592 Professor Sir J. J. Thomson [Jan. 17, 



terminals of a battery of storage cells, and by a magnetic force when 

 it passes between P and Q, which are the poles of an electro-magnet. 

 In the pencil before it passed under the influence of these forces there 

 might be many kinds of atoms or molecules, some heavy, others light, 

 some moving quickly, others comparatively slowly, but these would 

 all be mixed up together. When they are acted on by the electric 

 and magnetic forces, however, they get sorted out, and instead of 

 travelling along the same path they branch oif into different direc- 

 tions. No two particles will travel along the same path unless they 

 have the same mass as well as the same velocity ; so that if we know 

 the path of the particle we can determine both its mass and its 

 velocity. In chemical analyses we are concerned more with the 

 mass than with the velocity, and we naturally ask what is the con- 

 nexion between the paths of particles which have the same mass 

 but which move with different velocities. The answer is that all such 

 paths lie on the surface of a cone, and that each kind of particle 

 has its own cone ; there is one cone for hydrogen, another for 

 oxygen, and so on. Thus one cone is sacred to hydrogen, and if it 

 exists there must be hydrogen in the vessel ; so that if we can detect 

 the different cones produced from the original pencil, we know at once 

 the gases that are in the tube. Now, there are several ways of 

 identifying these cones, but I shall only refer to the one I have used 

 in the experiments I wish to bring before you this evening. These 

 moving electrified particles, when they strike against a photographic 

 plate, make an impression on the plate, and a record of the place 

 where they struck the plate can be obtained. Thus, when a plate is 

 placed in the way of the particles streaming along these cones, the 

 sections of these cones by the plate (parabolas) are recorded on the 

 photograph, hence we can identify these cones by the parabolic 

 curves recorded on the photograph, and these parabolas will tell us 

 what gases are in the vessel. 



The first application of the method which I shall bring before 

 you this evening is to detect the rare gases in the atmosphere. Sir 

 James Dewar kindly supplied me with two samples of gases obtained 

 from the residues of liquid air ; the samples had been treated so that 

 one might be expected to contain the heavier gases, the other the 

 lighter ones. I will take the heavier gases first. The photograph 

 for these is shown in Fig. 2. When the plate is measured up it shows 

 a faint line corresponding to the atomic weight 128 (xenon) ; a 

 very strong line corresponding to the atomic weight S2 (krypton), 

 a strong argon line 40 (argon), and the neon line 20. There are no 

 lines unaccounted for, and hence we may conclude tliat in the atmo- 

 sphere there are no unknown gases of large atomic weight occurring 

 in quantities comparable with those of xenon or krypton. This 

 result gives an example of the convenience of the method, for a 

 single photograph of the positive rays reveals at a glance the gases 



