Determination of Law of Reflexion of Gas Molecules. 301 



The first experiment was made with a tube of the form 

 shown in fig. 1 A. A droplet of mercury was placed in the 

 bulb and the tube highly exhausted, the walls being heated 

 with a Bunsen burner. After the occluded gas had been 

 driven from the wall, the tube was allowed to cool, and the 

 bulb heated until all of the mercury had distilled into the 

 cooler portion of the tube. It was then sealed off from the 

 pump, the capillary being heated very gradually, so as to 

 have it at a temperature just below the softening point of 

 the glass for some minutes before it finally collapsed. The 

 mercury was now gathered into a single drop which was 

 brought into the narrow neck of the tube. The whole was 

 then introduced into a flask of liquid air with the exception 

 of the turned-over neck. At the end of twelve hours a faint 

 circular deposit of mercury, of the same diameter as the con- 

 stricted portion of the tube just above the bulb, was observed. 

 In this case the distillation had taken place at room tem- 

 perature. The tube was now wound with insulated german- 

 silver wire from a point just above the surface of the liquid 

 air, to the closed tip. This was heated electrically to a 

 temperature which felt uncomfortably hot to the finger. 

 Ufider these conditions the phenomenon is reproduced in a 

 minute or two. The mercury vapour enters the upper por- 

 tion of the tube, and all of the molecules which are moving 

 sideways are condensed on the wall, the deposit being very 

 heavy at the top of the tube, and gradually thinning awav 

 to nothing a few centimetres below the surface of the liquid 

 air. Below this point the molecules are moving all in the 

 same direction, like bullets from a machine gun, and no 

 further deposit is found until the constricted portion of the 

 tube is reached. Here the molecular stream strikes the 

 sloping walls of the constriction, and a heavy deposit of the 

 metal occurs. Passing through the small opening, the gas 

 traverses the exhausted bulb in the form of a jet which 

 shows no tendency to spread out laterally, and deposits on 

 the wall in the form of a small circular patch with very 

 sharply defined edges (fig. 1). 



A photograph of the deposit, formed on an oblique plate 

 of glass within the bulb, is reproduced on PI. III. tier. 1. 

 In this case the tube was kept in liquid air for some time 

 before starting the vaporization, to allow the plate to take up 

 the low temperature. 



In the study of the reflexion of the jet the reflecting 

 surface must be kept at a relatively high temperature, to 

 prevent the condensation of the vapour. A large drop of 

 glass was ground off at an angle of 45°, and the oblique 



