THE TRANSFORMATIONS OF ELEMENTS 533 



chemical separations, followed by measurement of the radio- 

 activities of the different products. 



Early workers on thorium noticed that the amount of radia- 

 tion given off was subject to variation. Owens, in 1899, traced 

 this inconstancy to air-currents passing over the surface of 

 the thorium. Rutherford, next year, showed that it was due 

 actually to a gas — hence the effect of air-currents. It could 

 be drawn through tubes along with a current of air, passed 

 through cotton-wool, bubbled through solutions, and gave 

 thereafter an electrical effect. It thus behaved in every way 

 like a true gas, although it could not be isolated, and could 

 only be tested by its radioactive properties. Rutherford termed 

 it an emanation. It has a very short-life period. Its decay 

 was studied by drawing it into a cylinder by an air-current, 

 closing the cylinder, and noting the radioactivity from time to 

 time by the electrical method (the cylinder containing insu- 

 lated metallic discs connected to an electrometer). Its half-life 

 period is 54 seconds. 



This emanation of course disintegrates into something else. 

 When some quantity of it is allowed to decay in any closed 

 vessel, the products are deposited on the walls of the vessel. 

 They themselves give off radiations ; the phenomenon is known 

 as excited or induced activity, the deposit as the active deposit. 

 It differs in chemical and physical properties from all its pre- 

 decessors. The emanation can be washed out with air, and 

 the decay rate of the deposit studied. The curve is complex ; 

 mathematical analysis indicates at least two products. These 

 were postulated as thorium A and B. They were differentiated 

 more clearly by the work of Miss Slater, who studied the 

 effect of temperature on them. A platinum wire was exposed 

 for a considerable time to the action of the emanation, whereby 

 (relatively) a considerable amount of the after-products was 

 deposited on it. It was then heated by an electric current. 

 A lead cylinder surrounding it caught any volatilised matter. 

 Both wire and cylinder were then tested separately by an 

 electrometer, the decay of the radioactivity being studied. 

 Heated to about 700 C. the amount of radioactivity on the 

 wire remained constant, but the decay rate was faster than 

 usual. The cylinder was at first only very slightly active, but 

 the activity grew rapidly, the maximum being reached in 

 four hours; the decay rate thereafter was normal. These 



