228 



NATURE nA 



{'Jtdy 3, 1879 



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. 

 Spottiswoode, 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 appa- 

 ratus 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 box 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 indi- 

 vidual 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 con- 

 siderable extent the average distance that each can fly without a 

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

 When the bees are numerous the mean free path is very short ; 

 when the bees are few the mean free path will be longer, the 

 length being inversely proportional to the number of bees pre- 

 sent. Let us now imagine a loose diaphragm to be introduced 

 in the centre of the box, so as to diride the number of bees 

 equally. The same number of bees being on each side, the 

 impacts on the diaphragm will be equal ; and the mean speed of 

 the bees being the same, the pressure will be identical on each 

 side of the diaphragm, and it will not move. 



Let me now warm one side of this division so as to let it 

 communicate extra eaergy to a bee when it touches it. As 

 before, a bee will strike the diaphragm with its normal mean 

 velocity, but will be driven back with extra velocity, the reaction 

 producing an increase of pressure on the diaphragm. It will be 

 found, however, that although the diaphragm is free to move, 

 the extra strength of the recoil on the warm side does not pro- 

 duce any motion. This at first sight seems contrary to the law 

 of action and reaction being equal. The explanation is not 

 difficult to understand. The bees which fly away from the dia- 

 phragm have drawn energy from it, and therefore move quicker 

 than those which are coming towards it ; they beat back the 

 crowd to a greater distance, and keep a greater number from 

 striking the diaphragm. Near to the heated side of the dia- 

 phragm the density is less than the average, while beyond the 

 free path the density is above the average, and this greater 

 crowding extends to all other parts of the box. Thus it happens 

 that the extra energ)' of the impacts against the warm side of the 

 diaphragm is exactly compensated by the increased number of 

 impacts on the cool side. In spite therefore of the increased 

 activity communicated to a portion of the bees, the pressure on 

 the two sides of the diaphragm will remain the same. This 

 represents what occurs when the extent of the box containing the 

 bees is so great, compared with the mean free path, that the 

 abrupt change in the velocities of those bees which rebound from 

 the walls of the box produces only an insensible influence on the 

 motions of bees at so great a distance as the diaphragm. 



I will next ask you to imagine that I am gradually removing 

 bees from our box, still keeping the diaphragm warm on one 

 side. The bees getting fewer the collisions will become less 

 freqaent, and the distance each bee can fly before striking its 

 neighbour v'ill get longer and longer, and the crowding in front 

 of them will grow less and less. The compensation will also 

 diminish, and the warmed side of the diaphragm will have a 



' A short-hand report of a lecture delivered at the Royal Institution on 

 Friday, April 4, 1879. By William Crookes, F.R.S. Contributed by the 

 author. 



tendency to be beaten back. A point will at last be reached on 

 the warm side, when the mean free path of the bees will be long 

 enough to admit of their dashing right across from the diaphragm 

 to the side of the box, without meeting more than a certain 

 number of in-coming bees in their flight. In this case the bees 

 will no longer fly quite in the same direction as before. They 

 will now fly less sideways, and more forwards and backwards 

 between the heated face of the diaphragm and the opposed wall 

 of the box. Because of this preponderating motion, and also 

 because they will thereby less effectually keep back bees crowd 

 ing in from the sides, there will now be a greater proportionate 

 pressure both on the hot face of the 

 diaphragm and on that part of the box 

 which is in front of it. Hence the 

 pressure on the hot side will now ex- 

 ceed that on the cool side of the dia- 

 phragm, which will consequently have 

 a backward movement communicated 

 toil. 



I may diminish the size of the bees 

 as much as I like, and by correspond- 

 ingly increasing their number the mean 

 free path will remain the same. In- 

 stead of bees let me call them mole- 

 cules, and instead of having a few- 

 hundreds or thousands in the box let 

 me have millions and billions and tril- 

 lions ; and if we also diminish the 

 mean free path to a considerable ex- 

 tent, we get a rough outline of the 

 kinetic theory of gases. (I may just 

 mention that the mean free path of the 

 molecules in air, at the ordinary pres- 

 sure, is the ten-thousandth of a milli- 

 metre.) 



Three years ago I had the honour of 

 bringing before you the results of some 

 researches on the radiometer. Let 

 me now take up the subject where I 

 J.JJ. J then left off. I have here two radio- 



meters which have been rotating be- 

 fore you under the influence of a strong light shining upon 

 tliem. 



The explanation of the movement of the radiometer is this — 

 the light, or the total bundle of rays included in the term " light, " 

 falling upon the blackened side of the vanes, becomes absorbed, 

 and thereby raises the temperature of the black side : this causes 

 extra excitement of the air molecules which come in contact with 

 it, and pressure is produced, causing the fly of the radiometer to 

 turn round. 



I have long believed that a well-known appearance observed 

 in vacuum tubes is closely related to the phenomena of the mean 



Fig. 2. 



free path of the molecules. When the negative pole is examined 

 while the discharge from an induction coil is passing through an 

 exhausted tube, a dark space is seen to surround it. This dark 

 space is found to increase and diminish as the vacuum is varied, 

 in the same way that the ideal layer of molecular pressure in the 

 radiometer increases and diminishes. As the one is perceived 

 by the mind's eye to get greater, so the other is seen by the 

 bodily eye to increase in size. If the vacuum is insufficient to 

 permit the radiometer to turn, the passage of electricity shows 



