44:6 Scientific Intelligence. 



was collected as a droplet in this reservoir. Finally, the bulb 

 and all of the straight, vertical segment of the tube were im- 

 mersed in liquid air. At the expiration of twelve hours a faint 

 circular deposit of mercury, having the same diameter as the 

 lower constriction in the tube, was observed at the bottom of the 

 bulb. By winding a heating-coil around the lateral reservoir and 

 causing the mercury to distil at a temperature appreciably above 

 that of the room, the deposit in the bulb was established in a 

 minute or two. In the original paper is given a clear photo- 

 graph of the mercury which had condensed on a glass plate placed 

 in the bulb obliquely with respect to the long axis of the tube. 



The phenomenon is explained very clearly in the following 

 sentences. " The mercury vapour enters* the upper portion of 

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

 condensed on the Avail, the deposit being very heavy at the top 

 of the tube, and gradually thinning away to nothing a few cen- 

 timetres 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 con- 

 stricted 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." 



In order to study the reflection of the beam of molecules a 

 large drop of glass, at the extremity of a thin glass stem, was 

 ground down and polished plane at an angle of about 45° with 

 the axis of the stem. The reflecting surface was mounted con- 

 centrically with the evacuated bulb by sealing the end of the 

 stem to the inside of the lower, or drawn-out, portion of the bulb. 

 The insulating stem and the surrounding vacuum enabled the 

 reflecting surface to maintain its initial temperature, which was 

 well above the point of condensation of mercury vapor, for an 

 interval of time much longer than was necessary for the com- 

 pletion of the observations. Under these conditions, the deposit 

 began to appear in about three minutes after the heating-current 

 had been started, and in ten minutes it had become entirely 

 opaque to ordinary light. The photograph shows that the 

 deposit covers the upper oblique half of the bulb icith the excep- 

 tion of a narrow circular zone just above the plane of the reflect- 

 ing surface. " The results appear to show that the cosine law 

 is approximately followed, for the density is greatest on the line 

 of the normal, falling off gradually as the angle increases. At 

 angles greater than about 80 degrees there appears to be no 

 reflexion, at least in the case of the two surfaces which I have 

 examined." The second surface referred to was a sheet of mica 

 freshly split and mounted on a glass plate. The occurrence of 

 the blank zone was also found to be independent of the size of 



