132 SCIENTIFIC APPARATUS. 



any thermodynamic engine, with the same temperatures of source and 

 refrigerator, from a given quantity of heat. 



It is to be particularly observed here that Reversibility is the 

 sole test of perfection of an engine. Also in the working of a 

 reversible heat-engine nothing is said about the nature of the 

 working substance ; the temperatures of source and refrigerator, 

 and the quantity of heat supplied, are the sole determining factors 

 of the work which can be done. The importance of this proposi- 

 tion, as regards actual and proposed engines, cannot be over- 

 estimated. 



II. EFFECTS OF HEAT. 



These are so numerous that we can attempt to classify only a 

 few of the more important. For, in fact, far the greater part of 

 the energy which is at our disposal is due to the past or present 

 radiation from the Sun. Even Light is merely a high form of 

 radiant heat, but, except in so far as it is a form of energy it does 

 not come in for consideration in the present branch of our subject. 

 Without the radiant energy which the sun now supplies us, or has 

 supplied us in bygone times, our whole stock would consist 

 only in Tidal Energy, the Earth's internal Heat, and Primaeval 

 Potential Energy of uncombined chemicals such as native iron, 

 native sulphur, &c. The first of these is the only one which 

 promises to be of much use, but it has not to our knowledge been 

 utilised, at least to any great extent. 



We may classify the Effects of Heat as follows : 



(a) Change of Dimensions and Stresses of Solids, and of Volume 



or Pressure of Fluids. 



(b) Change of Molecular State. 



(c) Change of Temperature. 



(d) Electric Effects. 



(e) Chemical Changes. 



Let us consider these in order. 



