of the Heat of the Oxyhydrogcn- flame. 69 



of the change, have for the most part a greater stability than the 

 original bodies. 



One evidence of this greater stability is afforded by the develop- 

 ment of a quantity of heat (the heat of chemical action) from the 

 produced bodies having a smaller potential heat than the original ones. 



It results, both from reason and experiment, that in order to undo 

 or reverse any definite chemical action, just so much heat must be 

 directly or indirectly expended as was evolved by the original action. 



For the same quantity of heat evolved, the resulting temperature 

 varies with the mass and kind of matter heated, and with the rapid 

 or gradual evolution of the heat. 



When the evolution of heat is instantaneous, the resulting tempe- 

 rature may be calculated from the quantity of heat evolved and the 

 mass and specific heat &c. of the matter heated. 



By a unit of. heat is meant the quantity of heat necessary to raise 

 the temperature of one kilogramme of water one degree Centigrade, 

 or, more accurately, from 0° to 1°. 



II. 



Every 18 grammes of water is a combination of two 1-gramme 

 proportions of hydrogen H, with one 16-gramme proportion of oxy- 

 gen O ; and by the combination of 2 grammes of hydrogen with 1G 

 grammes of oxygen, there are developed 68 units of heat. 



Of these 68 units of heat, however, little more than 57 units are 

 really due to the chemical action, nearly 11 units of heat being 

 evolved by the contraction of the original mixed gas into two-thirds 

 its volume of steam, and by the further condensation of the resulting 

 steam into 18 cubic centimetres of water. 



While the quantity of heat evolved by the combination of a given 

 quantity of oxygen and hydrogen is invariable, the intensity of the 

 heat may vary from a scarcely recognizable rise of temperature up 

 to the highest temperature of the oxyhydrogen blowpipe flame, ca- 

 pable of fusing platinum and silica. 



A most remarkable effect of the intense temperature resulting from 

 the combination of oxygen and hydrogen into water, is the partial 

 decomposition of water into oxygen and hydrogen, discovered by 

 Mr. Grove in 1846. 



At this high temperature hydrochloric acid and carbonic anhydride 

 gases also undergo partial decomposition, into hydrogen and chlorine 

 and into carbonous oxide and oxygen respectively. 



Upon what do these singular decompositions by heat, of bodies 

 formed with great evolution of heat, depend ? or with what class of 

 chemical phenomena may they be associated ? 



III. 



Under certain familiar conditions, chemical action seemingly takes 

 place to its utmost possible extent in a single direction only, with 

 production of a maximum amount of the substance that is formed 

 with maximum evolution of heat. 



For example, taking atomic proportions in grammes, the heat of 

 formation of chloride of zinc, ZnCl 2 , is 101 units; and the heat of 



