The case is not as you suppose: a faint light is emitted which is very discernible in the dark; but the heat evolved is not sufficiently strong to be sensible: a whitish vapour arises from this combustion, which, uniting with water, condenses into liquid phosphorus acid. CAROLINE. Is it not very singular that phosphorus should burn at so low a temperature in atmospherical air, whilst it does not burn in pure oxygen without the application of heat? MRS. B. So it at first appears. But this circumstance seems to be owing to the nitrogen gas of the atmosphere. This gas dissolves small particles of phosphorus, which being thus minutely divided and diffused in the atmospherical air, combines with the oxygen, and undergoes this slow combustion. But the same effect does not take place in oxygen gas, because it is not capable of dissolving phosphorus; it is therefore necessary, in this case, that heat should be applied to effect that division of particles, which, in the former instance, is produced by the nitrogen. EMILY. I have seen letters written with phosphorus, which are invisible by day-light, but may be read in the dark by their own light. They look as if they were written with fire; yet they do not seem to burn. MRS. B. But they do really burn; for it is by their slow combustion that the light is emitted; and phosphorus acid is the result of this combustion. Phosphorus is sometimes used as a test to estimate the purity of atmospherical air. For this purpose, it is burnt in a graduated tube, called an _Eudiometer_ (PLATE XI. fig. 2.), and from the quantity of air which the phosphorus absorbs, the proportion of oxygen in the air examined is deduced; for the phosphorus will absorb all the oxygen, and the nitrogen alone will remain. EMILY. And the more oxygen is contained in the atmosphere, the purer, I suppose, it is esteemed? MRS. B. Certainly. Phosphorus, when melted, combines with a great variety of substances. With sulphur it forms a compound so extremely combustible, that it immediately takes fire on coming in contact with the air. It is with this composition that phosphoric matches are prepared, which kindle as soon as they are taken out of their case and are exposed to the air. EMILY. I have a box of these curious matches; but I have observed, that in very cold weather, they will not take fire without being previously rubbed. MRS. B. By rubbing them you raise their temperature; for, you know, friction is one of the means of extricating heat. EMILY. Will phosphorus combine with hydrogen gas, as sulphur does? MRS. B. Yes; and the compound gas which results from this combination has a smell still more fetid than the sulphuretted hydrogen; it resembles that of garlic.