220 



SCIENCE. 



A form of apparatus which I have found to answer very 

 well is shown in the accompanying woodcut (Fig. 2). 



A is the tube or retort containing the metal experimented 

 on in its lower extremity, and having a platinum wire sealed 

 into it at a distance of about two inches from the lower 

 end, the other end being drawn out and connected by a 

 mercury joint to an ordinary Geissler tube, which is con- 

 nected by another mercury joint to the Sprengel pump C. 



Another form of tube which I have used is prepared by 

 inserting two platinum poles into a piece of combustion 

 tubing sealed at one end, and after inserting the metal to 

 be experimented on, drawing out the glass between the 

 platinums to a capillary tube. 



I have also tried inserting the platinum pole at the end 

 of the retort, so that the spark passes from the surface of 

 the metal, but this arrangement did not answer at all. 



Some other modifications have been tried, but the first 

 form I have described is that which I have found to answer 

 best, so far as the trials have yet gone. 



D is the spectroscope. 



Eis the lens used for focussing the image of the Geissler 

 tube on the slit. 



oft" can be found bv examining the spectrum of this capillary 

 tube. 



I now give an account of the phenomena observed when 

 we were working with sodium, in order to show the kind 

 of phenomena and the changes observed. 



After a vacuum has been obtained the retort is heated 

 gradually. The pump almost immediately stops clicking, 

 and in a short time becomes nearly full of hydrogen. The 

 spectrum of the capillary then shows the hydrogen lines 

 intenselv bright. After some time the gas comes off far 

 less freely, and an approach to a vacuum is again obtained. 

 Another phenomenon now begins to show itself : on pass- 

 ing the current a yellow glow is seen, which gradually fills 

 the whole space between the pole in the retort and the 

 metal ; its spectrum consists of the lines of hydrogen and 

 the yellow line of sodium, the red and green line being 

 both absent until the experiment has gone on for some 

 time. 



As the distillation goes on, the yellow glow increases in 

 brilliancy, and extends to a greater distance above the pole, 

 and the red and green lines presently make their appearance 

 as very faint lines. 



FlC. 2.— Distill 



F is the spirit lamp for heating the retort. 



H is the battery. 



K and L an- the wires connected with the coil. 

 ' In the second cut (Fig. 3) the method of observing the 

 spectrum of the vapours close to the surface of the metal 

 is indicated ; the same letters apply, I)' being, however, in 

 this c.isc a direct-vision spectroscope, which was sometimes 

 employed foi convenient < . 



roi determining the exact positions of the lines in the 

 spectrum of the vapor in any part of the retort, a larger 

 Spectroscope, with its illuminated scale, was used in the 

 place of the diiect-vision spectroscope. 



'I In secondary wires of the coil were connected, one with 

 the pole in the upper bulb at H. and the othci with the plati- 

 num at A. 



H is an ordinary Geissler tube with two bulbs separated 

 bv a capillary tube. The greal advantage of this arrange- 

 ment is that this capillary portion can be used (or ascertain- 

 ing what gases 01 vapors are carried ovei by the pump 

 without any interference with the retort, both wins being 

 ( onnei ted with the Geissler tube. If, foi example, w< an 

 wo 1 kint; with sodium which contains an impurity oi hydro- 

 carbon, the moment at which it begins 01 leases to iipiim 



ation Apparatus. 



The upperboundary of the yellow is quite sharp, thelines 

 and fluted spectrum of hydrogen appearing above it. 



After the yellow glow-giving vapor (which does not at- 

 tack the glass) has been visible for some time, the pump is 

 stopped and the metal heated more strongly. On passing the 

 current a little while afterwards, a very brilliant leaf-green 

 vapor is seen underlying the yellow one, and connected 

 with il by a sap-green vapor. The spectra then visible in 



the tube at the same time are — 



Leaf-green ... Green and red lines of sodium and Cof 



hydrogen ; I > absent. 

 Sap-green ... Green, red, and yellow sodium lines of 



equal brilliancy ami C of hydrogen. 

 Yellow I) alone and ('. 



Bluish-green C and F and hydrogen structure. 



To observe the green sodium line alone it is necessary to 

 point the direct-vision spectroscope just above the surface 

 of the metal where the green is strongest. It is also neces- 

 sary to guard against internal reflections from the glass, as 



this iii.m BOmetimeS cause the I) line to be seen by re- 

 flection from the surface. 



