THOUGHTS ON NATURAL PHILOSOPHY. 45 



What determines the greatness of the above figures 

 and makes them at first sight improbable is the 

 enormous speed of the masses in play, a speed which 

 we cannot approach by any known mechanical 

 means. In the factor mV 2 , the mass of one gramme 

 is certainly very small, but the speed being immense 

 the effects produced become equally immense. A 

 rifle-ball falling on the skin from the height of a few 

 centimetres produces no appreciable effect in con- 

 sequence of its slight speed. As soon as speed is 

 increased, the effects become more and more deadly,, 

 and, with the speed of 1,000 metres per second given 

 by the powder now employed, the bullet will pass 

 through very resistant obstacles. To reduce the 

 mass of a projectile matters nothing if one arrives 

 at a sufficient increase in speed. This is exactly 

 the tendency of modern musketry, which constantly 

 reduces the calibre of the bullet but endeavours to 

 increase its speed. 



Now the speeds which we can produce are 

 absolutely nothing compared with those of the 

 particles of dissociated matter. We can barely 

 exceed a kilometre a second by the means at our 

 disposal, while the speed of radio-active particles is 

 100,000 times greater. Thence the magnitude of 

 the effects produced. These differences become 

 plain when one knows that a body having a velocity 

 of 100,000 kilometres per second would go from the 

 earth to the moon in less than four seconds, while 

 a cannon ball would take about five days. Taking 

 into account a part only of the energy liberated in 

 radio-activity, and by a different method, figures 



