But as the mean density exceeds that of water, and probably falls but little from the centre to the surface, these gases are gases only in the sense that if the pressure of neighbouring and outward parts gravitating towards the centre were relaxed, they would expand explosively, as we see happening in the eruptive prominences.
It burns with a brightly luminous flame, and is spontaneously inflammable at about too° C. When mixed with oxygen it combines explosively if the mixture be under diminished pressure, and is violently decomposed by the halogens.
All four of the halogens unite with hydrogen, but the affinity for hydrogen decreases as the atomic weight increases, hydrogen and fluorine uniting explosively at very low temperatures and in the dark, whilst hydrogen and iodine unite only at high temperatures, and even then the resulting compound is very readily decomposed by heat.
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