PROFESSOR AT KONIGSBERG 123 



of all natural processes, and the universe will be condemned 

 to a state of eternal rest. 



But there remains the great mystery of the origin of the 

 sun's heat, which keeps up the circulation of water on the 

 earth by means of cloud, and rain, and streams, which governs 

 all inorganic movement, and preserves the cycle of life by 

 the metabolism of plant and animal. The actual heat of the 

 sun, and the number of calories it gives out incessantly, could 

 be computed, but there was no valid hypothesis as to the 

 origin of this heat. Helmholtz set out from the Kant- Laplace 

 hypothesis, that the materials now distributed in the sun and 

 planets had originally occupied space in the form of a cir- 

 culating nebula, which acquired the multiform aspect of the 

 planetary system in virtue of its centrifugal and gravitational 

 forces. He assumed that the density of the nebulous mass 

 was at first a vanishing quantity in comparison with the present 

 density of sun and planets, and then calculated how much 

 work had been expended on this condensation, and how 

 much of this work still exists in the form of mechanical 

 energy, as the attraction of the planets towards the sun, and 

 the vis viva of their motions, after which he estimated by 

 means of the mechanical heat-equivalent, how much of that 

 work has been converted into heat. Helmholtz found that 

 only some 454th part of the original mechanical energy remains 

 as such, while the remainder transformed into heat suffices 

 to heat a mass of water equal to the mass of the sun and 

 planets taken together to 28,611,000 degrees of the centigrade 

 thermometer. ' The enormous quantity of heat lost from our 

 planetary system without compensation is not, however, lost 

 to the universe; it has radiated, and is daily radiating, into 

 infinite space, and we know not whether the medium, through 

 which the vibrations of light and heat are conducted, has any 

 limits at which the rays are compelled to turn back, or whether 

 they pursue their way for ever to Infinity/ 



It follows from the conclusions of Helmholtz, that even the 

 mighty primaeval dowry of the sun's heat, by whose shining 



I and heat-giving rays the immense wealth of organic and 

 inorganic processes upon the earth is constantly replenished, 

 must one day be exhausted, and that Humanity is threatened 

 with an eternal ice-age, even if, as Lord Kelvin suggests, the 



