102 
minutes after sunset ; November 3, a brilliant orange sunset 
ater a very clear day,” 
These observations were recalled by the receipt within a few 
days of a pamphlet from Mr. T. F. Claxton on ‘‘The Recent 
Sunset and Sky Glows.” This paper was presented to the 
Mauritius Meteorological Society on August 27, 1901. 
The first few paragraphs are as follows :— 
‘The gorgeous sunsets and sky glows of the past three months 
recall those vivid displays of 1883 and 1884 which were associated 
with the disastrous volcanic eruptions at Krakatoa, in the Straits 
of Sunda, and it is not surprising to learn that toward the end of 
May of this year similar, though less serious, eruptions oc- 
curred in about the same locality, according to the following 
cablegram which appeared in the Dazly Graphic ;— 
“«* Batavia, Jay 23, 1901.—The volcano of Keloet is in erup- 
tion. It is reported that there has been great loss of life among 
the natives. District of Kediri enveloped in total darkness.’” 
The sunset glows at Blue Hill followed this eruption, and 
the sky glows at Mauritius after about the same interval as 
similar but more brilliant glows in these latitudes followed the 
eruption of Krakatoa. It would be extremely interesting to 
knowif there are observations at intervening places. We should 
be glad to receive notes of such at the Blue Hill Observatory, 
Hyde Park, Mass., U.S. 
I wish also to call attention to the recent violent volcanic 
eruption in the island of Martinique, and suggest that observers 
be on the watch for the earliest optical phenomena. We 
should be glad to receive notices of such observations. There 
were some marked barograph undulations at Blue Hill on the 
morning of May 7 which are perhaps connected with this vol- 
canic eruption. H1enNrY HELM CLayTON. 
Blue Hill Observatory, Hyde Park, Mass., May Io. 
A Method of Showing the Invisibility of Transparent 
Objects under Uniform Illumination, 
As is well known, a perfectly transparent object is visible 
only in virtue of a variable illumination. This condition 
might be approximately realised, as Lord Rayleigh points out 
in his article on ‘‘ Optics” in the ‘* Encyclopzedia Britannica,” 
on the top of ahigh monument inadense fog. It is doubtful, 
however, if the experiment would be very successful even 
under these conditions, as the observer’s body screens the 
light in certain directions, making the illumination far from 
uniform. The following method I have found to give very 
good results :— 
The inside of a hollow sphere of metal, which can be 
separated into two cups, is thickly coated with Balmain’s lumin- 
ous paint. A small hole, not much larger than the pupil of 
the eye, enables the observer to view the interior and any 
objects within the sphere. I used for the sphere one of the 
metal floats which are used as automatic regulators in water 
tanks, and which can be obtained from any plumber. The 
float was made in two parts, which were easily separated by 
melting the solder. It is rather difficult to get a good uniform 
layer of the paint. Several coats are required, and even then 
it isapt to appear streaky in the dark. I am inclined to think 
that a better plan would be to mix the dry powder with boiled 
down Canada balsam, which will harden on cooling, and coat 
the outside of two glass hemispherical evaporating dishes with 
the hot mixture. The lips of the dishes would make the eye- 
hole. This mixture I have found produces much more uniform 
surfaces, and Iam employing it at the present time in some 
experiments in infra-red photography. 
If the inner surface is exposed to sunlight, and a transparent 
object such as a glass or crystal ball, a thick lens or a cut glass 
decanter stopper is put in the interior, it will be found to be 
practically invisible when viewed through a small hole, for light 
of equal intensity is incident in every direction. I have found 
that a large stopper with many facets does not quite disappear, 
some of the edge facets appearing darker than the diffused blue 
glow which fills the interior of the ball. This I believe to be 
due to the fact that the light reaching the eye from these facets 
by refraction happens to have undergone several internal re- 
flections and suffered a loss by absorption owing to a long path 
through the glass. The luminosity of the interior of the sphere 
is not quite uniform, however, and this may be sufficient to ex- 
plain the appearance of these facets. The observation is best 
made ina darkened room, the eye being brought close up to the 
small aperture. 
Since writing the above I have tried the balsam mixture on 
NO. 1700, VOL. 66] 
NA TORE 
[May 29, 1902 
the outside of hemispherical glass dishes. It is, however, better 
to scratch a small hole in the paint than to attempt to use the 
lips of the dish as an aperture, as in the latter case the line of 
union, which is always slightly darker than the rest of the 
surface, cuts directly across the field of view, which is a 
disadvantageous arrangement. q R. W. Woop. 
Johns Hopkins University, Baltimore. 
Misuse of Coal. 
THE tone of Prof. Perry’s letter in reply to Mr. Rosenhain is 
so acquiescent that it may seem to diminish the force of his 
original contention as to the national misuse of our stock of 
coal. There are two considerations which ought to be stated in 
reply to the plea that men may learn to grow their fuel as they 
go on, by a proper cultivation of the best vegetation. 
The first is this. The soil will not long continue to yield food 
if it be asked to provide fuel also. About three years ago- 
Sir W. Crookes devoted his address, as president of the British 
Association, to the consideration of the present position of the 
world’s food-supply question, and arrived at the conclusion that 
the outlook was not far from a gloomy one. 
In that conclusion he was but echoing Malthus, though with 
much better data and a more complete record as to what were 
in Malthus’ day unexplored countries. 
If examination of the food-yielding powers of the soil leads to 
such a result, it is evident that to add an additional demand for 
fuel will seriously injure both. Even though Malthus and Sir 
W. Crookes be only partially right, enough is left to prevent us. 
getting any long-lived satisfaction by growing fuel. There 
remains the possibility of ‘‘ intensive” cultivation, and this may 
be one form of the new engine Prof. Perry asked scientific men 
to look for. Already Lord Rayleigh has made a bold attempt 
to make this economically possible by preparing nitric acid from 
the air. Perhaps with the resources Prof. Perry asked for, Lord 
Rayleigh might succeed. 
The second point is this. Prof. Perry’s concern was mainly 
for British resources. The economic life of a large proportion 
of our people is bound up with an economic advantage in fuel 
and other minerals. Every scientific discovery which raises the 
efficiency of transformation from coal fuel to mechanical power 
helps to defer the day in which England’s mineral endowment 
will no longer be exceptional. The moment that oil or other 
natural fuel can compete with coal in the open markets, our 
prosperity must begin to decline. Similarly, if fuel can be 
grown to compete with coal, we lose position, simply because 
we cannot expect to grow so easily and well as many other 
countries. 
The motive impelling towards a constant search for improved 
efficiency in the use of coal is therefore doubly strong on our 
people and Government. Any improvement would be helpful 
to the whole world ; for us it would defer a calamity, possibly 
for a very long time. W. HIBBERT. 
101 Goldhurst Terrace, N.W., May 20. 
The Conservation of Weight and the Laws of 
Thermodynamics. 
In NatTurRE of May 15, Lord Rayleigh uses the laws of 
thermodynamics to prove the conservation of weight. 
In regard to the doctrine of the conservation of energy (the 
first law of thermodynamics) the following statement is made 
in Maxwell’s ‘‘ Theory of Heat,” p. 145, tenth edition: ‘*The 
evidence which we have of the doctrine is nearly if not quite 
as complete as that of the conservation of matter.” 
Taking this passage to imply that the two doctrines, conser- 
vation of weight and of energy, are to be held true as far as 
experiment has proved them true, and no farther, the question 
arises—To what extent have the laws of conservation been 
proved ? 
The experiments of Landolt (1893) and of Heydweiller (1901) 
show that the conservation of weight holds, in the cases inves- 
tigated by them, to one part in one hundred thousand. The 
accuracy of the law to one part in a million is left under 
suspicion. 
Energy being more difficult to measure than weight, it is 
unlikely that the conservation of energy has been proved to one 
part in one hundred thousand, At the present time, would not 
Maxwell say, ‘‘ The evidence which we have of the conser- 
vation of energy is not as complete as that of the conservation 
of weight” ? 
