184.8. J 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



275 



naval purposes. It was intended, as being most convenient for 

 the inquiry, to have adhered strictly to districts. In the experi- 

 ments tliis has hitherto been done, except in special cases at the 

 request of the Admiralty. 



The following table (Table II.,) contains an abstract of the 

 results, so far as regards the evaporative value of the fuel. This 

 table relates only to the economical value of the coals examined, 

 and to the steam generated by a unit of the respective coals, 

 without however implying a unit of time. The details with refer- 

 ence to time, which forms a most important element in the value 

 of the respective fuels, will be given hereafter. 



Table II. — Showing the Economic Values of the Coals. 



The economical results obtained by evaporation in the best- 

 applied practice are ascertained to be only a small part of the 

 theoretical result following from the actual quantity of heat capa- 

 ble of being generated. Still, as a comparative statement, it is 

 necessary to contrast the economical heat given out by a coal with 

 the theoretical quantity. The cause of tlie difference between the 

 aj)plied and theoretical quantities is, at least in a great degree, 

 obvious, and does not by the apparent difference prove the fallacy 

 of calculation. Before the comparison can be made, it is neces- 

 sary to liave a knowledge of the composition of the respective 

 coals : of this we subjoin a table. — {See Table III.) 



Chemists differ as to the mode of calculating the theoretical 

 heating values of coals, but, as an approximative rule, without in- 

 sisting on its absolute accuracy, tlieir calorific values are found to 

 stand in relation to the quantity of oxygen required for their com- 

 plete combu.stion. This may be estimated experimentally by 

 heating the coal with an excess of litharge ; or it may be deter- 

 mined by calculation from the known equivalents of the com- 

 bustible ingredients of the coal. From the quantity of lead 

 reduced by the coal, the oxygen employed in its combustion may 

 be estimated, and the calorific values stand in direct relation 

 to this quantity. The amount of oxygen necessary to consume the 

 combustible constituents may more accurately be determined 

 by elementary analysis ; and thus calculated, the results are gene- 



Table III. — Showing the Mean Composition of average samples of the Coals. 



* Included in the Ash. 

 rally found to be about ^ greater than those indicated by experi- 

 ment with the litharge. Tiie calculation from the elementary 

 analysis depends upon the circumstance, that 6 parts, or one equi- 

 valent, of carbon requires IG parts, or two equivalents, of o.xygen 

 for combustion, while 1 part of hydrogen requires 8 parts of 

 oxygen ; it is only necessary, therefore, to subtract from the 

 hydrogen a quantity corresponding to the oxygen contained in the 

 coal to enable the calculation to be made on these principles. 



As the calorific values are only relative, it is useful to refer 

 them to the heating power of pure carbon, one part of whicli 

 requires 2'666 parts of oxygen for combustion, and is capable, ac- 

 cording to Despretz, of heating 78^15 parts of water from its 

 freezing to its boUing point. The calculation may be simplified 

 by multiplying each part of lead obtained by 2-265, which gives at 

 once the weight of water capable of being heated between these 

 temperatures by a unit of the coal used in reducing the litharge. 

 On these principles the following table is constructed. — (iVe 

 Table IV. 



With regard to the practical application of fuel, such a table 

 could not supersede experiment, as tlie economical values of the 

 coal depend also on adventitious circumstances connected with 

 their physical as well as their chemical condition. This table, 

 while on the whole it agrees with and confirms the practical 

 results of experiments, still differs in a marked degree in one or 

 two instances : this difference arising as much from the chemical 

 as from the physical dift'erences of the coals. Thus, if by destruc- 

 tive distillation, which occurs in furnaces before combustion, a 

 large quantity of the constituents of the coal are rendered gaseous, 

 so much heat is expended in this act that the heat developed by 

 their after-combustion is frequently not greater than that ab- 

 stracted during their formation, in which case a thermo- neutrality 

 occurs. To ascertain the proportion of fixed and volatile products 

 in the various coals, a very difficult and elaborate process 

 was adopted ; but the tediousness and chances of failure in this 

 kind of analysis have induced us to include only a limited number 

 of coals (those given in Table V.), especially as for steam purposes 

 it was sufficient to determine the per centage of coke, as stated 

 in Table II. 



30* 



