the presence of oxygen 

 can be shown by the ignition of a red-hot splinter of wood. Such 

 effects are easily understood when we remember that the boiling-point 

 Vol. XVI. (No. 94.) 2 i 



