182 R. 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 
Sto 
If now a continually acting machine be in uniform action, all 
the changes which occur take place gctgmapch 2 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- | 
uently 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- — 
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 
> — re ae without condensers in which the steam A 
om the cylinder into the atmosphere, while, to supply its place, — 
o —_ portion of water is pumped from a anton me 
iler. ‘@ 
Furthermore, at least a partial application is also made in 
s 
Ee which are worked by two different vapors, as for instance 
y Water and the vapor of ether. In these the steam is con- 
densed only by contact with the metallic tubes which are inter: 
