Mechanical Effect from Chemical Forces. 3 



has demonstrated that if the heat evolved by compressing an 

 elastic fluid be equivalent to the force absorbed in the cQpipres- 

 sion, the proportion of heat converted into mechanical efiect by 

 any perfect thermo-dynamic engine will be equal to the range of 

 temperature divided by the highest temperature from the abso- 

 lute zero of temperature. Therefore, if in a perfect steam-engine 

 a be the temperature of the boiler from the absolute zero, and h 

 be the absolute temperature of the condenser, the fraction of the 

 entire quantity of heat communicated to the boiler which will be 



converted into mechanical force, will be represented by — — , 



which is analogous to the fraction representing the proportion of 

 chemical force converted into mechanical effect in the electro- 

 magnetic engine*. The extreme simplicity of this very imports 

 ant deduction which Professor Thomson has drawn from the 

 dynamical theory of heat, is of itself a strong argument in favour 

 of that theory, even if it were not already established by decisive 

 experiments. 



Now, estimating the heat generated by the combustion of a 

 grain of coal at l°-634 per lb. of water, its absolute mechanical 

 value will amount to 1261*45 foot-pounds; hence, according 

 to Professor Thomson^s formula, the work performed by any 

 perfect thermo-dynamic engine will, for each grain of coal 

 consumed, be represented by the equation 



^^, 1261-45 {a-b) 

 a 



which applies, as before intimated, not only to air-engines, 

 but also to those steam-engines in which the principle of ex- 

 pansion is carried to the utmost extent, providing always that 

 no waste of power is allowed to take place in friction, and that 

 the entire heat of combustion of the coal is conveyed to the 

 boiler or air-receiver. 



Professor Thomson was the first to point out the great 

 advantages to be anticipated from the air-engine, in consequence 

 of the extensive range of temperature which it may be made to 

 possess; and in a paper communicated to the Royal Society 

 soon afterwards, 1 described a very simple engine which fulfills 

 the criterion of perfection according to Professor Thomson's 

 formula. This engine consists of three parts, viz. a condensing 

 air-pump, a receiver, and an expansion cylinder; the pump 



* Referring to this analogy, Professor Thomson writes as follows : — " I 

 am incHned to think that an electric current circulating in a closed con- 

 ductor is heat, and becomes capable of producing thermometric effects by- 

 being frittered down into smaller local circuits or * molecular vortices.' " — 

 Letter to the Author, dated March 31, 1852. 



B2 



