vividly different as are those which our railway people find necessary.  There is a particularly beautiful double star of this kind in the constellation of the Swan.  You could make an imitation of it by boring two holes, with a red-hot needle, in a piece of card, and then covering one of these holes with a small bit of the topaz-colored gelatine with which Christmas crackers are made.  The other star is to be similarly colored with blue gelatine.  A slide made on this principle placed in the lantern gives a very good representation of these two stars on the screen.  There are many other colored doubles besides this one; and, indeed, it is noteworthy that we hardly ever find a blue or a green star by itself in the sky; it is always as a member of one of these pairs.

How We Find What the Stars are Made of.

Here is a piece of stone.  If I wanted to know what it was composed of, I should ask a chemist to tell me.  He would take it into his laboratory, and first crush it into powder, and then, with his test tubes, and with the liquids which his bottles contain, and his weighing scales, and other apparatus, he would tell all about it; there is so much of this, and so much of that, and plenty of this, and none at all of that.  But now, suppose you ask this chemist to tell you what the sun is made of, or one of the stars.  Of course, you have not a sample of it to give him; how, then, can he possibly find out anything about it?  Well, he can tell you something, and this is the wonderful discovery that I want to explain to you.  We now put down the gas, and I kindle a brilliant red light.  Perhaps some of those whom I see before me have occasionally ventured on the somewhat dangerous practice of making fire-works.  If there is any boy here who has ever constructed sky-rockets, and put the little balls into the top which are to burn with such vivid colors when the explosion takes place, he will know that the substance which tinged that fire red must have been strontium.  He will recognize it by the color; because strontium gives a red light which nothing else will give.  Here are some of these lightning papers, as they are called; they are very pretty and very harmless; and these, too, give brilliant red flashes as I throw them.  The red tint has, no doubt, been produced by strontium also.  You see we recognized the substance simply by the color of the light it produced when burning.

Perhaps some of you have tried to make a ghost at Christmas by dressing up in a sheet, and bearing in your hand a ladle blazing with a mixture of common salt and spirits of wine, the effect produced being a most ghastly one.  Some mammas will hardly thank me for this suggestion, unless I add that the ghost must walk about cautiously, for otherwise the blazing spirit would be very apt to produce conflagrations of a kind more extensive than those intended.  However, by the kindness of Professor Dewar, I am enabled to show the phenomenon on a splendid scale,