“Commercial Egg-Farming: From Practical Ex- 
‘equal income so easily in any other way.” 
_says the author on p. tro. 
secure for the work the serious consideration of 
_all poultry-keepers. 
-much information and no padding. 
SEPTEMBER 7, I916| 
NATURE — 5 
periences gatmed over a Period of Years. By 
S. G. Hanson. Pp. 62. (London: Constable 
and Co., Ltd.) Price 1s. net. 
“TI am not a poultry-farmer because I like hens, 
but simply because | do not know how to earn an 
So 
This fact alone should 
It is a good book containing 
We should, 
however, have liked more detail on several points, 
and are far from convinced as to the economic 
side of the large brooder-nouse. 
We note there is no balance-sheet. The cost 
of rearing pullets, even Leghorns, appears almost 
too modest at 2s. 6d. per head; and colony houses 
at about 2]. each (p. 39) also seems _ scarcely 
sufficient. : 
We like the author’s capital charge of 11. per 
bird, and this agrees with our own estimates and 
experience. No figures are given covering 
labour, rent, rates, and depreciation, considerable 
items on large plants. 
The book is well worth reading, being full of 
hints, and the figures on the income. side do not 
appear to be exaggerated; but it must be remem- 
bered that there are many drones in a large flock, 
and it is on the elimination of these that ultimate 
success depends. Although we are far from con- 
vinced as to the desirability of dry-mash feeding, 
there is much to be said in its favour on the score 
of economy of labour. 
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.] 
Observations on the Excitation of Helium Spectra. 
In the course of an examination of the properties 
of the electron discharge in an atmosphere of helium 
we have made some observations of the conditions 
affecting the excitation of the lines of the helium 
spectra which seem to be of considerable interest. The 
- source of electrons was an incandescent tungsten fila- 
_ wire about 8 mm. distant. 
' 2 mm. pressure. 
_ 20 volts. 
100 microamperes at 23 volts, when the arc spectrum 
ment, and the discharge passed to a parallel nickel 
The electrodes were 
mounted in a quartz tube filled with helium at about 
The helium was free from all con- 
taminants except a small amount of mercury vapour, 
the partial pressure of which was about o-oor mm. in 
the observations immediately following. 
In a particular experiment the current across the 
gap increased slowly from zero to 10 mitroamperes 
as the potential difference between the anode and the 
negative end of the filament was raised from o to 
The current then increased more rapidly to 
of mercury flashed out and the current jumped to 220 
microamperes, the potential necessary to maintain the 
discharge dropping at the same time to 21 volts. On 
raising the potential further. the current. increased 
rapidly to 690 microamperes at 23-5 volts, when the 
NO. 2445, VOL. 98] 
helium spectrum flashed out. With higher potentials 
most of the lines in the helium spectrum increased in 
intensity and the current increased, but at a gradually 
diminisning rate, to 1450 microamperes at 4o volts. 
In other experiments the helium spectrum has been 
found to be excited by 22-5 volts potential difference. 
If allowance is made for the initial kinetic energy 
of the electrons and for the drop of potential down 
the filament due to the heating current, this quantity 
is not increased by so much as one volt, and there are 
indications that it tends to approach a lower limit close 
to the ionisation potential value for helium found by 
Franck and Hertz and by Pawlow. In any event, the 
observations made would seem to destroy the special 
significance of the value (approximately 30 volts) of 
the minimum potential difference necessary: to excite 
the line spectra of helium given by Rau, ‘There is no 
doubt that these spectra can be excited by the impact 
of electrons having energies much less than the value 
of the ionisation energy of helium calculated by Bohr. 
We are unable to reconcile these results with Bohr’s 
theory except on the hypothesis that the impact ionisa- 
tion of helium is a more complex phenomenon than 
has been supposed. Possibilities which suggest them- 
selves are that the ionisation is the result of successive 
impacts or results from impacts on atoms in an 
abnormal condition caused by the absorption of radia- 
tion generated in other atoms in consequence of elec- 
tron impact. Experiments to test these possibilities 
are in preparation, 
In contrast to the lines of the mercury arc spectrum 
the different helium lines behave differently inter se 
when the exciting voltage is changed. Thus the blue 
line 4472 requires about half a volt, and the blue line 
4713 about a quarter volt more potential difference for 
excitation than the yellow line. The green line 4922 
of the first subsidiary series of parhelium seems to 
appear and disappear along with the blue line 4472 
of the first subsidiary series of helium. The order 
of excitation with voltage for the different lines is not 
simply a question of frequency, but depends partly on 
the series to which the lines belong. Most of the lines 
increase steadily in intensity with increasing voltage 
and current density, but the line 4713 of the second 
subsidiary series of helium increases rapidly in in- 
tensity to a maximum soon after excitation, then be- 
comes very faint as the potential difference is increased 
to about 4o volts, reappears with higher voltages, and 
then increases steadily in intensity with rising poten- 
tial difference. Several of these effects have been 
noted by Rau at higher voltages. 
We have examined the radiation from the helium 
spectrum in the extreme ultra-violet when excited by 
40 to 7o volts potential difference, using a photo- 
electric method, and have obtained definite indications 
of the presence of radiations having wave-lengths 
close to 600 and to 400 Angstrom units respectively. 
O. W. RicHaRDSON. 
C. B. Bazzont. 
Wheatstone Laboratory, 
King’s College, London. 
The late Prof. dames Geikie. 
A pioGrapny of the late Prof. James Geikie is now 
in course of preparation, and the work would be 
greatly facilitated if those who have letters or com- 
munications of general interest from him would kindly 
forward these to me at the Royal Scottish Geograph- 
ical Society’s Rooms, Synod Hall, Castle Terrace, 
Edinburgh. They will be carefully preserved and re- 
turned after being copied. Marion I, NEWBIGIN. 
Edinburgh, September 4. 
