i2 4 POPULAR SCIENCE MONTHLY. 



and so the scale of temperature thus defined may be termed absolute. 

 Moreover, such a scale leads at once to the idea of an absolute zero, for 

 no engine could be supposed to convert more heat-energy into work 

 than it received; and, choosing the temperature of the refrigerator 

 (calling it T), so that no heat-energy is rejected by the engine, but all 

 the heat-energy taken from the source is turned into work, it is impos- 

 sible for T to be negative, else the engine would have an efficiency 

 greater than 100 per cent. Therefore zero is the smallest algebraic 

 value the temperature of the refrigerator can have, and temperatures 

 reckoned from this zero are called absolute. The size of the degrees 

 on this scale is arbitrary, and has been conveniently chosen so that 

 there are one hundred degrees between the temperatures of boiling 

 and freezing water. If now a reversible engine be worked between 

 these temperatures and the quantities of heat-energy received and 

 rejected be measured, the temperature of boiling water on the absolute 

 scale may be found. It is not necessary actually to try the experiment, 

 for the work done by the expansion of a substance which obeys the 

 ordinary laws of gases may be calculated by the methods of the infini- 

 tesimal calculus. In this way Kelvin's thermodynamic scale has been 

 shown to be practically identical with that of a perfect gas ther- 

 mometer, which shows the absolute zero of the thermodynamic scale 

 to lie about 273 degrees below the zero of the Centigrade scale. 



Professor Tait has said : ' ' Without this work of Carnot, the modern 

 theory of energy, and especially that branch of it which is at present by 

 far the most important in practice, the dynamical theory of heat, could 

 never have attained in so few years its now enormous development. 

 Carnot 's claims to recognition are of an exceedingly high order, because 

 they depend not merely upon his method, which is one of startling 

 novelty and originality and is not confined to the subject of heat; but 

 upon the fundamental principle upon which he based his mode of com- 

 paring the heat employed with the work procured from it. Every 

 reasoner who has applied himself to the subject of heat since Carnot 

 has gone right so far as he attended to Carnot 's principle; but has 

 inevitably gone wrong when he forgot or did not attend to it." 



The two things which Carnot introduced, which were entirely orig- 

 inal with him and which left his hands in an almost perfect form, were 

 the idea of a ' Cycle of Operations ' and the further idea of a ' Eeversible 

 Cycle,' giving also the notion of a 'Eeversible and Perfect Engine,' 

 showing that the efficiency of such depends only on temperature. 



The peculiar merit of Carnot 's reasoning consists in the idea of 

 bringing the body back to its initial state as to temperature, density 

 and molecular condition, after a cycle of operations, before making 

 any assertion as to the amount of heat-energy gained or lost. This he 



