NATURAL PHILOSOPHY. 149 



given quantity of heat by applying it to the development and dilata- 

 tion of various elastic fluids, under the various conditions which are 

 practically realizable ? At the time when I entered upon these re- 

 searches, the question appeared to me more simple than it does at 

 present. But, as is usually the case in sciences of observation, in pro- 

 portion as I advanced in my studies, the subject has continually ex- 

 panded. Within the last few years the mechanical theory of heat has 

 occupied the attention of a great number of mathematicians. It has 

 been assumed that heat may be converted into mechanical action, and 

 reciprocally that mechanical action may be converted into heat. Ac- 

 cording to the old theory, the quantity of heat possessed by the elastic 

 fluid at its entrence into the steam-engine is found undiminished in 

 the elastic fluid which is discharged from it, the mechanical action 

 being produced solely by the passage of heat through the engine. Ac- 

 cording to the new theory, the whole of the quantity of heat does not 

 remain in the state of heat ; a portion of heat disappears during the 

 passage through the engine, and the motive power produced is in all 

 cases proportional to the quantity of heat lost. According to my ex- 

 periments, the quantity of heat possessed by the fluid at its entrance 

 into a high pressure steam-engine is 653 units ; that which it retains 

 at its escape amounts to 637. According to the theory of which I am 

 speaking, the quantity of heat rendered available for mechanical ac- 

 tion would be 653 minus 637, or 16 units; that is to say, only one-tbrtieth 

 part of the quantity of heat communicated to the boiler. In a con- 

 densing engine, the heat rendered available would be a little more 

 than one-twentieth. In air-machines, where the motive force is pro- 

 duced by the expansion of the air in the machine by heat, the action 

 should always be proportional to the difference between the quantities 

 of heat possessed by the air on entering and escaping from the engine ; 

 in other words, equal to the loss of that heat by the air in traversing 

 the engine. But as, according to Ericsson's system, the heat which 

 the air possesses at the time of its escape is deposited in substances 

 from which the fresh air in entering abstracts it, in order to convey it 

 back again into the engine, it is evident that theoretically speaking, in 

 these latter engines all the heat is rendered available for the mechan- 

 ical action, while in the best constructed steam-engine only one twen- 

 tieth of it is rendered available. It must, however, be remembered 

 that I do not here take into consideration the exterior losses, from 

 mechanical or industrial obstacles, which may present themselves in 

 practice. Messrs. Joule, Thomson and Rankine in England, Messrs. 

 Mayer and Chassius in Germany, setting out from different points of 

 view, have developed mathematically the mechanical theory of heat. 

 In the course of my researches I have encountered indeed at every 

 step anomalies which appeared to me inexplicable in accordance with 

 the theories formerly recognized. For the sake of illustration I will 

 quote one instance. 1. A mass of gas under a pressure of 10 atmos- 

 pheres is contained in a space which is suddenly doubled; the pres- 

 sure falls to 5 atmospheres. 2. Two reservoirs of equal capacity are 

 placed in a calorimeter, the one is filled with a gas under a pressure of 



