ARTIFICIAL ILLUMINATION. 157 
with explosive violence, as you hear, forming a very small quantity of 
highly heated water vapour which is ejected from the mouth of the 
flask. 
And now we come to the subject of the artificial production of light 
for practical purposes. What we almost universally want is white 
light. Let us consider for a moment the nature of light generally, 
and of white light in particular. We may say that light generali 
consists of rapid vibrations of the ether which pervades all space. If 
the vibrations are of a certain rapidity they may affect the eye with 
the sensation which we call red light; if they are of greater rapidity 
they may give us orange light, and so on through all the spectrum 
colours until we come to the waves which are of the greatest rapidity 
which the retina of our eyes can appreciate, and which give us the 
sensation of violet light. If rays emanating from a radiant body are 
of a less rapidity than those which give us red light, they are invisible 
* to our eyes, but they may give us the sensation of heat; if, on the 
contrary, they are more rapid than those producing violet light they 
are still invisible, but they may have a chemical or actinic effect. Now 
white light is the mixture of the visible colours of which the sunlight 
spectrum, or rainbow, is composed, and this white light is always, or 
nearly always produced by the incandescence, or glowing, of a highly 
heated solid body. If we put a poker in the fire, as it gets hot it 
radiates heat waves of low rapidity, but as yet gives no light; as it 
gets hotter, gradually waves of greater rapidity are given out and a 
dull red light begins to be seen; as the temperature rises the other 
colours of the spectrum are gradually added, and the light becomes 
whiter, until at last we have a pure white light evolved from 
the incandescent iron which is at very high temperature. The 
higher the temperature the more of the blue-violet rays, which are an 
important element in sunlight, are produced, and the whiter, in fact, 
is the light. If these blue-violet rays are absent to any degree the 
light has a colour determined by the predominance of the remaining 
colours of the spectrum, and is yellowish or reddish in its tone. What 
we desire in artificial illumination is pure white light, rich in blue rays 
like the sunlight, and, in order to obtain this, we must have our incan- 
descent solid body at the highest possible temperature. Look for a 
moment at the coloured papers which are hung on the wall in front of 
you. Why does this paper appear red to our eyes? It is because the 
nature of its surface is such that it practically stops all but the red 
rays which fall upon it from this white light which is illuminating it, 
and reflects only these red rays to our eyes. A similar explanation 
tells us why this other paper, is blue, and that other one is green. In 
order to see them in their true colours it is necessary to illuminate 
each with light which contains plenty of that particular ray which it 
has the property of reflecting. If, for instance, I illuminated this blue 
paper with light which contained no blue rays it could not reflect any 
rays at all to our eyes and would appear absolutely black. Let us try 
the effect of illuminating this row of variously coloured papers by alight 
which is rich only in yellow rays. I easily produce such a light, you 
see, by burping some spirits of wine on cotton wool sprinkled with 
