Chemical and Physical Papers. 91 



the application of a flame. It was found that some dusts are much 

 more explosive than others, and that when the dust is produced by 

 pulverizing coal the least friable portion of the coal is the most 

 explosive. The samples of the more and less friable portions have 

 been examined under the microscope and the size of the individual 

 grains measured, and it has been found that the more explosive 

 portion contains a large portion of very fine grains, averaging about 

 003 of a millimeter in diameter, while the less explosive portion 

 consists mostly of larger grains. By most explosive it is meant 

 that a lower proportion of coal-dust to air is explosive. It will be 

 seen that the surface of a grain 0.003 of a millimeter in diameter is 

 very large in comparison with the volume, and that the opportunity 

 •for the occlusion of gases is very favorable; and also that such a 

 grain could be heated in a very short time to the point at which 

 gases would be distilled. 



In the case of the only sample which has been obtained which 

 had not been exposed to the air for an appreciable length of time 

 it was found that the dust was not explosive, but that after about 

 four hours' exposure to the atmosphere it became explosive. This 

 sample was from the state mine at Lansing, and consisted of drill 

 dust taken from the solid coal, none of which came from coal lying 

 within three feet of the exposed face. Dust made by grinding 

 lumps of this coal which had been exposed for a considerable. time 

 had been previously found to be explosive. When this fresh dust 

 was collected it was sealed in a metal can. The can was so opened 

 as to collect any gas which might have been given off by the coal. 

 No such gas was found. Investigations are now under way to de- 

 termine what change takes place in the coal during its exposure to 

 the atmosphere. These investigations are incomplete, but show 

 that oxygen is absorbed from the air. This absorption takes place 

 even under a negative head of six inches of mercury. 



The explosiveness of some of the dusts has also been tested in 

 the presence of small quantities of natural gas, and it has been 

 found that in some cases, in which both the gas and the dust are 

 present in quantities too small for either one alone to be explosive, 

 the combination is explosive. The same test was made using car- 

 bon monoxid and the same result obtained. It is therefore evident 

 that the presence of even a small amount of explosive dust may be 

 a source of great danger in case the mine air should contain a 

 small amount of natural gas, or of combustible gases from any other 

 source. And it should be noted that the safety-lamp which is com- 

 monly used for testing for gas indicates nothing less than about 



