242 BODIES SMALLER THAN ATOMS. 



offered to the motion of the corpuscles through matter diminishes as 

 the velocity of the corpuscles increases, so that we can understand why 

 the rapidly-moving corpuscles from radium are able to penetrate sub- 

 stances which are nearly impermeable to the more -lowly moving cor- 

 puscles from cathode and Lenard rays. 



COSMICAL EFFECTS PRODUCED BY CORPUSCLES. 



As a very hot metal emits these corpuscles, it does not seem an 

 improbable hypothesis that they are emitted by that very hot body, 

 the sun. Some of the consequences of this hypothesis have been 

 developed by Paulsen, Birkeland, and Arrhenius, who have developed 

 a theory of the aurora borealis from this point of view. Let us sup- 

 pose that the sun gives out corpuscles which travel out through inter- 

 planetary space; some of these will strike the upper regions of the 

 earth's atmosphere, and will then, or even before then, come under the 

 influence of the earth's magnetic field. The corpuscles when in such 

 a. field will describe spirals round the lines of magnetic force. As the 

 radii of these spirals will be small, compared with the height of the 

 atmosphere, we may for our present purpose suppose that they travel 

 along the lines of the earth's magnetic force. Thus, the corpuscles 

 which strike the earth's atmosphere near the equatorial regions, where 

 the lines of magnetic force are horizontal, will travel horizontally, and 

 will thus remain at the top of the atmosphere where the density is so 

 small that but little luminosity is caused by the passage of the cor- 

 puscles through the gas. As the corpuscles travel into higher lati- 

 tudes, where" the lines of magnetic force dip, they follow these lines and 

 descend into the lower and denser parts of the atmosphere, w T here they 

 produce luminosity, which, on this view, is the aurora. 



As Arrhenius has pointed out, the intensity of the aurora ought to 

 We a maximum at some latitude intermediate between the pole and the 

 equator, for, though in the equatorial regions the rain of corpuscles 

 from the sun is greatest, the earth's magnetic force keeps these in 

 such highly rarefied gas that they produce but little luminosity, while 

 at the pole, where the magnetic force would pull them straight down 

 into the denser air, there arc not nearly so many corpuscles; the 

 maximum luminosity will, therefore, be somewhere between these 

 places. Arrhenius has worked out this theory of the aurora very 

 completely, and has shown that it affords a very satisfactory explana- 

 tion of the various periodic variations to which it is subject. 



As a gas becomes a conductor of electricity when corpuscles pass 

 through it, the upper regions of the air will conduct, and when air 

 currents occur in these regions, conducting matter will lie driven across 

 the lines of force, due to the earth's magnetic field, electric currents 

 will be induced in the air, and the magnetic force due to these currents 

 will produce variations in the earth's magnetic field. Balfour Stewart 



