“fumaroli” or the Solfatara at Pozzuoli. This alga is 
characterised by its remarkable resistance to heat and to the 
action of sulphuric acid. 
Mr. WILLIAM Foster, jun., writes in the Physzcal Review, 
viii. 5, on the conductivity and dissociation of some electrolytes, 
and confirms the values of the depressions of the freezing point 
as determined by Loomis, within as narrow limits as one should 
expect. The results also, with a few minor exceptions, conform 
to the dissociation theory as set forth by Arrhenius, and the 
author considers that in the present state of physical chemistry 
we are compelled to look upon this theory as the one that most 
nearly corresponds with the experimental data, thus affording, at 
least, a working hypothesis by which the most important 
generalisations can be made. 
To impress upon students the identity of the laws of force for 
magnetism and electricity at rest, it is necessary to obtain 
pictures of electrostatic curves analogous to those shown around 
magnets by means of iron filings. Mr. David Robertson 
describes in the Proceedings of the Royal Society of Edinburgh 
the way in which such dust figures can be produced, and the 
accompanying illustrations show the kind of results obtained. 
Electrostatic Dust Figures. 
A number of materials in the form of powders or short pieces 
were tried, but the best pictures were given by fine mahogany 
sawdust. To obtain a picture, a glass plate having one or more 
pieces of tinfoil stuck upon the under-face, is supported horizon- 
tally. Sawdust or other suitable material is spread as uniformly 
as possible over the plate by means of a sieve or muslin bag, and 
the tinfoil disc or discs are charged by being connected with a 
Wimshurst machine in action. The plate has to be tapped to 
assist the arrangement of the particles, and it is necessary to put 
the machine out of action when the dust begins to move out- 
wards from the tinfoil. By careful manipulation, distinct indica- 
tions of the directions of electrostatic lines of force around 
plane conductors of different shapes, and with like and unlike 
charges, are produced. The pictures can be fixed by means of a 
thin layer of paraffin wax on the upper side of the glass plate, 
or they may be photographed. The two pictures here repro- 
duced trom Mr. Robertson’s paper show the results obtained 
with a single disc and with two discs having unlike charges. 
A MONOGRAPH on “ After-images,”” by Mr. Shepherd Ivory 
Franz, forms a notable feature of the Psychological Review, iii. 
2. This monograph deals, firstly, with an experimental analysis 
of the conditions affecting the production, the duration, the 
latent period, the space relations, &c., of the after-image ; and, 
secondly, with a history of the phenomena and their relation to 
sensation, to imagination and to memory. The experiments 
described in the first portion were all made in the psychological 
laboratory of Columbia University, eleven advanced students 
being selected as subjects. Regarding the historical aspect, it 
NO. 1559. VOL. 60] 
NATORE 
[SEPTEMBER 14, 1899 
would appear that while after-images have formed the subject of 
numerous writings dating back from Aristotle’s De Somizs, the 
various theories are far from giving a complete explanation of 
the phenomena. 
A FURTHER blow has been dealt to Euler's proof of the 
Binomial Theorem, by Mr. R. F. Muirhead, writinz in the 
Proceedings of the Edinburgh Mathematical Society. This time 
it is not the omission of convergency considerations that is at- 
tacked, but the generalisation of the relation f(m) f(7z)=/(m+n) 
by the permanence of equivalent forms. Mr. Muirhead points 
out that the function /(z}(1— (x) sin 7) +(x) sin wm, where 
(x), Y(x) are any functions of x, is equal to (1 +.)” when we 
is a positive integer, and it would be equally valid to infer that 
functions of this type satisfied the Index Law for all values of 
m and 7, which is obviously false for most forms of (x) 
and wp (1). 
Mr. CHARLES S. SCHLICHTER has published, in the nine- 
teenth annual report of the United States Geological Survey, 
Part ii., a theoretical investigation of the motion of ground 
waters. In it he investigates the laws of motion of water 
through the interspaces of an agglomeration of equal spheres, 
and also discusses the general laws of flow of ground waters. 
In connection with the motion in horizontal planes, the well- 
known transformations by conjugate functions are applicable ; 
while the lines of flow in problems relating to the mutual inter- 
ference of two or more wells are closely analogous to the 
corresponding lines in two-dimensional hydrodynamical problems 
involving sourcesand sinks. The applications of known methods 
of mathematical analysis to the present problem might with 
advantage be studied to a greater extent than is done at present. 
A GUIDE to the Collection of Scottish Agates in the Edin- 
burgh Museum of Science and Art has been prepared by Mr. 
J. G. Goodchild. The collection was formed by the late Prof. 
Heddle, whose explanatory notes on the specimens have been 
incorporated in the descriptions now given by Mr, Goodchild. 
Dr. HENRY CHARLES LANG, well known as the author of 
“The Butterflies of Europe,” is bringing out a series of articles 
in Scrence Gosstp on ‘* Butterflies of the Palzearctic Region.” 
The article in the September number deals with the genus 
Papilio, and is illustrated by fine photographs of P. machaon, 
var. sphyrus, Ub., P. xuthus, L., type and variety xuthulus, 
Brem., and P. maackiz, Niéu. 
THE Archives of the Roentgen Ray—the only journal in 
which the transactions of the Rontgen Society of London are 
officially reported—has become an indispensable quarterly for 
all who are interested in the development of radiography, more 
especially in relation to medicine. The two numbers (vol. iii. 
No. 4, and vol. iv. No. 1), just received, contain several 
excellent reproductions of Rontgen-ray photographs, as well as 
descriptive text referring to them, and notes and articles on 
recent researches. 
AN illustrated price list of chemical apparatus has been re- 
ceived from Messrs. Brewster, Smith, and Co. Instruments and 
accessories for all branches of experimental and analytical 
chemistry appear to be included in the catalogue. With the list 
we have received a description of a simple spring balance— 
Moore’s Hydrostatic Balance—arranged to illustrate the prin- 
ciple of Archimedes, and specific gravity. The balance should 
be of service to teachers of the rudiments of physics. 
A GLANCE through a catalogue of Rontgen ray apparatus, 
just issued by the firm of Mr. H. W. Cox, Ltd., shows that the 
apparatus now available is of a very efficient and compact 
character. The instruments required to produce successful 
Rontgen ray pictures are so easily manipulated, and can be ob- 
tained at so reasonable a price, that there seems no reason why 
