138 Mr, William Crookcs [April 4, 



WEEKLY EVENING MEETING, 



Friday, Aj^ril 4, 1879. 



William Spottiswoode, Esq. D.C.L. LL.D. Pros. R.S. 

 Vice-President, in the Cliair. 



William Crookes, F.K.S. 



Molecular Physics in High Vacua. 



When I was asked, a month or two ago, to illustrate in this theatre 

 some of my recent researches on Molecular Physics in High Vacua, I 

 exclaimed " How is it possible to bring such a subject worthily before 

 a Royal Institution audience when none of the experiments can be 

 seen more than three feet off ? " If to-night I am fortunate enough 

 to show all the experiments to those who are not far distant, and if 

 I succeed in making most of them visible at the far end of the theatre, 

 such a success will be entirely due to the great kindness of your late 

 Secretary, Mr. SiDottiswoode, who has placed at my disposal his 

 magnificent induction-coil, — not only for this lecture, but for some 

 weeks past in my own Laboratory, — thus enabling me to prepare 

 apparatus and vacuum tubes on a scale so large as to relieve me of 

 all anxiety so far as the experimental illustrations are concerned. 



Before describing the special researches in molecular physics 

 which I propose to illustrate this evening, it is necessary to give a 

 brief outline of one small department of the modern theory of the 

 constitution of gases. It is not easy to make clear the kinetic theory, 

 but I will try to simplify it in this way : — Imagine that I have in a 

 large bos a swarm of bees, each bee independent of its fellow, flying 

 about in all manner of directions and with very different velocities. 

 The bees are so crowded that they can only fly a very short distance 

 without coming into contact with one another or with the sides of 

 the box. As they are constantly in collision, so they rebound from 

 each other with altered velocities and in different directions, and 

 when these collisions take place against the sides of the box pressure 

 is produced. If I take some of the bees out of the box, the distance 

 which each individual bee will be able to fly before it comes into 

 contact with its neighbour will be greater than when the box was 

 full of bees, and if I remove a great many of the bees I increase to a 

 considerable extent the average distance that each can fly without a 

 collision. This distance I will call the bee's ineaii free path. When 



