182 



i?. Clausius on the Application of the 



4. The expression that heat drives a machine, is of course not 

 to be immediately referred to the heat, but is to be understood as 

 signifying that some substance present in the machine, in conse- 

 quence of the changes which it undergoes by heat, sets the parts 

 of the machine in motion. We will call this substance the heat- 

 utilizing substance (den die Wirkung der Warme vermittelnden 

 Staff). 



If now a continually acting machine be in uniform action, all 

 the changes which occur take place periodically, so that the same 

 condition in which the machine, with all its single parts, is found 

 at a particular time, regularly recurs at equal intervals. Conse- 

 quently the heat-utilizing substance must be present in the ma- 

 chine in equal quantity at such regularly recurring instants and 

 must be in a similar condition. This condition may be fulfilled 

 in two different ways. 



In the first place, one and the same quantity of this substance 

 originally existing in the machine may always remain in it, in 

 which case the changes of condition which the substance under- 

 goes during the action of the machine must take place in such a 

 manner that at the end of every period it again returns to its 

 initial condition, and then begins again the same cycle of changes. 



In the second place, the machine may each time give off, ex- 

 ternally, the substance which has served during one period to 

 produce the action, and in its place may take up again from with- 

 out the same quantity of substance of the same kind. 



5. This last process is the more usual one in machines applied 

 in practice. It occurs, for instance, in the caloric air machines 

 constructed up to the present time, inasmuch as after every stroke 

 the air which has moved the piston in the cylinder is driven into 

 the atmosphere, and an equal quantity of air is supplied from the 

 atmosphere, through the feeding cylinder. The same is the case 

 in steam engines without condensers in which the steam passes 

 from the cylinder into the atmosphere, while, to supply its place, 

 a fresh portion of water is pumped from a reservoir into the 

 boiler. 



Furthermore, at least a partial application is also made in 

 steam engines with condensers of the usual arrangement. In 

 these the water condensed from the steam is partly pumped 

 back into the boiler, but not wholly, because it is mixed with 

 the cold water used for condensation, and a portion of this con- 

 sequently also passes into the boiler. The portion of the con- 

 densed water not again applied must be thrown out with the 

 rest of the water of condensation. 



The first process has recently been applied in those steam en- 

 gines which are worked by two different vapors, as for instance 

 by water and the vapor of ether. In these the steam is con- 

 densed only by contact with the metallic tubes which are inter- 



