The Radio-activity of Metals and their Salts. 283 



since for a slab composed of one material throughout k = 1 : hence 



Puvw ■ • ■ = v \kuPu + rC v p v + fC w p w + ...). 



We see then that if we know the ionisation caused by slabs com- 

 posed entirely of each of the elements we can predict the ionisation 

 caused by a slab containing them in any known proportion, on the 

 assumption that the activities are uninfluenced by their state of 

 chemical combination. The comparison of the activities calculated 

 with those observed will serve to test the truth of the assumption. 



3. The chief part of the apparatus used in these measure- 

 ments is a wooden box, the internal dimensions of which are 

 40 x 40 x 22 cms.: it is lined with zinc sheet: a drawer 8 cms. 

 deep, lined with aluminium foil, slides into the bottom of the box : 

 in this drawer are placed glass trays 0"15 cm. deep and 36 cms. 

 square, containing the material the activity of which is to be 

 investigated. A sheet of iron wire gauze of 01 5 cm. mesh is fixed 

 across the box at such a height that it just touches the surface of 

 the glass tra} 7 s when these are in position : in the space above the 

 gauze is an electrode of coarse wire netting, supported by a wire 

 passing through insulation in the side of the box and connected to 

 the measuring device described in Proc. Camb. Phil. Soc, Vol. xm. 

 p. 132, which proved exceedingly suitable for these experiments. 

 The lining of the top part of the box is kept at a potential 

 sufficient to send a saturation current through the gas : the lining 

 of the drawer is earthed. It is found necessary to keep a rapid 

 stream of air passing continually through the vessel in order to 

 keep the current constant : this air is deprived of its free ions 

 by passage through a tube in which it is subjected to a strong 

 electric field. 



4. It is clear that by measurements with such a vessel it is 

 only possible to find the difference of activity of two bodies placed 

 successively beneath the gauze. In order to find the absolute 

 activity of any one material it is necessary to use a material 

 which has no activity at all : but no such material is known. 

 Moreover it is necessary to have some solid body constantly in this 

 position, for ions can be dragged through the meshes of the gauze 

 and the current to the electrode changed thereby. But since it is 

 impossible to use a body without any activity whatever, the best 

 that can be done is to use a body the activity of which is as small 

 as possible. The most satisfactory material for this purpose was 

 found to be carefully cleaned glass plates : no material has been 

 investigated which possesses a smaller activity. The error intro- 

 duced by assuming that the glass has no activity will be that 



caused by regarding - } as equal to r, where c is at most one- 



