sure, repay any one who takes the trouble to repeat them, and will be extremely useful in illustrating the subject of anomalous dispersion to small classes of advanced students, I shall describe in some detail the manner of preparing and using these dispersion tubes which I have found most advantageous. The ends of the tubes are first warmed and thickly coated with sealing-wax ; one of the glass straws is then placed in position, and a small piece of plate glass, previously warmed, pressed against the wax, any crevices around the straw being closed with wax. The leading-in tube is next placed in position, and a piece of freshly cut sodium (about 5 mm. on a side) inserted. The other window is then cemented to the tube, and the current of hydrogen started as soon as possible. Some experience is necessary properly to regulate the hydrogen stream during the experiment. When the tube is first heated much white smoke forms. If a stream corresponding to about one bubble per second is allowed to flow, the smoke will usually clear up in a few minutes and give little trouble. The tube should be heated by means of a Bunsen burner turned down low, the tip of the flame playing against the bottom of the tube. If a sodium flame is placed behind the tube the formation of the vapour can be watched, for it appears almost jet black against the flame, though quite colour- less in white light. The behaviour of the vapour is somewhat peculiar. It grows out from the sodium globule as a dark atmosphere with a sharply defined surface, which clings to the globule with great tenacity. It resembles at first a thick growth of mould more than anything else that I can think of, and a sudden gust of hydrogen scarcely moves it at all. A wire pushed up through it drags a certain amount above the free surface in much the same manner as a stick pushed up through the surface of thick molasses would do. If the tube be inverted the black cloud clings to the upper surface, behaving on the whole like a very viscous mass. It is even possible to dip some of it up on a wire.
These peculiar physical properties of the metallic vapour I have as yet only studied in a very superficial manner, and I mention them now, only because it appears to me that there is some connection between them and the optical behaviour of the medium.
I am of the opinion that the apparent viscosity is an illusion, and that the sharply-defined surface is merely the boundary at which either condensation or chemical action (the hydrogen not being pure) is taking place. The process of dipping the vapour up on the wire might be explained by condensation on the wire followed by vapori- sation. A more careful study of the physical behaviour of the vapour will be made some time in the future.
The apparatus employed in the study of the dispersion of the vapour was essentially identical with that used by Becquerel. The light of an arc lamp was focussed on the horizontal slit of a collimator,
