October 1, 1894. 



KNOWLEDGE 



219 



P is gently heated by a spirit lamp flame until a small 

 faintly luminous flame is seen, indicating combination of 

 the last traces of oxygen with the phosphorus. After this 

 has gone on a sufficient length of time, and the apparatus 

 may be considered to be absolutely free from oxygen, 

 water, or carbonic aoid, the stopcock on A is again closed 



Experiment II. — A pellet of clean sodium was placed 

 in the steel spoon and the gas (coal gas) allowed to 

 circulate, the phosphorus being heated. The line seen 

 in Experiment I. was watched, and some time after it had 

 quite disappeared the current was stopped, and the sodium 

 pushed into the hot part of the tube. Instantly the central 



and the steel spoon carrying the sodium is pushed into the 

 hot part of the tube H, turned over, the sodium shaken 

 out and again withdrawn past the entrance of the side tube. 

 Now the tube carrying the prism is pushed down into line 

 with the tube H, and the white flame of a paraffin lamp is 

 placed close alongside the glass T, so that a ray of white 

 light can be made to traverse the glowing vapour in H. 

 One may now observe at will the absorption or emission 

 spectrum of the glowing sodium by the simple operation 

 of turning the lamp flame up or down. 



I will now proceed to describe some of the experiments 

 actually performed. 



Experiment I. — With the porcelain tube strongly heated 

 a slow current of coal gas, not specially freed from oxygen, 

 was allowed to circulate in the apparatus, no sodium being 

 admitted. A distinct and fine sodium line was visible in 

 the spectroscope, which increased in brightness when a 

 little air was mixed with the gas, but which gradually 

 faded to invisibility when the phosphorus tube was heated 

 so as to eliminate oxygen. 



The explanation of this result is quite simple ; the 

 hydrogen combines with the trace of oxygen when it 

 reaches the hot tube, and the " flame " so formed (which, 

 however, is not visible as such except when a large 

 quantity of oxygen is present) becomes tinted with sodium 

 derived from the tube itself just as the Buusen flame 

 outside is tinted. This line, then, may be classed as the 

 result of chemical " luminescence," whatever physical 

 meaning may be ascribed to that expression. 



bore of the porcelain was flUed with light, which in the 

 spectroscope was found to be perfectly continuous, but 

 crossed by a very wide black line at D. Gradually the 

 continuous spectmm faded, and as it became fainter the 

 dark D line was seen to ba bordered with a fringe of light 

 on each side; and as the vapour became less and less dense, 

 owing to the distillation of the s odium into cooler parts of 

 the tube, the D line went through the changes represented 

 in Fig. 2, in the order a, h, c, d, finally persisting as a rather 

 wide" bright line in which a very fine dark line could 

 usually be made out. But at any stage of the experiment, 

 the dark central line could easily be extinguished by 

 allowing a gentle current of gas to push back the cooler 

 absorbing layer into the hotter regions. Now the question 

 to decide was, whether this bright, broad, hazy D line was 

 or was not the result of chemical activities. 



Experiment III. — The sodium spectrum was examined as 

 in Experiment II., but without heating the phosphorus, a 

 considerable trace of oxygen, therefore, remaining in the 

 gas. With the current stopped, the phenomena observed 

 were the same as in Experiment II., but as s.'on as a slow 

 current was allowed to impinge on the sodium by partly 

 opening the stopcock on A, the centre of the broad bright 

 line became much brighter, and could be seen projected on 

 the adjacent continuous spectrum from the glowing sides 

 of the tube. 



* The dispersion of the spectroscope being insufficient to separate 

 the components of the D line, it is evident that when a " wide " line 

 was seen the two components of the pair were fused into one. 



