MECHANICAL EFFECT FUOM CHEMICAL FORCES. 



175 



one degree, is equal to the mechanical force which is able to 

 raise a weight of 772 lbs. to the height of one foot, or 

 according to the expression generally used, to 772 foot- 

 pounds. Therefore the work developed by a grain of zinc 

 consumed in a Daniell's battery is given by the equation, 



a 

 We now come to the third class of engines, or those in 

 which the chemical forces act through the intervention of 

 heat. In the most important of these the immediate agent is 

 the elasticity of vapour or permanently elastic fluids. In 

 a very valuable paper on the dynamical theory of heat, 

 Professor Wm. Thomson has demonstrated that if the heat 

 evolved by compressing an elastic fluid be equivalent to the 

 force absorbed in the compression, the proportion of heat con- 

 verted into mechanical effect by any perfect thermo-dynamic 

 engine will be equal to the range of temperature divided by 

 the highest temperature from the absolute zero of tem- 

 perature. Therefore, if in a perfect steam engine a be the 

 temperature of the boiler from the absolute zero, and b 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 

 a 



proportion of chemical force converteu into mechanical effect 

 in the electro-magnetic engine.* The entreme simplicity of 

 this very important deduction which Professor Thomson has 

 drawn from the dynamical theory of heat, is of itself a strong 

 argument in favor of that theory, even if it were not already 

 established by decisive experiments. 



• Referring to this analogy, Professor Thompson writes as follows : — " I am 

 inclined to tbink that an electric current circulating in a closed conductor w heat, 

 and becomes capable of producing tliermometric effects by being frittered down 

 into smaller local circuits or 'molecular vortices.'"— Letter to the Author, dated 

 March 31st, 1852. 



