, 
4 
FEBRUARY 12, 1914] 
sequence. He is first introduced to the astronomy 
of the ancients, and then of to-day. The his- 
torical account of the telescope is followed by the 
practical forms of to-day, leading up to the 
famous observations of the present time. Then 
follow concise statements about the sun, moon, 
planets, comets, stars, coloured and multiple, 
clusters and nebulz, &c., all of which are sufficient 
to give the reader an interest in the subject and 
a wish to know more about them. Many practical 
hints and much good advice are given which will 
be serviceable to those who are making use of 
small telescopes. Numerous well-chosen illustra- 
tions, many of which are from the pencil of the 
author, accompany the text. A glossary of astro- 
nomical terms, a brief bibliography of the more 
elementary astronomical books and an index bring 
this practical little book to a conclusion. 
The Petrology of the Igneous Rocks. A Summary 
of the modern Theories of Petrogenesis, a 
_ Description of the Rock-forming Minerals, and 
a Synopsis of the chief Types of the Igneous 
Rocks and their Distribution as illustrated by 
the British Isles. By Dr. F. H. Hatch. 
Seventh edition, revised. Pp. xxiv + 454. 
(London: George Allen and Co., Ltd., 1914.) 
Price 7s. 6d. net. 
ATTENTION has been directed in these columns 
to this now well-known text-book on two previous 
occasions. On May 14, 1891 (vol. xliv., p. 25), 
the first edition was reviewed at length, and on 
May 20, 1909 (vol. Ixxx., p. 337), the fifth edition 
was noticed. It will be sufficient to say of the 
present edition that it has undergone consider- 
able revision and that new chapters on the pyro- 
clastic rocks and the metamorphic derivatives of 
the igneous rocks have been added, together with 
numerous new illustrations. 
LETTERS TO THE EDITOR. 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications.] 
Active Nitrogen. 
A PaPER appears in the Berichte (vol. xxxvi., 17, 
P. 4095, 1914), by E. Tiede and Domcke, in which 
it is again maintained that the glow characteristic 
of active nitrogen is not seen in the absence of oxygen. 
In the experiment chiefly relied on, the authors pre- 
pare nitrogen in an exhausted apparatus, by heating 
barium or potassium azide; and they lead it through a 
cooled vessel straight into the discharge tube, also 
exhausted. They state that after careful washing out 
with nitrogen, no afterglow appears. We do not 
know how to account for their conclusion, but we 
can only state that in our hands the experiment gives 
exactly the opposite result. We used potassium azide, 
and after most thoroughly heating the glass and the 
electrodes and washing out the vessel repeatedly with 
nitrogen, the glow remained absolutely undiminished 
in intensity. 
We have also tried a new experiment. Some of the 
NO. 2311, VOL. 92] 
NATURE 659 
liquid alloy of sodium and potassium was placed in a 
discharge bulb, which was charged with rarefied 
nitrogen. The surface of the alloy is quite bright, 
and the nitrogen has been standing over it for three 
weeks, but the afterglow is as good or better than 
ever. Fuller details of these experiments will be pub- 
lished later. 
Finally, even if what Tiede and Domcke say were 
true (which we entirely deny), we do not see that it 
would alter the fact that a gas has been obtained by 
one of us capable of reacting in the cold with, e.g. 
hydrocarbons to form hydrocyanic acid. If this is not 
active nitrogen, what is it? 
H. B. Baker. 
R. J. Strom 
Imperial College of Science, February 1o. 
Weather Forecasting. 
Mr. Dertey’s plea (Naturr, January 29) for in- 
creased aid to meteorology certainly deserves serious 
consideration. Under the present condition of affairs 
it is not possible to issue forecasts for more than a 
day or two in advance with much hope of success, 
but there is no reason why this should continue in- 
definitely. Seasonal forecasts would be of immense 
importance to agriculture, more so, indeed, than fore- 
casts for the following week, because, were the char- 
acter of an ensuing summer known, it would be nearly 
always possible to plant crops that would thrive under 
the expected conditions. It is quite possible that if 
we had a sufficiency of good charts covering the 
greater part of the earth, seasonal forecast might be 
made empirically, just as daily forecasts now are, and 
were they as successful as the present daily forecasts, 
the occasions on which a fairly good harvest could not 
be secured would be few, for it is seldom that the 
weather is not favourable for some one or two crops. 
The key to the whole situation lies in being able to 
foretell the distribution of pressure. Being given a 
chart with the isobars on it, it is possible to fill in a 
great deal more with fair certainty. 
But at present we are hopelessly in the dark as to 
the reasons of cyclones and anticyclones, why they 
form and why they move. The investigation of the 
upper air has led to this, that all our old theories 
about cyclones and anticyclones must go to the scrap- 
heap. It has been usual, and is still for that matter, 
to explain a high or low barometer by saying that 
the air over them is respectively cold or hot. The 
exact opposite is the fact. If the barometer in Europe 
at a certain place is very low it is a practical certainty 
that the greater part of the mass of air lying over that 
place will be very much colder, and therefore heavier, 
than usual. Facts of this sort have to be explained 
before we can hope to advance much farther. 
But there is very good hope for the future. If the 
upper air investigation has entirely altered our ideas 
as to the cause of pressure changes, it has also shown 
that the conditions prevailing above are far more 
simple than they are below. If an isobaric chart 
for a height of 9 km. for Europe could be drawn 
for a given date, that chart would enable us to fill 
in with fair accuracy the temperatures, and therefore 
also the pressures, for the space covered between the 
heights of 1 km. and 20 km. Higher than 20 km. 
the observations do not go, and below 1 km. the 
conditions are exceedingly complex, but within the 
limits given the temperatures do not differ very far 
from linear functions of the pressure at 9 km. 
It seems, therefore, as though the surface changes 
are a sort of by-product of the changes occurring 
above, but the outstanding puzzle is what produces 
and maintains the changes of pressure above. 
