indeed, the prime constituents of the air suffer a like fate. The responsible substance which plays the part of the familiar mercury, or alcohol, in Dr. Travers's marvellous thermometer is hydrogen gas. The principle by which it is utilized does not differ, in its rough essentials, from that of ordinary thermometers, but the details of its construction are much too intricate to be elaborated here. But if you would see the most wonderful things in this laboratory--or rather, to be quite accurate, I should say, if you would stand in the presence of the most wonderful things--you must go with Professor Ramsay to his own private laboratory, and be introduced to some little test-tubes that stand inverted in cups of mercury decorating a shelf at one end. You would never notice these tubes of your own accord were you to browse ever so long about the room. Even when your attention is called to them you still see nothing remarkable. These are ordinary test-tubes inverted over ordinary mercury. They contain something, since the mercury does not rise in them completely, but if that something be other than ordinary air there is nothing about its appearance, or rather lack of appearance, to demonstrate it. But your interest will hardly fail to be arrested when Professor Ramsay, indicating one and another of these little tubes, says: "Here you see, or fail to see, all the krypton that has ever been in isolated existence in the world, and here all the neon, and here, again, all the zenon." You will understand, of course, that krypton, neon, and zenon are the new gases of the atmosphere whose existence no one suspected until Professor Ramsay ferreted them out a few years ago and isolated them. In one sense there should be nothing mysterious about substances that every air-breathing creature on the globe has been imbibing pretty constantly ever since lungs came into fashion. But in another view the universal presence of these gases in the air makes it seem all the more wonderful that they could so long have evaded detection, considering that chemistry has been a precise science for more than a century. During that time thousands of chemists have made millions of experiments in the very midst of these atmospheric gases, yet not one of the experimenters, until recently, suspected their existence. This proves that these gases are no ordinary substances--common though they be. Personally I have examined many scientific exhibits in many lands, but nowhere have I seen anything that filled my imagination with so many scientific visions as these little harmless test-tubes at the back of Professor Ramsay's desk. Perhaps I shall attempt to visualize some of these imaginings before finishing this paper, but for the moment I wish to speak of the _modus operandi_ of the discovery of these additions to the list of elements.