Page:Radio-active substances.djvu/53

Experiments made at long distances with a product considerably more active than that which was used for the preceding series:—

$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	$d$, in centimetres.........	14	30	53	80	98
	Percentage of undeflected rays	12	14	17	14	16
$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	$d$..................	124	157
	Percentage of undeflected rays	14	11

It is thus evident that after a certain distance the proportion of undeflected rays in the radiation is approximately constant. These rays probably all belong to the γ species.

The following is another series of experiments in which the radium was enclosed in a very narrow glass tube, placed below the condenser and parallel to the plates. The rays emitted traversed a certain thickness of glass and air before entering the condenser:—

$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	$d$, in centimetres.........	2·5	3·3	4·1	5·9	7·5	9·6	11·3
	Percentage rays not deflected	33	33	21	16	14	10	9
$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	$d$..................		13·9	17·2
	Percentage rays not deflected		9	10

As in the preceding experiments, the number of the second line approximate to a constant value, when the distance $d$ increases, but the limit is reached for smaller distances than in the preceding series, because the α-rays have been more completely absorbed by the glass than the β- and γ-rays.

The following experiment shows that a thin sheet of aluminium (0·01 m.m. thick) absorbs principally α-rays. The product being placed 5 c.m. from the condenser, the proportion of rays other than β, when the magnetic field is acting, is about 71 per cent. When the same substance is covered with the sheet of aluminium, the distance remaining the same, the radiation transmitted is found to be almost totally deflected by the magnetic field, the a-rays having been absorbed by the aluminium. The same result is obtained when paper is used as the absorbing screen.

The greatest part of the radiation of radium consists of α-rays, which are probably emitted principally by the superficial layer of the radiating matter. When the thickness of the layer of radiating matter is varied, the intensity of the current increases with this thickness; the increase is not proportional to the thickness for the whole of the radiation; it is, moreover, more considerable for the β-rays than for the α-rays, so that the proportion of β-rays increases with the thickness of the active layer. The source of