392 Prof. Thomson, On the presence of 



Wide prevalence of radio-active substances. The method used 

 to detect whether or not a substance was the source of a radio- 

 active emanation, was to reduce the substance under examination 

 to as fine a powder as possible and then shake the powder up in 

 distilled water which did not contain any radio-active gas. The 

 presence of a radio-active gas given off by the substance was 

 tested in two ways. In the first filtered air was bubbled through 

 the mixture of the powdered substance and distilled water, and 

 then passed through a vessel within which a gold-leaf electroscope 

 was enclosed, the gold leaves of the electroscope were observed 

 through a microscope with a micrometer eye-piece. The rate 

 of movement of the leaves when charged gave a measure of the 

 ionization of the gas in the closed vessel. Measurements were 

 made of the rate of fall of the leaves before and after the intro- 

 duction of the air which had bubbled through the mixture, the 

 existence of a radio-active emanation from the substance mixed 

 with the water showed itself by the increase in the rate of leak 

 of the electroscope after the introduction of this gas. 



The other method of testing for the emanation was to boil the 

 water with which the powdered substance was mixed, to introduce 

 the gas expelled from the mixture by boiling into the vessel 

 containing the electroscope and to see whether this increased the 

 rate of leak of the electroscope. By either of these methods an 

 emanation whose life is longer than the time required to transfer 

 the gas extracted from the water into the testing vessel could 

 readily be detected; a very short lived emanation such as that 

 given off by actinium might however escape detection by these 

 methods. 



A large number of substances were tested, of which the follow- 

 ing gave unmistakable evidence of containing a radio-active 

 emanation. Soil from the garden adjoining the laboratory, Cam- 

 bridge gault, gravel from a pit at Chesterton, powdered bricks, 

 powdered glass, sea sand from the beach at Whitby, Yorks. (this 

 was exceedingly rich in the emanation), blue lias from Whitby; 

 one specimen of powdered silica contained a very large quantity 

 of the emanation, other specimens little or none; one specimen 

 of wheaten flour contained an appreciable amount of emanation, 

 other specimens none ; one or two specimens of flowers of sulphur 

 gave off emanation, while other specimens obtained from a differ- 

 ent source did not. Practically all the clays, sands and gravels 

 tested gave off the emanation. The emanation is continually 

 being given off by these substances, for though the water contain- 

 ing the powdered substance can be temporarily deprived of the 

 emanation either by boiling or bubbling, yet if the water with 

 the powder in it is allowed to stand for a few days, the emanation 

 gradually accumulates and can be collected by the same process 



