230 
NATURE 
[APRIL 29, 1915 

tion. The songs of the folk are classified under 
tne heads of narrative ballads, love and mystic 
songs, pastorals, the songs of the poacher and 
the highwayman, the soldier and the sailor, the 
pressgang, hunting, sporting and labour songs, 
carols, singing games of children, the popular 
street ballad, and the broadside. 
As for the folk-dance, much of the knowledge 
which we possess is due to the enthusiasm of Mr. 
Cecil Sharp, who has hunted down the veteran 
dancers, and taught them how to instruct the 
minor societies which have been started in almost 
every large centre. The authors deny that the 
Morris dance is connected with the Moors, and 
prefer without good reason a Celtic derivation 
which makes it to mean “noble.” The suggestion 
that it may be in part due to the Salii or leaping 
priests of Rome is hazardous; there is more to be 
said for the theory which connects some of the 
figures with a primitive sun-worship. The sword- 
dance seems to be derived from some primitive 
rites of fertility. The manual is a good introduc- 
tion to the subject, and the bibliography and 
illustrations leave little to be desired. 
Masonry as Applied to Civil Engineering. 
I. N. Taylor. Pp. xi+230. (London: Con- 
stable and Co., Ltd., 1915.) Price 6s. net. 
Tue early part of this book contains descriptions 
of the various kinds of stones used in civil en- 
gineering, together with the methods of work- 
ing and handling them. This portion cannot be 
described as altogether complete in itself: there 
are many references to another book by the 
same author. There is a certain off-hand manner 
in the treatment which has led to one, at least, 
unfortunate result. On p. 23 calculations appear 
giving the load which can be raised by a set of 
Weston’s blocks when a given pulling force is 
applied. No allowance has been made for fric- 
tion, hence the result of the calculation is alto- 
gether misleading. 
In the later portions of the book the author 
deals with retaining walls, dock walls, dams, 
bridges, towers, monolithic and block concrete 
construction, shoring, and underpinning. It is 
probable that the reader will derive more benefit 
from the descriptions and drawings given in these 
portions.than from the very brief theoretical dis- 
cussions and calculations. On p. 43 there is an 
illustration showing a surcharged wall, together 
with a few lines of description supposed to indi- 
cate how to obtain the thickness of the wall. 
There appears to be some omission in the illus- 
tration; it is impossible to grasp the author’s 
meaning. The treatment of the wedge method 
on p. 45 is imperfect, and again there appears 
to be omissions in the illustration. 
In general, the author gives formule without 
any explanation of how these are derived. We 
do not think, therefore, that the book is likely to 
meet the requirements of modern students, who, 
for the most part, desire to be perfectly clear 
regarding the derivation, limitations, and as- 
sumptions involved in any formula which they are 
called upon to use in design. 
NO. 2374, VOL. 95] 
By 


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 
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taken of anonymous communications. ] 
The Nature of Gas Ions. 
Our knowledge of the nature of gas ions is derived 
mainly from measurements of their mobility in an 
electric field. Experimental evidence has shown that 
as the pressure (p) is altered the mobility (k) of the 
ion alters generally in accordance with the law, 
pk=constant; when this law holds good it is safe 
to conclude that the ion remains unaltered at the 
different pressures. One of the most interesting 
results of experiment is that in the case of the nega- 
tive ion formed in air the product pk increases as 
the pressure is diminished below about 10 cm.; this 
result indicates a simplification in the nature of the 
negative ion at the lower pressures. Prof. Townsend 
and his students have examined this phenomenon in 
great detail, and have come to the conclusion that the 
velocity (v) of the ion should be expressed as a func- 
tion of the field (X) and the pressure (pf) in the form 
v=f(X/p), indicating that the nature of the ion de- 
pends on the field and the pressure. In his recently 
published treatise Prof. Townsend concludes that the 
negative ion in a dry gas is in general a cluster of 
molecules which fora certain range of electric forces 
and pressures passes through a transition stage until 
finally, when X/p exceeds a certain value, the negative 
carriers are practically all electrons. 
In some experiments recently undertaken to deter- 
mine the mobilities of the ions formed by the radia- 
tion from polonium in thoroughly dried air, I have 
been led to conclusions essentially different from those 
now generally held. From a pressure of one atmo- 
sphere down to about 8 cm. the results were normal; 
at pressures below 8 cm. the negative carriers were 
found to consist of two kinds, electrons and ions, 
the former increasing in number relatively to the latter 
with diminishing pressure. In no instance was there 
any evidence of an intermediate or transition stage, 
the separation between the faster and the more slowly 
moving carriers remaining throughout clear and dis- 
tinct. I was able to measure the velocity of the 
negative ion at all stages until resolution between the 
two kinds of carriers was no longer practicable, and 
it was ascertained that the value of pk remained con- 
stant from one atmosphere down to the lowest pres- 
sure employed (1/7 mm.), indicating that the negative 
ion remains unaltered in nature over this wide range 
of pressures. : 
The fact that electrons exist in dry air at the 
lower pressures is, of course, well known, but at a 
pressure of 12 mm. the ions constitute at least 80 per 
cent. of the negative carriers and at 1/7 mm. more 
than 50 per cent. The proportion of ions and elec- 
trons depends solely upon the pressure, the effect of 
the field being for the most part small or negligible. 
For the negative ion at all pressures v was found to 
be directly proportional to X/p; there seems to be no 
necessity for introducing any unknown function of 
The fact that the electrons travel through several 
centimetres of a gas at pressures as high as 8 cm. 
without attaching themselves to neutral molecules is 
remarkable, and seems to imply that the distribution 
of carriers between ions and electrons must be deter- 
mined immediately after the act of ionisation; it would 
appear that the electron has to be fired into the mole- 
