BY THE ACTION OF A RADIO-ACTIVE SUBSTANCE, ETC. 265 
the open ends of the tubes B x and hh A short brass tube L was soldered near the 
end of the tube A, and was connected to the manometer M, so that the pressure of 
the air in' A could be found. The air from the room was admitted to the apparatus 
through capillary tubes K. having first passed through a tube of glass-wool G, to 
prevent the admission of any dust which might alter the resistance of the tubing. 
The capillary tubing was connected to the drying tubes, and the rest of the connec¬ 
tions were made with wide tubing. The tube u leading from B was connected to 
two large vessels W x and Wo, which were exhausted by means of a water-pump. 
In order to obtain a stream of air through A at a given pressure P, and with a 
velocity between suitable limits, the stopcock Sj was closed and the whole apparatus 
was exhausted until the pressure was a few millims. below the pressure P. The 
tube G was connected to the delivery tube of a gasometer, the movable cylinder of 
which was adjusted so as to be on the point of moving downwards when the gaso¬ 
meter was open to the air. The vessel W 2 was then connected with the water-pump, 
and Sj was turned on for a few minutes. The velocities V (through the tubing T) 
which were desirable necessitated a larger supply through the apparatus than could 
be taken out by the water-pump, so that the pressure, as shown by the manometer, 
gradually rose at the rate of about 3 millims. per minute. The stopcock Sj was turned 
oft' when the pressure was as much above P as it was below P at the beginning of the 
experiment. The velocity V in the tubes T can be accurately found by observing the 
volume of air that escaped from the gasometer and the time during which the stop- 
VOL. CXCV.—A. 2 M 
