57 
Sir H. Davy's researches on flame. 
brought into contact with a mixture of oxygene and hydro- 
gene, it only rarefies them, but does not explode them; but 
this depends upon the degree of heat communicated by the 
coal : if it is red in day light and free from ashes, it uniformly 
explodes the mixture ; if its redness is barely visible in shade, 
it will not explode them, but cause their slow combination : 
and the general phenomenon is wholly unconnected with rare- 
faction, as is shown by the following circumstance. When the 
heat is greatest, and before the invisible combination is com- 
pleated, if an iron wire heated to whiteness be placed upon the 
coal within the vessel, the mixture instantly explodes. 
Light carburetted hydrogene, or pure fire-damp, as has 
been shown, requires a very strong heat for its inflammation; 
it therefore offered a good substance for an experiment on 
the effect of high degrees of rarefaction by heat on combus- 
tion. I mixed together one part of this gas and eight parts 
of air, and introduced them into a bladder furnished with a 
capillary tube. I heated this tube till it began to melt, and 
then slowly passed the mixture through it into the flame of 
a spirit lamp, when it took fire and burned with its own 
peculiar explosive light beyond the flame of the lamp, and 
when withdrawn, though the aperture was quite white hot, 
it continued to burn vividly. 
That the compression in one part of an explosive mixture 
produced by the sudden expansion of another part by heat, 
or the elestric spark, is not the cause of combination, as has 
been supposed by Dr. Higgins, M. Berthollet, and others, 
appears to be evident from what has been stated, and it is 
rendered still more so by the following facts. A mixture of 
MDCGCXVII. I 
