Mr. J. Gill on the Dynamical Theory of Heat. 113 



exterior work, or interior molecular motion, in re-establishing the 

 previous state of equilibrium. It is not requisite to suppose the 

 direct or immediate reappearance of the vis viva in restoring 

 equilibrium in any one operation ; for equilibrium may be equally 

 restored by a transfer of force which may disappear from one body 

 to reappear in another under the same form; but sooner or 

 later in the rotation of phenomena the transformation takes place, 

 for when one form of energy disappears altogether, it must be 

 substituted by another. 



But if high-pressure steam be quietly condensed in the vessel 

 which contains it, by simply withdrawing the heat of conversion, 

 or change of state, the excess of energy above mentioned disap- 

 pears entirely. What becomes of it ? I submit that this question 

 must be answered before the dynamical theory of heat can be 

 received as satisfactory or rational, as far as human knowledge 

 at present extends. 



The recent experiments of M. Hirn showed that when high- 

 pressure steam expands tumultuously into a condenser, heat is 

 created; that is to say, more heat appears in the condenser than 

 was contained originally in the steam. The fact is not disputed, 

 but it is explained by saying that the particles of the steam rush 

 into the condenser with immense velocity, while the water which 

 results from their condensation is quiet ; and as the vis viva co- 

 existent with velocity in the motion of translation of the particles 

 remains in the condenser, it must assume the form of molecular 

 motion known as heat. I submit that this explanation is not 

 satisfactory, though it may be a correct statement of the mole- 

 cular phenomena. The steam, as high-pressure steam, contains 

 only the heat (molecular motion) of conversion, or change of 

 state, which is allowed to be a nearly constant quantity under 

 different circumstances of temperature and pressure; and this is 

 the quantity which appears when the steam is condensed under 

 pressure. But the vis viva or molecular motion of the steam is 

 more after tumultuous expansion than before, and therefore steam 

 so expanded is superheated, as directly proved by experiments of 

 Siemens, and by my own ; consequently it must be allowed that 

 in its compressed state it possesses something which is not quan- 

 titative thermometric heat for the time, though always accom- 

 panied by higher temperature in the steam, but which produces 

 work if the steam expands under moderated resistance, and heat 

 if it expands tumultuously. 



Analogous to the foregoing case are Joule's remarkable expe- 

 riments of 1845, in which compressed air was expanded to an 

 equal degree by two different methods — first, by a gradual en- 

 largement of the containing space under moderated pressure, by 

 which exterior work was performed, accompanied with (apparent) 



Phil. Mag. S. 4. Vol. 26. No. 173. Aug. 1863. I 



