APRIL: 11, 1907 ] 
increase of the surface, by enclosures of air, will increase 
the radiating power. It is my intention to make some 
measurements of the intensity of the light radiated from 
the ends of long and short cylinders of red-hot glass. 
The hydraulic analogy of radiating bodies which we will 
now consider occurred to me during a lecture on radi- 
ation, and proved quite useful in explaining the different 
behaviour of various types of radiators. 
The radiator is represented by a tall hollow cylinder, 
open at the top and closed at the bottom, provided with a 
number of outflow pipes of different sizes as shown in 
Fig. 1. Water flows into the 
cylinder at a certain definite rate 
from a horizontal pipe or flume, 
the height of which above the 
base of the cylinder (1,) repre- 
sents the temperature of the 
flame. Obviously the level of the 
water in the cylinder will rise 
until the rate at which the water 
flows out exactly equals the rate 
at which it flows in. This 
height (1,) is the temperature 
which the radiator acquires in 
the flame. The jets of water 
which issue from the tubes re- 
present radiation of - different 
wave-lengths, the small jets re- 
presenting the short waves. 
We will first suppose our 
hydraulic radiator to represent 
a black body, say a lump of carbon. In this case all 
the pipes at the bottom are wide open, and we have the 
maximum outflow of all wave-lengths for any given 
temperature, t.e. for any given height of the fluid within 
the cylinder. If we take the cylinder empty and plunge 
it into water, jets will squirt into it through the pipes, 
that is, it is a perfect absorber for all wave-lengths. 
With all the pipes open, however, the level of the water 
within the cylinder will not rise to any great height, 
owing to the limited rate at which water flows in from 
the horizontal pipe. This means that the lump of carbon 
in the flame does not rise to a yery high temperature 
because it radiates energy at a high rate. At the low 
temperature there is comparatively little visible light in 
the radiation, for the shorter waves only appear in 
quantity at high temperatures. We can imitate this con- 
dition in our hydraulic model if we choose by putting 
valves on the inside of the tubes, those on the small tubes 
opening only at high pressures. 
To make our model imitate the 
salt we plug up all the pipes. The cylinder now re- 
presents a transparent body. If immersed in water it 
absorbs nothing through the pipes, and no matter how 
high the level of the water rises ip it there is no emission 
of fluid, in other words, no radiation. The body rises in 
temperature until the temperature is equal to that of the 
flame, but there is no radiation. Take next the case 
of the lime in the oxyhydrogen flame. It is a partially 
transparent substance, and we can imitate it by plugging 
the tubes with glass beads or cotton. Owing to the lesser 
rate at which the water now flows out through the tubes, 
the level rises much higher than when the tubes are all 
open, and owing to the greater pressure (temperature) we 
have liquid jets through the small tubes (short wave-length 
radiation). The inferiority in the emissivity is more than 
made up for by the higher temperature which the body 
can acquire. We are now ready for the Welsbach 
mantle. 
It has been conclusively shown by Rubens that the 
peculiar brilliancy of the thorium mantles, caused by a 
small trace of cerium, is due to the fact that the cerium 
makes the thorium selectively absorbing for the short waves 
at high temperatures. If we wave a Bunsen flame over 
a mantle in a brilliantly lighted room, it will be seen to 
turn yellow at a temperature a little below a red heat. 
In other words, it becomes a strong absorber for the short 
waves. It is, however, transparent for the long waves, 
consequently it does not emit energy at anything like the 
rate at which a black body does, and in consequence can 
rise to a high temperature in the flame, exactly as a pure 
NO. 1954, VOL. 75] 
bead of microcosmic 
NATURE 
559 
thorium mantle. Its band of absorption in the blue region 
enables it to pour out visible radiations nearly as power- 
fully as those which a black body at the same tempera- 
ture would emit, hence its enormous brillianey. Our 
hydraulic model, with its tubes all plugged with cotton, re- 
presents the mantle of pure thoria, while to transform it 
into the Welsbach mantle we have only to pull out the 
porous plugs from some of the smaller tubes. In this 
condition, owing to the impeded flow in the large tubes, 
the water wifl rise in the cylinder to a great height, and 
we get very powerful jets from the small tubes which we 
have opened, much more powerful than in either of the 
previous cases considered. Of course, with all the tubes 
open we could get equally intense small jets if we poured 
the water in at the top at a sufficient rate. There is a 
limit to this rate, however, for it is obvious that the rate 
at which the water is poured in at the top corresponds 
to the rate at which the flame can pour energy into the 
radiating body, a circumstance which depends on the con- 
ductivity of the body for heat and other things. 
It is not necessary to make the hydraulic apparatus, of 
course, for its action is so easily understood that a 
diagram answers every purpose. Its utility lies in the fact 
that it fixes in the mind of the student the behaviour of 
different types of radiators when plunged into a flame. 
It could be made, perhaps, to illustrate the displacement 
of the point of maximum energy in the spectrum which 
accompanies a rise in temperature, but it is doubtful 
whether any such complications would -prove beneficial. 
It seems best, on the whole, not to try to illustrate too 
much with it, as its relation to a radiating body is at 
best rather far-fetched. R. W. Woop. 
Johns Hopkins University, Baltimore. 
Retardation of Electroscopic Leak by means of 
recognised Radio-active Substances, 
IN a communication made to the Royal Society on 
April 5, 1906, and subsequently published in the ‘* Archives 
of the Middlesex Hospital,’’ vol. vii., I described certain 
experiments which I regarded as showing that substances 
exist which retard the leak of an earthed metal electro- 
scope. I further asserted that an aluminium plate which 
had been kept in proximity to, but not in contact with, 
uranium, thoria, or pitchblende, also retards the electro- 
scopic leak. This retardation does not necessarily occur 
immediately after introduction of the modified aluminium 
plate into the electroscope, for after proximity to thoria 
there is a period, lasting three or four days, during which 
the leak is accelerated, and after proximity to radium I 
failed to find any evidence of retardation whatever. My 
results were received with scepticism, except by Sir 
William Ramsay, who had independently observed the 
same phenomenon in his laboratory. It is impossible to 
occupy your space with details, but it may be stated that 
gold-leaf electroscopes made of }-inch lead were used, 
that the earthing of electroscopes and aluminium was 
complete, that effects of induction and alteration of 
capacity of the electroscope were eliminated, and that the 
general conditions were kept as constant as possible. 
Since reading the paper I have repeated the experiments 
in the most stringent way of which I am capable in a 
pathological laboratory, and have obtained identical results. 
Further, using the same apparatus, I have exposed the 
aluminium plates to X-rays for a period of three hours, 
and have found a complete absence of any change in the 
rate of leak, whether in the direction of retardation or 
of acceleration. Full details of these experiments will be 
published in the forthcoming number of the Archives of 
the Middlesex Hospital. Below I give the salient points 
of an experiment which was carried on continuously from 
August 10 to December 24, with the exception of intervals 
August 27 to September 11, and September 19-30, during 
both of which the electroscopes were left undisturbed. 
The values given represent percentages of the mean leak 
of the electroscope during twenty-four hours under normal 
conditions corrected by the leak of the control electro- 
scope on the day for which the observation is given. 
Lowest corrected percentage during period August 10 to 
