522 THE POPULAR SCIENCE MONTHLY 



tides. This pencil is acted on by electric forces when it passes between 

 the plates L and M, which are connected with the 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 traveling along the same path they branch 

 off into different directions. 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 

 connection 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 vessels. 



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 corre- 

 sponding to the atomic weight 128 (xenon) ; a very strong line corre- 

 sponding to the atomic weight 82 (krypton), a strong argon line 40 

 (argon) and the neon line 20. There are no lines unaccounted for, 

 and hence we may conclude that in the atmosphere there are no 

 unknown gases of large atomic weight occurring in quantities com- 

 parable with those of xenon or krypton. This result gives an example 



