Prof. Tyndall's Nates on Scientific History, 27 



is the heat usefully applied ; that is to say, this difference ex- 

 presses the heat which has been converted into mechanical effect. 

 The more perfect the machine, the less will be the amount of 

 heat obtainable from the condensation of the steam. The best 

 engines give a difference of about 5 per cent. ; that is to say, 

 100 lbs. of coal, burnt in such a machine, give no more heat 

 than 95 lbs. which are burnt without doing any work. 



(Considering the sort of criticism to which he has been re- 

 peatedly subjected*, the manner in which Mayer establishes the 

 result last mentioned is worthy of particular attention. It 

 will be observed that he deliberately chooses a substance which 

 experiment proves to be suited to his purpose — a substance, 

 that is, in which the whole of the heat rendered latent is consumed 

 in exterior work.) 



5. To prove this important proposition, we must investigate 

 the relationship of elastic fluids to heat and to mechanical work. 

 Gay-Lussac has proved by experiment, that when an elastic 

 fluid passes from one vessel into a second one of the same size, 

 but exhausted, the vessel from which the elastic fluid issues is 

 cooled, while that into which it enters is warmed by exactly the 

 same number of degrees. This experiment, which is distin- 

 guished for its simplicity, shows that a given weight and volume 

 of an elastic fluid may expand to double, quadruple, &c. of its 

 previous volume without experiencing any change of temperature, 

 or, in other words, that for the simple expansion of the gas 

 no expenditure of heat is necessary. 



6. Let a cubic inch of air at Of and under a pressure of 28 

 inches of mercury, be heated to 274°, and let the quantity of heat 

 required to warm the air be x. When it streams into another 

 exhausted recipient of the same volume, the air will retain its 

 temperature of 274°; the medium surrounding the vessels will 

 undergo no change of temperature. Again, let a cubic inch of 

 air, not at constant volume but under a constant pressure of 28 

 inches of mercury, be heated from 0° to 274°, a greater quantity 

 of heat is now needed than before: let the quantity be x + y. 

 If the air be permitted to cool in the two cases, it will give back 

 the heat communicated to it. The air which is not followed by 

 a pressure will, on cooling from 274° to 0°, give out the heat x; 

 that which cools under a constant pressure will yield the heat 

 x + y. 



7. Steam in the engine, where it expands under the piston, 



* As an example, see 'Good Words,' October 1862, p. 604, note: — 

 " Mayer's statements imply its indiscriminate application to all bodies in 

 nature, whether gaseous, liquid, or solid." Not what Mayer's words 

 " imply," but what they are is stated in the text. 



t All through his papers Mayer uses Centigrade degrees. 



