430 , Rev. Mr, Eminett on Combustion, [Dec. 



substances," than to propose limitations, which make a number 

 of laws of action out of one ; and then we must consider every 

 case of chemical combination which excites the temperature of 

 ignition, as a case of combustion ; whether the resultant be an 

 oxide, chloride, ioduret, sulphuret, phosphuret, or whatever else 

 it may be. It is also universally allowed that the caloric results 

 from, or is evolved in consequence of the condensation which 

 takes place ; * therefore its origin is not to be traced to either 

 substance singly, but to a diminution of the specific heat which 

 is produced by the condensation ; and hence the specific t heat 

 of the product of combustion is less than the sum of the specific 

 heats of its component parts. The capacity for heat is generally 

 less than the sum of the capacities ; and the difference between 

 the sum of the specific heats before combustion and that of the 

 product, whether oxide, chloride, sulphuret, &c. is a measure of 

 the heat evolved during the combustion ; yet in the present state 

 of chemical science, the change which takes place in the capa- 

 city cannot be made a measure of the intensity of the force, nor 

 of the quantity of heat evolved : for let a be any very small 

 interval of temperature ; at the first interval from the true zero, 

 the temperature will be a, and let the capacity or quantity of heat 

 absorbed in elevating the body to that temperature be c ; when 

 the temperature is a + a, or 2 a, let the capacity be c^ ; at tem- 

 perature 2 a •{- a = 3 a, let the'capacity be c'',J and so forth, to 

 any assumed temperature. Now at any given temperature, 7^ a, 

 the specific heat is the sum of the quantities which have been 

 absorbed, i. e. the sum of all the capacities =zc -{- c' -i- c" + d'^ 

 -f- &c. + c'"" * ; whilst the capacity at that temperature is simply 

 c"*~^ If then we make the temperature the abscissa of a curve, 



• I am aware that the phenomena of gunpowder, of oxymuriate of potash (I use the 

 old-fashioned words, not being able to discover any advantage of potassa over potash, 

 platinum over platina) mixed with sulphur or other inflammables, and of similar com- . 

 pounds are apparently irreconcilablewith this statement. But we must remember that 

 the same gas, as it exists in different solid compounds, has net a constant density (as 

 will appear towards the conclusion of this paper) ; and the quantity of heat remaining 

 will be the greatest where the gas is least condensed, i. e. when united with bodies which 

 have least attraction for it generally. Now in the manufacture of the oxymuriate, or 

 hyperoxymuriate, or chlorate of potash, little heat is evolved ; hence the gas retains 

 much of its elastic force, or is but slightly condensed ; hence it retains much of itg 

 native energy. Similarly, nitrate i)f potash, or of copper, or silver, retains much of the 

 energy of oxygen ; but when united to iron or zinc, little remains. Here we recog- 

 nise one law of action : when a gas, which is a supporter of combustion, enters into 

 combination with solid matter, and is not highly condensed, it must retain much of its 

 peculiar energy ; and hence, much gas, containing but little specific heat, may be 

 evolved, even by the combustion of a mixture of solid matters. Nitre must contain 

 much specific heat ; for azote is not inflammable : its union with oxygen requires the 

 aid of external heat : the union of the gaseous compounds of azote with oxygen evolve 

 little heat ; the gas is not highly condensed ; therefore the acid in nitre retains much 

 ©f its energy. 



-|- By specific heat I mean tlie whole quantity of caloric or latent heat which is con- 

 tained in a body. I mention this, because by many writers, specific heat and capacity 

 for heat are made synonimous. 



X ^-i^-i c"} c'» - » are supposed to increase or decrease according to some definite law. 



