368 Prof. R. W. Wood on the Condensation 



was blown out into a small thin-walled bulb, the surface 

 of which was touched with a pellet o£ cotton wet with 

 liquid air. A film formed on the cooled spot, as shown in 

 the photograph, proving that the vapour could traverse the 

 bent tube without condensation. Tubes with over a dozen 

 bends were used, proving that a large number of reflexions 

 were possible, the number of bends being the minimum 

 number of reflexions sufficient to carry the molecule to the 

 condensation bulb. 



It was found that a much longer time was required to form 

 a film in the bulb when a large number of bends were intro- 

 duced, which indicated that a portion of the molecules had 

 'been condensed on the way down the tube. In the case of 

 the tube shown on Plate VII. fig. 4 the deposit in the bulb 

 was produced in fifteen minutes. The cotton pellet was then 

 applied at the bend marked A, and a deposit of equal density 

 was obtained in two minutes. Transferring the cotton to the 

 point B gave an equal deposit in about 10 seconds. 



It is evident that the density of the cadmium vapour is 

 considerably decreased by its passage down the bent tube, 

 but as no visible deposit could be seen on the walls, it was 

 not at once evident what became of it. 



A glass stopcock was introduced between the bulb and the 

 straight tube containing the cadmium, which was then heated 

 above its melting-point. The stopcock was then closed and 

 at the same instant the cotton pellet wet with liquid air was 

 applied to the bulb, but no film was produced, even after 

 many trials. This seemed to show that the vapour was in- 

 capable of existing as vapour in a bulb at room temperature 

 for any appreciable time. 



A very thin- walled bulb of exceptionally clean glass was 

 then blown on a tube, and after forty minutes heating of the 

 cadmium, it was removed from the pump and examined in a 

 strong light against a black background. The wall of the 

 bulb diffused a small amount of light, as if a light smoke had 

 deposited on it, and examination under the microscope with 

 Y^-inch oil-immersion objective showed widely separated 

 aggregates of the metal which appeared to be clusters of 

 crystals (fig. 2 a). More frequently, however, deposits such 

 as shown by fig. b were obtained. When we consider the 

 excessive thinness of metal films which are partially trans- 

 parent to light, it is clear that an amount of cadmium 

 sufficient to form a film of moderate opacity over the entire 

 surface of the bulb, if collected into a single crystal, would 

 probably not be much larger than one of these small 

 aggregates. 



