PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 501 



tion established at that time with reference to the indestructibility and 

 increatibility of matter. These two great acquisitions, namely, the 

 propositions relative to the permanence of matter and energy, give to 

 modern physics a solid foundation. 



Lecture II. 



In the former lecture we have learned how, by transformation of me- 

 chanical work into heat, their equivalence has been demonstrated ; we 

 will now show how this equivalence has been ascertained by reversing 

 the process, i. e. by turning heat into work. A transmutation of heat 

 into work is observed in the expansion of bodies by heat, also in the 

 melting and evaporation of substances, external as well as internal work 

 being performed in these cases. However, it cannot be proved directly 

 and plainly, with bodies expanded by heating, that in the transformation 

 of heat into work a quantity of heat in proportion to the latter is used, 

 because no decrease of temperature, but, on the contrary, an increase of 

 it, takes place. We shall therefore be obliged to look for examples in 

 which for the work performed by a body a corresponding quantity of 

 the heat contained in it is consumed, and is thus diminished. 



This was the case in the following experiments by Gay-Lussac, and 

 at a later period by Joule. They introduced a current of compressed air 

 within an exhausted chamber; no loss of heat was perceptible, because 

 no work had been performed. There was however a loss of heat upon 

 the compressed air flowing into a chamber filled with air, because now 

 the pressure of the air had to be overcome ; consequently labor had to 

 be performed. Joule deduced from the amount of heat lost in this meas- 

 ured work the heat equivalent (1845). 



In steam-engines the steam is the transmitter of the heat, which per- 

 forms work indirectly. If the law of the mechanical equivalent of heat is 

 to stand, it has to be proved that the quantity of heat produced in the 

 steam of the boiler is greater than the amount of heat contained in 

 the condenser after the work has been done. Him, in Colmar, has 

 given this most difficult proof by highly ingenious experiments with 

 a steam-engine (1855), and Clausius deduced the heat equivalent from 

 these experiments (1855). 



We have now learned the possibility of the transformation of heat 

 into mechanical work; it can further be demonstrated that heat may be 

 transformed and again reduced into other actual or possible forms of 

 motion, which arc called light, electricity, magnetism, or chemical action. 

 For instance, by means of steam electric machines and thermopiles heat 

 is transformed into electricity. By means of the heat in a steam-engine,) 

 magneto-electrical machines may be made to rotate, and to produce 

 powerful electric currents; the latter again may produce mechanical, 

 chemical, magnetic, thermal, and optical effects ; electricity then no 



