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SOME PHYSICAL PROPERTIES OF COAL. BY BENJ. HOLGATE, F.G S. 
The behaviour of coal m the different circumstances under 
which it is used is so varied and complex, and yet of so much 
importance to our well-being, that I offer no apology for recording 
observations made with regard to its physical properties and behaviour. 
If we first examine the different conditions under which coals 
are used, and then examine the properties of the coals which have 
been found best suited to these conditions, we shall learn at least 
something of the general principles which should guide us in the 
selection of coals for our different re(piirements ; and, on the other 
hand, we may, from the behaviour of the coal whilst in use, gather 
some information as to its origin and the alterations to which it has 
been subjected since it was deposited. 
In this country some coals are used in open fire grates for 
domestic purposes which burn at a comparatively low temperature, 
some in boiler furnaces at a higher temperature for the making of 
steam, and others in reverbatory furnaces at a very high temperature 
for the heating or smelting of glass, iron, steel, &c. These are all 
used with a free admission of air. 
Again, other coals are used for gasmaking, which I may remind 
you is performed by the introduction of coals into an air-tight vessel, 
this, being heated, distils its contents, giving off gas and other sub- 
stances and leaving a residue of somewhat spongy coke. 
Other coals are used for nothing but making coke. In this case 
the coal is thrown into a chamber, the walls of which have been 
previously heated to redness, the opening is then built up so as to 
almost entirely prevent the admission of air. The heat of the 
chamber slowly communicates itself to the coals, in which operation 
their structure is entirely changed, some of the volatile gases are 
allowed to escape at a comparatively low temperature, the remainder 
forming new chemical combinations as they slowly permeate through 
the thick body of the coal and solidify into the coke, giving it density, 
hardness, and strength. The different kinds of coke are required for 
