318 
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Penetrating Rays from Radio-active Substances. 
THE permanent radio-active substances uranium, thorium and 
radium all give out two types of rays, one easily absorbed and 
non-deviable by a magnetic field and the other more penetrating 
in character and deviated by a magnetic field. In addition to 
these rays, Villard, using the photographic method, first drew 
attention to the existence of some very penetrating rays from 
radium non-deviable by a magnetic field. This result was 
confirmed by. Becquerel. 
I have recently examined all these radio-active substances by 
the electrical method, and have found that thorium, and also the 
excited radio-activity produced by thorium and radium, emit 
some rays as penetrating in character as those from radium. 
Uranium, in comparison with thorium and radium, emits little, 
if any, of this radiation. 
These rays are extraordinarily penetrating in character, and 
pass readily through great thicknesses of matter. They are cer- 
tainly as penetrating as the most penetrating rays given out by 
a hard X-ray tube. The amount of ionisation produced by 
them is only a very small fraction of that produced by the other 
two types of radiation. Using testing vessels of ordinary size, 
the ionisation due to the penetrating rays is of the order of 1 
part in 100 of that due to the deviable rays and 1 part in 10,000 
of that due to the easily absorbed rays. 
In the experiments on radium, 0°7 gram of radium chloride, 
of activity 1000 times that of uranium, was used. The radiation 
from this, after its passage through 1 cm. of lead, caused a 
rapid movement of the needle in the sensitive electrometer 
employed. The radium was placed in a thick-walled lead 
vessel and a piece of aluminium waxed tightly over the top to 
prevent the escape of the emanation. The following numbers 
illustrate the diminution of the rate of leak in a testing vessel, 
placed above the radium, with the thickness of the lead traversed 
by the radiation :— 
Thickness of lead. : Current. 
“72 cm. ® Fs 1 NT 
72 -40-O2ems hse ef a me “S60 
Pee ary yy 673 a wt fea 0337, 
» +1°86 ,, hi sis se dreh UES 
PEAS OME as aie oa cen es) 
The current with *72 cm. of lead over the radium is taken 
as unity. It will thus be seen that the current falls off approxi- 
mately in a geometrical progression with the thickness traversed, 
and that after passing through 1°86 cm. of lead the intensity is 
reduced to about one-quarter. 
The following table shows the thickness of different metals 
traversed before the intensity is reduced to one-half :— 
Metal. Thickness in cm. 
Mercury 75 
Lead 9 
Tin 1°8 
Copper 22 
Zinc ec Sig ae anaes 
Iron oh 0 so ane SOS 
Assuming this law of absorption to hold, the rays would pass 
through a thickness of about 7 cm. of lead, 19 cm. of iron 
and about 150 cm. of water before the intensity would be 
reduced by absorption to one per cent. of its original value. 
The amount of the penetrating radiation from thorium is 
about the same as for radium, taking into account the ratio of 
their radio-activities. As the radium employed was about 1000 
times as active as thorium, it was necessary to work with a 
kilogram of thorium nitrate to obtain about the same amount of 
rays as from the '7 gr. of radium. 
Experiments were also made to see if the excited radio-activity, 
due to thorium and radium, which gives out deviable and non- 
deviable rays, also emits_these penetrating rays. In order to 
get measurable effects, it was necessary to obtain intense excited 
activity. For this purpose a zinc plate was exposed as kathode 
in a closed vessel containing 300 gr. of thoria. A lead wire was 
also made very active by exposure.as kathode for six hours in a 
NO. 1709, VOL. 66] 
NATURE 
[JuLy 31, 1902 
vessel containing a large amount of radium emanation, obtained 
by bubbling air through a solution of radium chloride. The 
excited radiation from these two sources was found to include 
rays about as penetrating in character as those from radium and 
thorium. The intensity of these rays diminished with the time, 
rapidly for radium and more slowly for thorium excited radia- 
tion. This diminution with time is probably directly connected 
with the rate of decay of the other known types of radiation from 
excited bodies. 
Since the penetrating rays are present in thorium and radium, 
and also in the excited radiations due to these bodies, and are 
absent in uranium, it seems probable that the penetrating rays 
in both radium and thorium are due to the excited radio-activity, 
produced in the mass of the compound by the emanations which 
are unable to escape into the air. According to this view, the 
production of penetrating rays is a function of that portion of 
radio-active matter which causes excited radio-activity. 
Connection between Absorption and Density.—Some experi- 
ments were made to see how the absorption of the rays by 
matter varied with the density. The coefficient of absorption 
A was determined by noting the ratio of the intensities of the 
rays after passing through a known thickness of matter. The 
following table illustrates the results :— 
Penetrating rays. | Deviable rays from 
uranium, 
Substance. - - i = api me 
rN | AY | ee fe ce 
| density | density 
eeu = 
Water S00 033 033 | —- |}; =— 
Glass »e1 |, O86 7035 14:0) ian i57i 
Tron... os 28 *036 44 aig 
Zinc... Be ‘28 039 — — 
Copper se ei "035 60 iT, 
int. =~ Bo | “38 [OS 2) ae OD. 13-2 
Lead eae eer 77/ 068 122 10°8 
Mercury 14) golem icons _ = 
| 
A comparison table on the right is added for the deviable rays 
given out by uranium. It will be seen that the quotient of 
absorption by density is in neither case a constant, but the 
differences are no greater for the non-deviable penetrating rays 
than for the deviable rays of uranium. It is interesting to 
observe that the value of A divided by the density is for both 
types of rays twice*as great for lead as for glass or iron. It 
will be seen from the above table that the penetrating rays from 
radium, compared with the deviable rays of uranium, pass 
through a thickness of glass about 160 times greater for the same 
reduction of intensity. 
Comparison of penetrating Rays with Rontgen and Kathode 
Rays.—The question at once arises as to whether these very pene- 
trating rays are projected particles like kathode rays or a type 
of Rontgen rays. The fact that the penetrating rays are not 
deviable by a magnetic field seems, at first sight, to show that 
they cannot be kathode rays. I have repeated the experiments 
of Villard, and have been unable to obtain any appreciable 
deviation of the rays, which had passed through ‘6 cm. of 
lead, even in a very strong magnetic field. The photographic 
method was used, and four days’ exposure of the plate was 
necessary to get an appreciable impression. In some other 
respects, however, the rays seem more closely allied to kathode 
than to Roéntgen rays. It is well known that Rontgen rays 
produce much greater ionisation in gases like sulphuretted 
hydrogen and hydrochloric acid gas than in air, although the 
differences in density are not large. For example, sulphuretted 
hydrogen gives six times and hydrochloric acid gas nine times 
the conductivity of air. On the other hand, with kathode rays 
the conductivity observed is only slightly greater than for air. 
The experiment was made of filling the testing vessel with 
sulphuretted hydrogen, when it was found that the current 
for the penetrating rays from radium was only slightly greater 
than for air. Both this experiment and the results for the 
variation of absorption of the rays with the density of matter 
seem to show that the penetrating rays have a closer resemblance 
to kathode than to Roéntgen rays. 
It must, however, be remembered that the observations on 
the relative conductivity of gases and the relative absorption of 
