468 Mr. J. Evershed's Experiments on 



Y-tube attached, one branch of the Y leading to the appa- 

 ratus and the other connecting A with B by means of a 

 rubber tube carrying a clip. The S at the foot of the diagram 

 is a small direct-vision spectroscope, and L is a lens focussing 

 the central parts of the tube H on to the slit of the instru- 

 ment ; both are attached to a strip of hard wood movable 

 horizontally about an axis placed between the lens and the 

 slit. This enables one to instantly push aside the spectro- 

 scope into the position shown by dotted lines and observe the 

 glowing tube directly. 



To observe the sodium-spectrum, one fills the gas-holder A 

 with some indifferent gas containing no oxygen, or only a trace 

 of that element, such as nitrogen or hydrogen, or ordinary 

 coal-gas. When full it is disconnected with the gas-generator 

 or gas-main, as the case may be, and connexion is made with 

 the apparatus. Next, weights are put on A until sufficient 

 pressure is obtained to drive a current of gas through the 

 wash-bottles, drying-tube, <fec. into B. Then the Bunsen is 

 lighted under the porcelain tube, which it presently heats up 

 to a bright incandescence for about two inches of its length. 

 After sufficient dry gas has passed through, and all trace of 

 moisture has gone, the current is stopped by closing the 

 stopcock on A, and a small pellet of sodium is dropped into 

 the steel spoon through the side tube of the glass T, the 

 stopper with the inner tube and prism being removed for this 

 purpose and quickly replaced. The current is then restarted, 

 to drive away any oxygen that may have diffused in by the 

 operation, and at the same time the tube 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 free from 

 oxygen, water, and carbonic acid, the stopcock on A is again 

 closed and the steel spoon carrying the sodium is pushed 

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

 out, and the spoon again withdrawn past the entrance of the side 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. 



The experiments actually performed with this apparatus 

 may be thus briefly described : — 



1. With the porcelain tube strongly heated, a slow current 

 of coal-gas, not specially freed from oxygen, was allowed to 



