THE NATIONAL GEOGRAPHIC MAGAZINE 



431 



Photograph from U. S. Bureau of Mines 



A TINY GUARDIAN OF THE MINER'S WELFARE 



As susceptible as men are to the overwhelming effects of mine gases, the canary bird is 

 much more so. The result is that in many disasters the birds are made the outposts of the 

 invading army of restoration. They are overcome long before man can detect the presence 

 of the gas and therefore warn the men of the dangers ahead. 



goes still higher in carbon, with 93 to 97 

 per cent. Then comes graphite, with still 

 more carbon, and finally the diamond. 



Here again Nature has shown us how 

 she made the different kinds of coal. 

 Occasionally in a bituminous bed we 

 come across a little section of anthracite, 

 and always there is basalt accompanying 

 it. In some great volcanic eruption liquid 

 lava was thrown out and it ran over the 

 bituminous coal, driving out, by its in- 

 tense heat, exactly as the coking process 

 does, most of the volatile matter and 

 transforming the bituminous coal into 

 coke, which under great pressure hard- 

 ened into anthracite. 



Again, if anthracite or coke be sub- 

 jected to the heat of an electric furnace, 

 as it is by the abrasive manufacturers at 

 Niagara Falls, it becomes an impalpable 

 black powder. So, also, in the earth do 



we find places where anthracite under- 

 went such intense heat that even the little 

 gas it contained could not resist expul- 

 sion, with the result that the anthracite 

 became graphite, which is widely mined 

 and which the world uses alike for lubri- 

 cating machinery, making lead pencils, 

 polishing stoves, and shining shoes. 



That the diamond, the head of the car- 

 bon household, was formed in the pres- 

 ence of iron, under tremendous pressure, 

 was a theory arrived at by M. Henri 

 Moisson, an eminent French chemist. 

 Analyzing a great number of small stones, 

 he found always a trace of iron present. 

 He held that molten iron, cooling in the 

 presence of carbon deep in volcanic 

 depths, where there was little elbow room 

 for it to undergo expansion in assuming 

 a solid form, would exert a tremendous 

 pressure upon the particles of carbon it 



