174 
NATURE 
[APRIL 15, 1915 

the use of alcohol, the raising of the standard of 
parental fitness, and similar problems. In a de- 
tached appendix the dangers of venereal diseases 
are frankly pointed out. The author’s general 
position is that much is to be attained by greater 
cleanliness, increased control of the hours and 
conditions of labour, and improved environment 
for children, but that there must also be some 
way of preventing the decisively unfit from be- 
coming parents. The book is competent and 
wise, but some of the statements seem to us to 
require safeguarding. The citations as to child 
labour ‘in the greatest canning factory” in 
America are so terrible that we hope there is some 
mistake. The date should have been given. The 
book is dedicated to “Boys and Girls, the 
Guardians of the Next Generation,” but we hope 
we are right in understanding that it is meant 
only for the teacher’s use. 
Principles of Physical Geography. Bye Goes 
Pry, Bp. xeeasie (ondonl:= Win Bs Clive 
1915.) Price 1s. 6d. 
Tuis little book contains that part of the author’s 
text-book of geography which deals with physical 
geography, with some additions on such subjects 
as map drawing, climate, and the crust of the 
earth, as well as a new chapter on man and his 
work. It contains no definite instructions for 
practical exercises to be worked by the pupil, but 
the descriptive treatment will prove suitable for 
students preparing for the examinations men- 
tioned in the author’s. preface. 
ELEERERS TO DEE VE DimOR: 
[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 Naturr. No 
taken of anonymous communications. | 
notice is 
The Thermionic Gurrent. 
Ir a carbon filament lamp is silvered inside, and a 
platinum wire through the side of the glass makes 
electric connection with the belt of silver, it is easy to 
experiment with the thermionic current, using a tele- 
phone receiver with one terminal to the platinum wire 
and the other to the water mains. 
With an alternating current at 110 volts, a loud 
note is heard, depending on the frequency of alterna- 
tion. With a direct current from a dynamo there is 
sufficient variation in the voltage to obtain a sound 
just audible at 100, loud at 110, very loud at 130, 
and it might be described as an uproar at 140 volts. 
It might be expected that the intensity would increase 
until the lamp burnt out. Nothing of the sort. At 
142 volts the uproar is replaced by dead silence, which 
continues up to 165 volts, as high as the lamp would 
stand. 
The explanation may be gathered from Langmuir’s 
paper (Physical Review, December, 1913). The 
thermionic current does not increase with the tem- 
perature, according to Richardson’s law, unless the 
vacuum is of a high order. With a moderate vacuum 
the volume charge between the filament and _ silver 
NO. 2372, VOL. 95| 

causes the thermionic current to remain at a value 
nearly constant, as the temperature is raised above 
a certain value. The thermionic current begins by 
b 
obeying Richardson’s law (¢=aVTe~T),and then later 
approximates to a steady value. 
Thus at low voltages variations of voltage cause 
variations of temperature and consequent fluctuations 
of thermionic current, heard through the telephone. 
Above 140 volts, however, for the particular lamp 
in question, a change of voltage and of temperature 
produces no change of current, and hence no sound 
can be heard in the telephone. 
It is possible that this method may prove very con- 
venient for testing the electron emission from various 
substances in different gases, and it suggests a method 
of measuring a high vacuum. A. S. EvE: 
McGill University, Montreal, March 31. 

A Mistaken Butterfly. 
WHILE waiting for a car at Pacific Grove, Monterey 
County, California, on February 12 (Lincoln’s birth- 
day) of the present year, I noticed that a man stand- 
ing near me had the brightly-coloured ‘“‘eye”’ of a 
peacock’s feather in the band at the back of his hat- 
While looking at this I saw a butterfly floating above 
the man’s head. It suddenly lighted on the ‘‘eye’” 
and apparently began trying to extract food from it. 
I directed the man’s attention to it; he removed his 
hat, and we watched the insect for several minutes 
as it tried to secure food from the feather. It then 
flew away, as if satisfied that it had made a mistake. 
I do not know the name of the butterfly, but it was 
one of many of a light brown colour that seem to 
be plentiful at Pacific Grove at that season. I was 
told that these butterflies at a certain time regularly 
alight in thousands upon a special pine tree (one of a 
great many) in the western edge of the town, and 
from this fact they have called it the ‘‘ Butterfly 
Tree.” I do not know whether these insects seek 
their food from flowers by the sense of smell or that 
of sight, but it was evident in the present case that 
this one was guided entirely by sight. 
E. Barnarpb. 
Yerkes Observatory, Wisconsin, U.S.A 
March 29. 

BRITISH SUPPLY OF DRUGS AND FINE 
CHEMICALS. 
PX a British Association meeting about twenty 
years ago an eminent physicist received 
some rough handling from his chemical colleagues 
on account of the impurities which were mani- 
festly present in the materials he had used in his 
experiments. He replied, in effect, that chemists 
should employ themselves in purifying chemicals 
for physicists to use. Nowadays chemicals such 
as he would have desired are made by the ton, 
chiefly by three firms in Germany. For ordinary 
chemical, and even physical, research such fine 
materials are turned out that it’ may be doubted 
in many cases if the work done with them is 
worthy of such refinement. Frequently the chemi- 
cals used are better than the chemist who works 
with them could have produced for himself. For 
work of the highest degree of refinement, such as 
the determination of atomic weights, the chemi- 
cals which can be purchased cannot be used with- 
