The burning. Of magnesium in a bell jar of oxygen is a familiar class room experiment. An intense bluish white light is produced and the compound known as magne sium oxide forms the residue of the combustion. This is the fundamental principle of ?ashlight photography. Atmospheric air not being pure oxygen — ir is, of course, known to all that it is diluted with another gas, nitro gen — we get a light of diminished brilliancy when the magnesium is burnt in the ordinary atmosphere, and so various bodies are mixed with it in order than its actinic efficiency may be increased by the presence of oxygen and other accelerators, if I may so term them. For, as Mr. Rau, of Philadelphia, long since pointed out, anything which increases the heat of the burning magnesium will also increase the actinic light. The ribbon or wire burns comparatively slowly when ignited in the ordi nary air, because the supply oi oxygen is diluted; but it glows brilliantly when burned in a jar of pure oxy gen. Hence, bodies which quickly and readily supply it with oxygen during its combustion increase its energy. But this increase of energy introduces the element of danger, or, in other words, the explosive catalytic, chlorate of potash.