( 345 ) 



shut air-tight when the rims are filled with cement. When the 

 furnace is in action a steady current of water, passing through the 

 two mantles M and M' , keeps the ends of the tube cool. Each of 

 the two caps has a rectangular plate glass window and also, on both 

 sides of this, openings a and b {b' and a'), placed diametrically 



)=• 



Fig. 1, 



opposite to each other and pro\ided with short brass tubes, the 

 purpose of which will appear presently. Moreover in one of the two 



caps (see also fig, 2) two other short 

 tubes c^and d are fastened in openings: 

 through c the porcelain tube of a Le 

 Chatei.ier pyrometer is fitted air-tight, 

 while on d a glass cock with mercury 

 lock is cemented, leading to a mano- 

 meter and a Geryk air-pump. As soon 

 as the sodium (a carefully cleaned 

 piece of about 7 grammes) had been 

 pushed to the middle of the tube in 

 a small nickel dish provided with elas- 

 tic rings, the tube had been immedi- 

 ately closed and exhausted. 

 We shall now describe the arrangement by which inside the mass 

 of vapour arbitrary inequalities in the density distribution were pro- 

 duced. It consists of tw^o nickel tubes A and B of 0,5 cm. diameter, 

 leading from a to a' and from b to b' and so bent that in the heated 

 middle part of the wide tube they run parallel over a length of 30 

 centimetres at a distance of only 0.8 cms. In the four openings of 

 the caps, ^1 and B are ftistened air-tight by means of rubber packing. 

 This kind of connection leaves some play so that by temperature 

 differences between the wide and the narrow tubes these latter need 



Fig. 2. 



