SECTION III. 
OF THE GENERATION AND CONDENSATION OF STEAM, 
AND THE APPARATUS FOR THOSE PURPOSES. 
Art. 176. Steam is generated by the application of heat, and it is condensed by 
cold (art. 71.); and we have now to consider the best sources for obtaining the 
heat for its generation with economy, and the means of applying it so as to obtain 
the most effect. Our section therefore naturally divides itself into an inquiry con 
cerning combustion and fuel ; the effect of and application of fuel ; the structure 
of boilers and fire-places ; the principles of condensation, and the apparatus. 
Of Combustion and Combustibles. 
177. There are various substances, which, when heated to a certain tempera 
ture depending on their nature, begin to give out heat, and continue to do so till 
the whole of the substance be completely changed into new products, most of them 
gaseous, which in ordinary cases are dissipated in the atmosphere. A substance 
which undergoes this change is termed a combustible, or burning body ; and if it be 
commonly used for producing heat, it is called fuel. 
178. The quantity of heat given out during combustion is the difference 
between that which the matter operated upon contained before, and that which it 
contains after combustion. This is an invariable quantity when the same quantity 
of matter is operated upon, and proportional simply to the quantity of fuel used ; 
unless indeed the process be imperfectly managed, or that we could render the 
products of such a nature, that they would contain a less quantity of heat than 
those usually produced. The latter would perhaps be a fruitless research ; but 
chemistry is making rapid advances in the means of fully establishing this point. 
It is however of the utmost importance, since the application of steam to naviga 
tion, to determine the effect of the mixture of combustible bodies, both with a view 
to fix on those which contain most heat to a given quantity of matter, and to 
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