ON THE CONSERVATION OF FORCE. 317 



were made to co-operate, could not be done without a know- 

 ledge of our law in all its generality. The possibility of a per- 

 petual motion was first finally negatived by the law of the con- 

 servation of force, and this law might also be expressed in the 

 practical form that no perpetual motion is possible, that force 

 cannot be produced from nothing; something must be con- 

 sumed. 



You will only be ultimately able to estimate the importance 

 and the scope of our law when you have before your eyes a 

 series of its applications to individual processes in nature. 



What I have to-day mentioned as to the origin of the 

 moving forces which are at our disposal, directs us to something 

 beyond the narrow confines of our laboratories and our manu- 

 factories, to the great operations at work in the life of the earth 

 and of the universe. The force of falling water can only flow 

 down from the hills when rain and snow bring it to them. To 

 furnish these, we must have aqueous vapour in the atmosphere, 

 which can only be effected by the aid of heat, and this heat 

 comes from the sun. The steam-engine needs the fuel which 

 the vegetable life yields, whether it be the still active life of the 

 surrounding vegetation, or the extinct life which has produced 

 the immense coal deposits in the depths of the earth. The 

 forces of man and animals must be restored by nourishment ; 

 all nourishment comes ultimately from the vegetable kingdom, 

 and leads us back to the same source. 



You see then that when we inquire into the origin of the 

 moving forces which we take into our service, we are thrown 

 back upon the meteorological processes in the earth's atmosphere, 

 on the life of plants in general, and on the sun. 



