WHAT WE OWE TO THE SUN. 47 



planation of such an occurrence. Suppose that two bodies, 

 each dark and cold, were to strike together in space with 

 such velocities as are ordinarily possessed by celestial 

 bodies, then a vast generation of heat would be a conse- 

 quence of the collision. Each of the bodies possesses a 

 certain quantity of energy in virtue of its motion; but 

 an important law of mechanics tells us that, though the 

 motion may seem partly or wholly annihilated by the 

 collision, yet the energy is not lost; it must reappear 

 in one of the other garbs which energy can so rapidly 

 assume. In such a case as this it will immediately take 

 the form of heat, and the two bodies will be heated by 

 the blow. Other things being equal the quantity of 

 heat that will be produced depends upon the speed with 

 which the two colliding objects rush together. With 

 high speeds the heat that can be thus generated is much 

 greater than when the speeds are low. Thus, for in- 

 stance, if the speed be doubled the heat that would be 

 generated by the collision is increased fourfold ; or if 

 the speed of approach be increased tenfold, then the 

 consequent production of heat is made ten times ten, that 

 is, one hundredfold. Speaking generally, we might say 

 that the heat generated by the collision is proportional to 

 the square of the relative velocity with which the two 

 bodies fly together. 



Considering the enormously high velocities with which 

 the heavenly bodies are animated, it is obvious that in 

 the possibility of collision we have a source of heat 

 adequate for the production of the most gigantic phe- 

 nomena. For example, let us take our own earth, which 

 has at present a speed of eighteen miles a second. Sup- 



