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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