APRIL 4, 1907 | 
NATURE 53 
o>) 
quence as compared with No. 2, the strawberry 
mildew. The now famous American gooseberry 
disease is illustrated. To prevent its spread, the use 
of one ounce of potassium sulphide dissolved in three 
gallons of water is recommended. With this solution 
the bushes should be sprayed just before the leaves ex- 
pand, and the spraying should be repeated at intervals 
as necessary. It is unfortunate, we think, that the 
destruction of affected bushes by fire is not also re- 
commended. A descriptive pamphlet, for which one 
penny is asked, is supplied with the diagrams. 
La Mécanique des Phénomeénes fondée sur les 
Analogies. By M. M._ Petrovitch (Belgrade). 
* Scientia ’? Phys.-Math. Series, No. 27. Pp. 96. 
(Paris: Gauthier-Villars, 1906.) Price 2 francs. 
Dr. J. W. MELtor, in his ‘‘ Chemical Statics and 
Dynamics,’’ p. 19, gives the following. as the four 
stages of a physical theory : hypothesis, differential 
equation, integration, observation. While this 
sequence is well illustrated in the study of dynamical 
phenomena, these, after all, constitute but a small 
proportion of the large number of effects in which 
changes are brought about by the action of definite 
causes. This book, while not containing any very 
novel and striking features, puts matters in a some- 
what fresh light by giving prominence to the more 
philosophical aspect of the equations of mathematical 
physics and allied branches of science. Thus the 
motions determined by a constant force, a positive, 
and a negative force varying as the distance, are all 
characterised by different known forms of the in- 
tegrals of the equations of motion. If in any pheno- 
menon the changes which occur can be represented 
by equations of the form of one of these integrals, 
then conversely the relation between cause and effect 
may be of the same form as the corresponding law 
of foree. The mathematical portion of the bools is 
comparatively simple, and about the hardest problem 
considered is that of forced oscillations in a resisting 
medium. The book appears suitable for placing in 
the hands of such science students as have not the 
time to pursue an extended course in mathematics, as 
they would doubtless get many hints from its perusal. 
It may be doubted whether much is gained by the 
inclusion of physiological problems, such as_ the 
action of bacteria, in the present discussion, or 
whether such problems can indeed be adequately 
treated without introducing statistical considerations. 
But there are many cases where, even if the analogy 
be not exact, it is more easy to picture the progress 
of phenomena by associating them with dynamical 
or other analogues, and the ‘book will be useful if it 
teaches students to think in this way. 
The Steam-table. A Table of the Thermal and 
Physical Properties of Saturated Steam Vapor and 
of the Specific Heat of Water. Compiled from 
various sources by Prof. Sidney A. Reeve. Pp. 
iit+42. (New York: The Macmillan Company; 
(London: Macmillan and Co., Ltd.) Price 1s. 6d. 
net. 
Tuts is a very elaborate table from 4oo |b. per sq. 
inch and 445° F. down to 0.18 lb. per sq. inch and 
32° F. Usually we know a pressure in round 
numbers or a temperature in round numbers, and 
two separate tables are needed; Mr. Reeve’s table 
contains both, and there is an ingenious arrange- 
ment for making interpolation easy. There are 
entries for every degree, and also for every pound 
per sq. inch. The usual error of such tables, using 
Regnault’s heats with a value of Joule’s equivalent 
which does not agree with Regnault’s unit of heat, 
seems to be avoided by Mr. Reeve, and this steam- 
table seems to us likely to prove of very great value 
to steam engineers. 
NOWMIO5 2, VOL 75)| 
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.] 
Ionisation by Spraying. 
IN a paper published in the Philosophical Magazine 
(February) I noted that positive and negative ions could 
be observed in large quantities by an Ebert apparatus if 
fine spray from water were produced profusely in its neigh- 
bourhood. Whilst much work has been done in connec- 
tion with. electrification caused by the bubbling of air 
through water and the splashing of drops, the effects due 
to spray do not appear to have received much attention. 
A description of a simple method of studying the ionisa- 
tion by spraying, with a preliminary note of some of the 
results obtained, may therefore be of some interest. A 
strong current of air, filtered through cotton wool, is 
passed for a definite time, usually half a minute, through 
a small glass sprayer, as supplied by Beckmann for intro- 
ducing salts into a flame for spectroscopic work; but in 
the present case the air and spray pass together into the 
large lower chamber of an electroscope containing an insu- 
lated cylinder connected with a gold-leaf system in a small 
upper chamber. ‘The leaf remains steady when air alone is 
driven into the lower chamber, except for a slight natural 
leak, which remains constant in spite of much spraying. 
When spray has been introduced into the electroscope for 
half a minute the fall of potential is observed for that 
time, and from minute to minute until the leaf steadies 
to the natural leak. The effect terminates in two or three 
minutes in the case of water, but in the case of acetic 
acid, chloroform, ether, and the alcohols the ionisation 
effects do not disappear for ten to fifteen minutes, so that 
there are large inert ions, both positive and negative, pre- 
sent, some with a velocity of the order 10-° cm./sec. in 
a field of 1 volt/em. Similar results have been found by 
Aselmann for salt solution. 
In most cases the positive and 
generated in nearly equal quantities, 
negative ions are about 1-5 times as numerous. as the 
positive. The same ratio holds for ether, of which the 
negative ions are more quickly removed than the positive. 
The most important point, however, is that small quantities 
of liquids can be examined by the help of these small 
sprayers, and definite results obtained for He substances, if 
care is taken to avoid impurities. 
The following is a preliminary statement of the results 
obtained :— 
negative ions are 
but with water the 
Substances Negative Positive 
Mercury ae aa Ben oo 6 to 
Toluene 2 0-02 0-02 
Sea-salt and water ... 0-01 0-015 
Hydrochloric acid and water 0-04 0-04 
Pentane 0-07 0-03 
Phenetol 0-08 0-08 
Benzine 2 O-14 0-08 
Ammonia water 0-45 0-30 
Tap water fe ae se 0:65 0-50 
Distilled water ce isp cps HE 1-0 
Ether... uD 2°5 
Chloroform, pure . 
Chloroform, impure 
Acetic acid ... 
Methyl iodide 
Methyl alcohol 
Ethyl alcohol 
Amyl alcohol 4 
The figures are taken to an uanniteaee Bandera and are 
expressed in terms of the positive ionisation due to distilled 
water. It may be better ultimately to select ethyl alcohol 
as a standard of comparison, because the ionisation due to 
water varies sharply when any impurity is introduced. 
It is remarkable that volatile substances like benzine, 
pentane, phenetol, and toluene should give rise to little or 
no ionisation when sprayed, whilst ether, chloroform, 
alcohol, and aldehyde should so profusely form both posi- 
tive and negative ions. A. S. Eve: 
McGill University, March 22. 
* 4 6) 2 62 Wf WS UD 
LENCE RLEN EE) RATT 
SOOKE EN 
mnoonnw 
