42 Capt. A. 0. Egerton on the Vapour Pressure 



glass it is not at once condensed but undergoes a series of 

 reflexions. It is also noticed that it is more readily condensed 

 on itself. In support of this, in some experiments it has 

 been found that the cadmium condenses in separate sticky 

 grey molecules or filaments, which are seen to be composed 

 of a number of small irregular crystals when viewed under 

 the microscope. Further, it has already been mentioned 

 that mercury condensed by the aid of liquid air forms an 

 opaque black film on the condensing surface, which becomes 

 practically invisible on warming to atmospheric temperature. 

 The mercury coalesces on warming. 



It might be possible to attain better results at very low 

 pressures by cooling to a lower temperature, or by using a 

 tube on which metal is already condensed and which has 

 been carefully weighed. At pressures above 10~~ 4 mm., the 

 repeated reflexions do not affect the results, as a compara- 

 tively large difference of pressure on either side of the 

 aperture is maintained. 



The zinc deposit is a beautiful bluish-white lustrous film, 

 but if the quantity is very small it generally presents itself 

 as a grey powder. The cadmium deposit is considerably 

 greyer in colour, but beautifully lustrous. For a small 

 quantity, the powdery form is yellowish grey. The deposit 

 is streaky and appears to follow streaks in the glass or silica. 

 The metals condense on the walls of the tube in a band of 

 about 1^ cm. in breadth on that portion of the tube as it 

 emerges from the hot-box. There is a distinct and fairly 

 narrow region of greatest thickness when the deposit is 

 obtained on a cooled inner tube, the end of the tube opposite 

 the hole is sometimes left bare, showing that reflexion occurs 

 to a considerable extent ; the thickest part of the deposit is 

 just where the tube becomes straight. There is also generally 

 a small deposit on the walls of the tube. In all cases the 

 shape of the deposit is such that the straight line emission 

 demonstrated by Dun oyer is exhibited ; in one experiment 

 with a narrow condensing tube it has been noticed that the 

 metal deposited in bands of distinctly different thickness, as 

 if particles of different range were being condensed. Many 

 interesting points arise in an investigation of the condensation 

 of these metals, but it is a study apart from the scope of this 

 paper*. 



* Prof. R. W, Wood, in an interesting paper (Phil; Mag. xxxii. p. 364), 

 has recently made a study of the subject which bears out the above 

 observations. 



