498 PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 



heat is furnished to it from outside. In the first instance the velocity 

 of the oscillating molecules will be increased, consequently the kinetic 

 energy of the latter is intensified; this fact manifests itself by the rising 

 of the temperature of the body. We will call the work employed in this, 

 measured in work-units, (according to Zeuner,) the oscillation-work. 

 There is besides a work process going on in the interior. This produces 

 a change, as a rule the extension of the mean intervals, i. e., the medium 

 distances of the oscillating, corporeal atoms, whereby in the expansion 

 of bodies by heat the particles of bodies recede from each other, or they 

 even entirely change their places, as is the case in the processes of melting, 

 boiling, and evaporating. There is, lastly, an exterior work connected 

 with the expansion or aggregation change of bodies by heat in general, 

 as usually the pressure of the atmosphere, or some other pressure on the 

 surface of the body from without, has to be overcome. This exterior 

 work produces the useful effect of caloric and steam engines. 



The eutire work done by a certain amount of heat is equal to the sum 

 obtained by the addition of the exterior and interior amount of the 

 oscillation work. In assuming an infinitely small change of the body 

 by the increase or decrease of an infinitely small amount of heat we 

 assume its changeableness, and so succeed in subjecting those quantities 

 to mathematical calculation, and in deducing thereby those truths of 

 the mechanical theory of heat which are of such high importance, theo- 

 retically and technically ; in regard to this subject we would refer to 

 the works of Clausius, Zeuner, and Hirn. 



According to the mechanical theory of heat the true nature of specific 

 heat will be easily understood. In specific heat, the variable assumes 

 smaller values with reference to oscillation-work (temperature) for the 

 addition of a certain amount of heat, as the specific heat of the body is 

 greater; that is, if the body requires more heat both as to exterior and 

 interior work, for the variable. In melting, boiling, and evaporating the 

 further change of oscillation-work under uniform pressure from without 

 is zero, that is, no increase in the thermometrical indication is obtained, 

 beeause all the work employed is expended in the dispersion or segre- 

 gation of the substance. 



In regard to the latter point it is necessary to be more explicit in 

 order to be understood. If a mercurial thermometer is placed in melt- 

 ing ice or snow the quicksilver of the thermometer will soon reach 0° 

 C. (32° ¥.), where it will stand until all the ice is melted ; it will be the 

 same with other melting bodies. The melting temperature of every 

 substance remains constant while it melts. 



The same will be the case in boiling fluids. The temperature at which 

 a fluid begins to boil, and continues to do so, is called its boiling tem- 

 perature or its boiling point. This varies with the material quality 

 of the fluid, and also lias relation to a fixed pressure of the air. As long 

 its a fluid boils with the pressure unchanged, its boiling point remains 

 constant; no matter how much the fire is enlarged, there will only be 



