EVAPORATIVE POWER OF COAL. 27 



water is again taken, the highest reading being accepted. 

 The temperature of the water at the commencement should be 

 as near GOdeg. F. as possible: if too hot, the gases are not 

 properly cooled before they escape. If the test is properly 

 conducted, the contents of the copper furnace ought to dissolve 

 out in water and only leave a white ash ; but if there is any 

 black unburnt coal left, the ash and what remains of the coal 

 are collected, weighed and ignited in a platinum crucible, and 

 the loss deducted from the amount of fuel taken. A British 

 thermal unit is the quantity of heat necessary to raise lib. of 

 water one degree F. when at or near 39deg. F. It requires 

 967 B.T.IJ. to convert one unit of water at 212deg. F. into 

 steam at that temperature: in other words, the latent heat of 

 steam is 967deg. F. If each grain of fuel is burnt in the midst 

 of 967 grains of water, then since 30 grains of fuel are taken we 

 require 967 x 30, or 29,010 grains of water, so the glass cylin- 

 der is graduated to hold this quantity. By deducting the ori- 

 ginal temperature of the water in the cylinder from the tem- 

 perature after the experiment, we do not obtain the true heat- 

 ing power of the fuel, since the copper cylinders absorb a 

 certain amount of heat, and some more is lost by the imperfect 

 absorption, of heat by the water. By careful comparative tests 

 with the bomb calorimeter it is found that a fair correction is 

 to add one-tenth of the increase of heat to the difference be- 

 tween the temperature before and after the test. For 

 example 



Temperature of water after experiment .... 71 deg. F. 

 Temperature of water before experiment 60 deg. F 



11 deg. F. 

 Plus l-10tli of 11 deg. 1.1 deg. F. 



Evaporative Units 12.1 deg. F. 



which signifies that lib. of coal would evaporate 12. lib. of 

 water previously heated to 100 deg. C. or 212 deg. F. The 

 calorific power is better expressed in evaporative units than as 

 British thermal units, as the latter would give a large and un- 

 wieldy figure. 



When the results of an analysis are obtaned, one should 

 be able to interpret them so as to determine the value of the 

 fuel for the purpose required. 



Volatile Matter. The amount and quality of the volatile 

 combustible matter decides whether the coal is suitable for the 

 manufacture of illuminating gas or not. There should not be 

 too much or too little volatile hydrocarbons in a coal required 

 for coking, otherwise the coke will be too spongy or the walls 

 of the cells will be too thick. Coals that give off too much gas 



