MAKING DARKLESS OUT OF DAYLIGHT.

King Country Chronicle, Volume V, Issue 392, 2 September 1911, Page 7

MAKING DARKLESS OUT OF DAYLIGHT.

+ THE UNIVERSE IS BLACK. The notion of making darkness, in :hunks, so to speak, out of ordinary laylight, seems rather picturesque, [f it be understood that the darkness is of so real and permanent a character that no light, no matter now powerful, can possibly illuminite it, the phenomenon becomes exceedingly interesting. Yet it is en:irely practicable to manufacture such darkness. Science has now learned how to produce it very easily ; and you yourself can make it, if you wish, without much difficulty. The receipt, is simple enough. In order to make a piece of absolute and permanent darkness, you must have, air that is optically pure. This means that it must contain neither dust nor moisture. In a properly equipped laboratory such air would be obtained by passing a quantity of ordinary air through a white-hot platinum tube, which burns up the dust and decomposes the moisture particles. Suppose that you have a glass receiver, filled with optically pure air obtained in this manner. If a ray of electric (or other light) be sent through it, it is not illuminated. The interior of the receptacle remains dead black. It is filled, in a ;vord, with permanent and absolute darkness.

You can make the experiment for yourself, however, in a much easier and simpler way. Take a coverless wooden bos and stand it on a table, on one of its sides. Replace the absent cover with a pane of glass ;and make two little windows (likewise with small panes of glass set in them) in the opposite ends of the bos. On the side which rests upon the table spread a piece of cloth that has been saturated with glycerine. This, of course, on the inside of the box—into which you must also introduce £.< few bits of lime. A BOXFUL OF BLACKNESS. Leave the box alone for five or six days, and at the end of that rime the air it contains will be optically pure. All of the moisture that was in it will have been absorbed by the lime, and all of the dust particles will-hava fallen to the bottom and clung to the glycerine. S T ow you are ready for your experiment.

Place a candle (better still an incandescent bulb) outside one of the little windows, and, with the help of an ordinary hand mirror, try to throw a ray of light through the box. The experiment, of course, ought to be performed in the dark. As a result, it will be seen that the ray enters one little window and comes out at the other, but that portion of it which passes through the box is invisible. The light enters and the,' light comes out, but. the interior of the box —as you look into it through the glass front—remains perfectly black.

; All the light in the world would not illuminate the boxful of blackness. !It is absolute and permanent dark- ! ness —the same kind of darkness as I that with which (as will presently ;be shown) all interstellar space is i filled. It is black for the same reason —because it contains no particles i of dust or moisture to reflect light. Light may pass through it, but cannot illuminate it. The real interest of the little ex- ■■ periment described lies in its bearing upon the great and wonderful phenomenon of the blackness of the universe. To us on the earth it seems as if all of interstellar space was ' filled with light. But such is not the case at all. On the contrary, it is filled with darkness —a darkness | so absolute that nothing, can illumin- ; ate it.

Persons who have ciimed very high mountains' have often testified that the most striking thing they observed had to do with the colour of the heavens. The sky, as they reached great . altitudes, lost its familiar blueness and became black. If they had been able to ascend a few miles farther, the sky would have become absolutely black, and they would have seen the stars shining brilliantly, even though it were mid-day. WHY STARS CANNOT BE SEEN BY DAY. The general illumination, characteristic of daylight, by which we a re surrounded, is an effect due to the thin gaseous veil called the atmosphere which envelops the earth. This envelope of gases is full of tiny particles of dust and moisture, which reflect and distribute the rays of the sun in all directions. But, at an elevation of a few miles, there is comparatively little dust and moisture, and so the blackness that is overhead begins to appear. The scattering of the sun's rays by the particles of dust and moisture in the atmosphere, while producing the effect of general illumination with which we are familiar, prevents us from seeing the stars in the day time. But. if, even at mid-day, we were able to rise 100 miles above the earth, the stars, as we behold them, would shine with extraordinary brightness. The sun, too, would be far more brilliant than it has ever seemed to us ; and, instead of being reddish-yellow, it would appear, as it really is, of a beautiful blue colour.

But though sun and stars would blaze so brilliantly, it would be seen that all. space was filled with blackness—a veritable void of darkness so profound as to be insusceptible of illumination. With neither dust nor moisture, there is nothing in it to reflect light. It is naught but sb&er emptiness, through which: light

may pass, but ""which can "have no brightness of its own. The dust in the air is not visible to our eyea under ordinary circumstances But let a sunbeam strike ini o a dark room, through a crack in a shutter, perhaps, and immediately it is sem to be filled with floating particles that are in constant motion.—"Popular Science Siftings."