Matter present in the Atmosphere. 101 



a maximum amount o£ activity was obtained. A Wimsburst 

 influence-machine, driven by a small electric motor, was 

 used, and the potential o£ the wire varied from about —9000 

 to — 11,000 vo]ts. It was necessary to work with a large 

 vessel in a building into which no radioactive matter had 

 been introduced. The conditions were satisfied by the 

 Engineering Building at MoGrill University, which is situated 

 at a considerable distance from the Physics Building. In 

 the hydraulic laboratory there is a large water-tank 808 cms. 

 high, and 152 cms. square, with a total volume o£ 18*7 cubic 

 metres. The sides are made of iron 3*5 cms. thick. The 

 tank is filled from time to time with water supplied by the 

 City of Montreal, and derived from the rivers St. Lawrence 

 and Ottawa. The river water is practically free from radio- 

 active matter, nor could any appreciable activity be discovered 

 in the mud and slime deposit left at the bottom. The tank 

 was placed at the writer's disposal by the courtesy of the 

 authorities of the Engineering Building, and. a series of 

 experiments were made, extending from November to April. 

 In all cases the activity was measured ten minutes after the 

 Wimshurst machine was stopped. For the sake of simplicity 

 the activity is expressed in terms of the scale-divisions of the 

 reading-microscope used to observe the fall of the gold-leaf 

 of the electroscope. It may be noted that one scale-division 

 was about equal to a fall of 3' 6 volts, and that the capacity 

 of the system as shown in fig. 2 was 3*5 E.S. units. The 

 wire, after removal from the tank, was rapidly coiled on a 

 metal reel, somewhat similar to the outer part of a large 

 fishing-reel, and was then placed in a zinc cylinder connected 

 to earth. Along the axis of the cylinder was a brass rod, 

 insulated by a sulphur support, and directly connected with 

 the gold-leaf system of an electroscope placed beneath the 

 zinc cylinder. 



The fall of potential per minute, due to the active matter 

 collected on a wire 8 metres long placed in the iron tank, 

 ten minutes after removal, corresponded to 3*6 scale-divisions. 

 This is the mean of the results of a large number of obser- 

 vations, taken on different days, and the readings varied from 

 2'ti to 4*3. Thus the active matter collected from 18*7 cubic 

 metres of air caused a fall of potential measured by 3*6 

 scale-divisions per minute. 



A relatively small, air-tight, zinc cylinder 154 cms. high 

 and 25 cms. in diameter was then taken, and the emanation 

 derived from 2 x 10~ 4 mgs. of pure radium bromide was 

 introduced into it. The volume of the cylinder was 76,000 

 c. cms., and the emanation was distributed through the 



