AT THE PHYSICO-TECHNICAL INSTITUTE 433 



1 Conceptually the luminiferous ether will always come under 

 the same category as the ponderable bodies. 



' Of the magnitudes comprised in the concept of immaterial 

 substances I will here only mention the one we are most 

 fully acquainted with, that of which you have in all probability 

 heard most frequently, and the idea of which will be the most 

 familiar. I mean the supply of energy, of effective working 

 power, which is operating in the world, a Proteus capable of 

 being manifested under the most various forms, and of 

 changing from one to another while still unalterable in its 

 quantity, indestructible, incapable of being added to. 



'A weight lifted from the earth represents to us energy which 

 we can utilize to drive a clock and all manner of small machines. 

 A stream flowing down from a mountain can drive powerful 

 engines. From these instances we obtain the fundamental 

 unit of energy ; we measure it by the weight raised, multiplied 

 by the height to which it is lifted, and by the force of gravity, 

 i.e. by kilogrammetres. 



' The velocity of a mass in motion is again the equivalent of 

 a driving force, which we denote by an old term as its vis viva. 

 A falling weight descends with increasing velocity, i.e. the 

 energy of its height is converted into vis viva while the 

 height itself is lost, and if eventually compelled to convert its 

 falling motion by a curved path back into a rising motion, it 

 can thereby regain the height whence it descended. The 

 stretched spring of a clock contains a store of energy, and so 

 does a compressed volume of gas or steam. The latter drives 

 our steam-engines and hot-air machines. 



1 We obtain the proper quantity of compressed gas and steam 

 in these last machines by addition of heat. Heat is energy. 

 It may be derived from mechanical motion when this is 

 destroyed by friction and inelastic impact, and is reconverted 

 into mechanical energy in the above machines. 



'Chemical force is energy, and of a very high equivalent. 

 It provides us with the largest part of our artificially produced 

 supply of heat for practical purposes. It can also be converted 

 directly through electricity into mechanical driving power 

 without any equivalent production of heat, and thus reproduce 

 chemical energy. 



' The more carefully these thousandfold interchanges between 



