78 Relations between Chemical? Change, [January, 



the further question of the economy of electro-dynamic 

 engines. 



Let us first examine the first part of the theory — namely, 

 that a definite amount of chemical change is interchangeable 

 with a definite amount of heat and of electric force. I think 

 we may safely grant that the doctrine of the equivalence of 

 definite amounts of chemical change, heat, and electric force, 

 is true when the chemical change takes place under precisely 

 the same circumstances. This may be true ; but it is a 

 truth practically useless. 



2. But does a definite amount of chemical change 

 correspond with the same definite amount of production of 

 heat and electric force under different circumstances, say of 

 atmospheric pressure, surrounding temperature, contiguity 

 of different substances, and other external influences ? 

 There may, or may not, be a priori reasons why it should, 

 and such have been urged as conclusive reasons derived from 

 the axiom of conservation of energy ; but I think I shall 

 show that there are practical reasons for doubting the 

 universal applicability of this principle except in mere 

 mechanics. 



3. But, before doing so, let me quote an instance which 

 shows the great practical importance of the question. Take 

 the case of the production of iron by the hot and cold blast. 

 With the cold blast certain quantities of ore, flux, and 

 fuel are mixed together; combustion is induced, cold air 

 passed through the mixture, and the various chemical com- 

 positions and decompositions take place. With the hot 

 blast, part of the fuel is used first to heat the air which 

 passes through the furnace, and part is placed as before in 

 the furnace. It is found that in this way a much greater 

 effect is produced by a smaller quantity of fuel. This, 

 of course, does not prove that more total heat is thus 

 produced, but it shows that it is produced in a more effective 

 manner. 



4. Now let us take the case of the production of heat in a 

 galvanic battery. The theory at present generally accepted 

 is this : — 



(1). That the total amount of heat produced in a circuit 

 depends upon the amount of zinc or other metal oxidised, 

 though the distribution of the heat in this circuit depends 

 upon the resistance of the several parts ; being directly pro- 

 portioned to this resistance. Thus, if H be the amount of 

 heat and nM the units of weight of the metal consumed, 

 HanM. And if I be the quantity of electricity circulating in 

 each section of the circuit and r lt r 2i the resistance of two 



