CATHODE RAYS AND RONTGEN RAYS. 277 



is rectilinear unci th:it it is quite sensitiv^e i)erpendicular to the surface 

 of the electrode. Again, the mechanical eli'ects produced l)y these 

 rays are of great interest, owing- to the support which they seem to 

 give to the theorj^ of the emission of matter. They are shown by a 

 beautiful experiment. Two rails formed of glass rods and placed in 

 the path of the cathode rays support the axle of a paddle wheel. This 

 little machine begins to move, revolves continuously as soon as elec- 

 trical communication has been established, as if the flanges received 

 blows — a l)oml)ardment, according to the expression used by Crookes — 

 of material particles issuing from the negative electrode. On revers- 

 ing the direction of the current the wheel revolves in the opposite 

 direction. The ballistic explanation seems so reasonal)le that it natu- 

 rall}' insiimates itself into the mind and gives rise to a belief in cath- 

 ode projectiles. However, on reflection, the argument is by no means 

 conclusive. Everyone has seen in the show windows of opticians the 

 little instrument which is called a radiometer, which was itself an 

 inv^ention of Crookes. It forms a kind of windmill, exceedingly light, 

 and inclosed in a bulb of glass that has been exhausted of air. It 

 begins to move in the same way as the water wheel of the preceding 

 experiment, but under the action of luminous rays — that is to say, of 

 vibrations of the ether, without suggesting this time a bombardment 

 of projectiles. 



A second property of cathode rays, an unexpected and very remark- 

 able one, is that the}'^ are attracted 1)\^ a magnet. Making the pencil 

 visible by means of a phosphorescent screen placed within the tube it 

 is seen to bend nway on approaching a magnet; it can be attracted and 

 repelled at will by varying the position of the magnetic agent. The 

 amount of the deflection depends partly on the strength of the magnet 

 and parth' on the velocity of the cathode rays, a velocity which can 

 be determined by varying the pressure of the gaseous residue that 

 fills the bulb. On giving proper motion to the magnet it is easy to 

 conceive that one might succeed in twisting the pencil into a spiral. 

 This obedience to the directive force of the magnet goes so far as to 

 allow it to form a circle upon itself. In this experiment the cathode 

 ray behaves like an electric current of which the negative pole would 

 be the cathode and which runs along a metallic wire. This magnetic 

 deflection is easily explained by the emission theory; the rays would 

 be formed by a row of electrified material' particles following each 

 other rapidly and carrying an electric charge. This transportation of 

 electricit}' by the transportation of matter is called a current b}- con- 

 vection. Rowland, Rontgen, and other physicists have shown that 

 currents of this nature are similar to ordinary currents by conduction. 

 On the other hand, deflections produced by a magnet are unknown in 

 etherial, calorific, luminous, and actinic radiations. 



In the third place the cathode ray is electrified. This we assumed a 



