Page:The wonders of optics (1869).djvu/260

caused by the carbon and hydrogen in a state of luminous vapour, which only have a few of the colours of the spectrum, which, when passed through the prism, fall into their proper places. All substances with which we are acquainted are capable of being converted into luminous vapour by means of heat, and when thus burnt produce flames of more or less faint luminosity, generally characteristically coloured. A piece of soda inserted in the wick of a spirit lamp gives a yellow tinge to the flame; a morsel of saltpetre (nitrate of potash) or nitrate of strontia will give a purple and crimson tint respectively. These hues are caused by the metals sodium, potassium, and strontium contained in these salts being converted into luminous vapour. On analyzing these coloured flames with a prism, as before, we should find in the case of the soda a single broad yellow line, situated just in the middle of the yellow portion of the spectrum, the rest of the space where the spectrum should be being perfectly dark. The reason of this is pretty simple. Sodium burns with a pure yellow flame, consequently when passed through a prism it cannot split into any other colours, but takes its place in the position belonging to yellow of that particular hue. Were it a little more orange or green in tint, it would take its place nearer to the red or violet end of the spectrum. The light from saltpetre, which contains potassium may next be examined. It will be found to tinge the flame with the spirit-lamp of a beautiful purple. We can almost guess what will happen when this flame is submitted to the action of the prism. We shall find that the purple light emitted will split into red and violet, which will immediately arrange themselves in their proper positions according to their hues. If in like manner we substitute nitrate of strontia for saltpetre, we shall get a splendid crimson flame which is decomposed by the prism into red, orange, or blue.