228 Dr. P. Zeeman on the Influence of Magnetism 



of a current of water, the copper caps and the glass plates 

 may be kept sufficiently cool while the porcelain tube is 

 rendered incandescent, In the neighbourhood of the glass 

 plates, side-tubes provided with taps are fastened to the 

 copper caps. With a large Bunsen burner the tube could be 

 made incandescent over a length of 8 cm. The light of an 

 electric lamp, placed sideways at about two metres from the 

 electromagnet, in order to avoid disturbing action on the arc, 

 was made to pass through the tube by means of a metallic 

 mirror. The spectrum of the arc was formed by means of the 

 grating. With the eyepiece the D-lines are focussed. This may 

 be done very accurately, as in the centre of the bright D-lines 

 the narrow reversed lines are often seen. Now a piece of 

 sodium was introduced into the tube. The Bunsen flame is 

 ignited and the temperature begins to rise. A coloured vapour 

 soon begins to fill the tube, being at first of a violet, then of 

 a blue and green colour, and at last quite invisible to the 

 naked eye. The absorption soon diminishes as the temperature 

 is increased. The absorption is especially great in the 

 neighbourhood of the D-lines. At last the two dark D-lines 

 are visible. At this moment the poles of the electromagnet 

 are pushed close to the tube, their distance now being about 

 24 mm. The absorption-lines now are rather sharp over the 

 greater part of their length. At the top they are thicker, 

 where the spectrum of the lower, denser vapours was observed. 

 Immediately after the closing of the current the lines widen 

 and are seemingly blacker; if the current is cut off they 

 immediately recover their initial sharpness. The experiment 

 could be repeated several times, till all the sodium had dis- 

 appeared. The disappearance of the sodium is chiefly to be 

 attributed to the chemical action between it and the glazing 

 of the tube. For further experiments therefore ungiazed tubes 

 were used. 



8. One may perhaps try to account for the last experiment 

 (§ 7) in this direction: — It is true that the tube used was 

 not of the same temperature at the top and at the bottom ; 

 further, it appears from the shape of the D-lines (§ 7) that 

 the density of the vapour of sodium is different at different 

 heights. Hence certainly convection-currents caused by 

 difference of temperature between the top and bottom were 

 present. Under certain plausible suppositions one may cal- 

 culate that, by the putting on of the electromagnet, differences 

 of pressure are originated in the tube of the same order of 

 magnitude as those caused by the difference of temperature. 

 Hence the magnetization will push e. g. the denser layer at 

 the bottom in the direction of the axis of the tube. The lines 



M 



