NovEMBER 5, 1903] 
NATURE 5 
ERTTERS TO THE EDITOR. 
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Variation of Atmospheric Absorption, 
I sHoutp be pleased to know how far some observations 
on the change in the average absorption of the terrestrial 
atmosphere in this country during the last two years have 
been confirmed by observations elsewhere. The following 
table gives the mean of the best of these, made at Wash- 
ington in the autumn of 1901, the spring and autumn of 
1902, and in the winter, spring, and summer of 1903. 
Coefficients of Transmission for Zenith Sun. 
| Hama ce |e » |» | # M bo 
Wave-length... 0750 | 0°60 | 0°70 | 0°80 | 0'90 | 1’00 | 1°20 | 1°60 
Mean of observa- | | | 
tions, 1901-1902. | 07765 | 0°76y | 0°857 | 0°897 | O'gTO | O'g2T | 0°933 0°930 
Mean of observa- | | 
ticns during 1903 | 0°627 | 0°692 | 0°753 | 0°797 | 0°825 | 0847 | 0°874 | 0909 
Excess of transmis- 
sibility of 1901— | 
1g02 over that of | 
1903 20% | 10% | 13%] r2% | 10% | 84% | 6°5% | 2°3% 
The decrease in the transmissibility of the air this year 
as compared with the last is so marked that some local 
effect on climate and vegetable growth might seem to be 
probable. Whether the unusual coolness of the summer, 
reported both in America and abroad, is connected with it 
may be a subject for speculation. S. P. LANGLEY. 
Smithsonian Institution, Washington, October 22. 
Heating Effect of the Radium Emanation. 
Tur very important and fundamental experiments de- 
scribed by Profs. E. Rutherford and H. T. Barnes in 
Nature of October 29 will have been read with the greatest 
interest. Owing to the importance of the subject, I should 
like to direct the-authors’ attention to some points in their 
comment and explanation which do not appear to me to 
be quite clear, and if I can draw from them some more 
detailed discussion this letter will have served its purpose. 
The general conclusion arrived at by the authors is that 
““more than two-thirds of the heating effect is not due to 
the radium at all, but to the radio-active emanation which 
it produces from itself.”’ If I understand the description 
of their experiments correctly, these seem to me, however, 
to point to the fact that it is the ‘‘ excited activity ’” and 
not the emanation that is the cause of the heating. 
Apparently de-emanated radium gives out an amount of 
heat at a rate which falls in a few hours to a minimum 
and then slowly recovers. Now the emanation itself begins 
to form again at once, so that on the authors’ hypothesis 
the heating effect should start with a minimum and then 
gradually increase. The activity of the radium measured 
by electric methods follows the course of the heating effect, 
and, as Messrs. Rutherford and Soddy have explained 
(Phil. Mag., April, not May as quoted by the authors), this 
is due to the fact that the de-emanated radium has still 
the excited activity attached to it, and this activity decays 
in the course of a few hours. When the excited activity is 
gone there is nothing but radium left, and the further 
changes are due to the re-formation of the emanation and 
its subsequent change into excited activity. During the 
course of the first few hours there is, therefore, very little 
emanation, but there is excited activity which falls to a 
minimum and then slowly grows again. Does not the ex- 
planation which holds for the activity also hold for the 
heating effect, and would it not follow that the parallelism 
of heating effect lies with the amount of the excited activity 
present, and should be assigned to it rather than to the 
emanation ? 
Similarly, the emanation, according to the authors, does 
NO. 1775, VOL. 69] 
not give its full heating power at first, but the heating 
effect rises to a maximum in the course of the first few 
hours. If the emanation is the cause of the heat, why this 
slow rise? Here again the effect seems proportional to 
the amount of excited activity present, and not to the 
amount of the emanation. The connection of heating 
power with the emission of a rays also requires further 
elucidation, and the information given by the authors is 
not, I believe, sufficient to prove their case. It is only with 
great diffidence that I address these remarks to you, because 
Prof. Rutherford knows the whole subject at first hand, 
and his judgment is more likely to be correct than mine. 
Nevertheless, one likes to know whether others have felt 
the same difficulty, and whether the apparent disagreement 
is one of misunderstanding or has some more deep-seated 
cause. ARTHUR SCHUSTER. 
The Owens College, Manchester, November 2. 
Radium and Plants. 
Tue sensibility of protoplasm towards the radiations of 
radium is a matter of so much importance that a few 
preliminary experiments I have carried out on plants may 
be of interest. 
The first experiment I made in this direction was 
with cress seedlings. About 100 seeds were uniformly dis- 
tributed over the surface of some moist sand contained in 
a flower saucer, and a tube containing 5 mgrs. of pure 
radium bromide supported at a height of 1 cm. over the 
centre of the sand surface. During the experiment the 
saucer, covered with a glass shade, was kept in the dark. 
It was hoped that this arrangement would show whether 
the radiations are harmful or not to the sensitive cells of 
seedlings, and at the same time indicate if they are able 
to act as a stimulus to evoke positive or negative 
curvatures. 
After the germination of the seeds, which took place 
within two days nearly simultaneously all over the sand, 
the growth of all the seedlings was nearly uniform. But 
close comparison showed that the seedlings immediately 
under the radium tube were to some small extent retarded 
in their development. The retardation was apparent in the 
seedlings situated within a radius of about 2 cm. from the 
radium bromide. Besides being smaller, these seedlings 
developed somewhat fewer and shorter root-hairs than those 
nearer the margin of the sand. 
In the subsequent growth the presence of the radium 
evoked no curvatures in the little plants close by it, or 
in those more removed. Nor did it appear to exercise any 
noxious effects, other than the retardation just described, 
on the seedlings within the period of the experiment, viz. 
thirteen days. The plants grew up beside it and against 
the glass containing it, neither influenced by it nor hurt 
by it, so far as one could see. 
This experiment was repeated on two other occasions 
(one experiment lasting three days after germination and 
the other lasting four days) with the same result, viz. no 
curvature was evoked, but the seedlings close under the 
radium bromide were slightly retarded in their growth. 
In order to determine if motile organisms are sensitive 
to the radiations I enclosed the radium tube in a vessel of 
water containing large quantities of Volvox globator. 
Extraneous light was cut off from the experiment. After 
twenty hours many of the Volvox colonies had sunk to the 
bottom of the vessel, but they were evenly distributed over 
the bottom, and were neither aggregated under the tube 
nor dispersed away from it. Those that were still 
swimming in the water were also uniformly distributed 
through it, some actually in contact with the radium tube 
and some far away from it, but showing no sign of being 
attracted towards it, or of being repelled from it. 
It is apparent from these few experiments that the radi- 
ations emitted by radium bromide are not able to produce 
marked effects in a short time on these vegetable cells and 
tissues. Even the phosphorescent light (which is quite 
perceptible to the eye under suitable conditions) emitted by 
the radium bromide is too feeble to be effective in calling 
out a phototactic response. Henry H. Drxon. 
Botanical Laboratory, Trinity College, Dublin. 
