432 Light Scattering by Air and Blue Colour of the Sky. 



that apparatus for its exhibition to students should be in every 

 physical laboratory. For demonstration purposes and for 

 the investigation of rare gases, which it is difficult to keep 

 pure in metal tubes with cemented windows, the following 

 form of apparatus will be found superior in many respects 

 to the branched metal tubes employed by Strutt in his inves- 

 tigationsi The tubes are made wholly of glass, and can be 

 hermetically sealed, insuring the continued purity of the 

 gas, and they can be set up and exhibited at a moment's 

 notice. They are prepared in the following way : — 



A glass tube about 25 mm. in diameter and 25 or 30 cm. 

 in length, is drawn down in an oblique direction at each end, 

 as shown at A, fig. 2. 



Fiir. 2. 



The flame of the blast-lamp is then directed against one 

 end as shown in the figure, and the end blown out into a 

 bulb, as shown at B. 



This method of preparation obviates the necessity for 

 sealing in lateral tubes for the admission of the gas, and will 

 be found a great saver of time in the construction of all tubes 

 for the study of the optical properties of gases and vapours. 

 The other end of the tube is left as shown at A in the figure, 

 as the sloping wall obviates the back reflexion of light from 

 the bottom of the dark cave which this end of the tube is to 

 form. Two lateral bulbs are next blown out from the wall 

 of the tube : these should be as large as possible consistent 

 with strength. Two tubes should be prepared, one filled with 

 air filtered through densely packed cotton and the other 

 with ether vapour, which has over twenty times the scattering- 

 power of air. The air can be forced into the tube through 

 the filter with a tire-pump, and the small lateral tubes 

 sealed-off with a flame. A small quantity of liquid ether is 

 introduced into the other tube, and the entire tube warmed 

 by sweeping it with a bunsen flame, or better by immersion 

 in water heated to a temperature of about 40° C. The jet 

 of vapour which escapes from the small tube can be ignited 

 after waiting a few moments for the expulsion of the air. 

 As the tube cools the flame dies down, and just as it is on the 

 point of going out the small tube is sealed. The tubes are 



