22 NATURE 
is that the light is accompanied by heat, so that it is necessary 
to shield the patient from the heat without interfering with the 
passage of the light. A condenser spark between iron electrodes 
is useful because it gives a large amount of ultra-violet radiation 
without much heat. Dr. Turner showed that this light is capable 
of discharging bodies whether positively or negatively electri- 
fied. He then showed that glass and mica are opaque to the 
radiation while pure rock salt is transparent.—Mr. Wilson Noble 
exhibited a mechanical break similar to the one already shown. 
A roller and a disc, with their axes parallel, are placed in con- 
tact and made to rotate in the same direction by a motor. 
Longitudinal slots are cut upon the surfaces of both, and the 
break occurs when a slot in the roller comes opposite a slot in 
the disc. Since the two are moving in opposite directions at 
their point of contact the break is very sudden. To vary the 
length of the break without altering the rate of rotation, the slot 
in the roller is wider at one end than the other, and the disc can 
be placed soas to touch the roller at any point of its length. — 
Mr. R. S. Whipple exhibited a temperature indicator for use 
with platinum thermometers, in which readings are automati- 
cally reduced to the gas scale. The instrument is very similar 
to the well-known Callendar and Griffiths’ temperature indi- 
cator, with the exception that it is so arranged that the 
readings obtained are automatically reduced to the gas 
scale, thus avoiding the necessity of applying a correction. 
It consists of a simple Wheatstone’s bridge with equal 
ratio arms, the other arms being the thermometer and a long 
helical bridge wire together with the compensating leads. A 
travelling contact is moved round the wire until a balance is 
obtained. The bridge wire is wound on an ebonite drum 
on the outer surface of which a helix has been cut. 
The contact piece, which is connected electrically with the 
galvanometer, is carried from the inside of a cylinder fixed toa 
shaft. A white celluloid tube on which the scale is divided is 
fixed to the outer surface of the cylinder. A screw of the same 
pitch as the helix on the ebonite drum is cut on the shaft, so 
that by rotating the shaft the contact is caused to travel along 
the bridge wire, and at the same time the scale is carried past 
an index placed above it. The scale has been so constructed 
that the reading at the index gives directly the temperature of 
the thermometer reduced to the gas scale. The instrument 
reads from 0° to 1400° C.—Mr. S. A. F, White read a note on the 
compound pendulum. In the determination of the length of 
the equivalent simple pendulum for a compound pendulum the 
form of which is a symmetrical bar and bob with one fixed, one 
movable knife-edge and no sliding weight it is convenient to 
make the mass of the movable knife-edge small. In this case, 
small displacements of this knife-edge will not materially alter 
the position of the centre of gravity or radius of gyration of the 
pendulum about an axis through its centre of gravity. The 
time of swing about the fixed knife-edge will therefore remain 
practically constant. The best determination of the correct 
position of the movable knife-edge for an equal time of oscilla- 
tion will be given when for the smallest displacement of this 
knife-edge there is the greatest variation in the time of oscilla- 
tion about it. The author has determined the position which 
at 5 3 jj . 
makes wh maximum, / being the distance of the axis of sus- 
pension from the centre of gravity. He has also drawn the 
curve showing the relation between dt and! hi The calcula- 
dh 
tions have then been applied to the determination of the 
position of the movable knife-edge in a particular pendulum. 
The experimental value of the ratio of 4 to & deduced from 
this pendulum when the movable knife-edge is adjusted to its 
right position agrees well with that predicted by the theory. 
The author states that when the length of the equivalent simple 
pendulum is about a metre, it should be possible witha stop- 
watch reading too 2 second to determine ‘‘ ¢” to about *I or ‘2 
per cent. If the fixed knife-edge were made the movable 
knife-edge, the value of = would be very large, but there 
ak 
would be difficulties in the way of measuring the small time of 
swing and the small equivalent length. 
Chemical Society, April 17.—Prof. Tilden, F.R.S., in 
the chair. —Dimercurammonium nitrite and its haloid derivatives, 
by Dr. P. C. Ray. This salt was prepared by the addition of 
aqueous ammonia to a solution of sodio-mercuric nitrite. On 
solution in hydrochloric acid the new compound furnishes a 
NO. 1696, VOL. 66] 
[May 1, 1902 
mercuric ammonium chloride of the formula 2HgCl,.NH,Cl, 
and with hydrobromic acid the corresponding bromide. These 
salts in turn, with sufficient potash, furnish respectively the 
chloride and bromide of dimercurammonium, ‘The author’s 
observations on these substances support the Rammelsberg- 
Pesci representation of the general structure of ammoniated- 
mercury salts.—Preparation and properties of 4-isopropyl- 
dihydroresorcinol, by Dr. Crossley. A correction in the 
nomenclature of this substance is made from 2:6-diketo-4-iso- 
propylhexamethylene to that given above, since further inves- 
tigation has shown that its usual structure is thereby better 
indicated. —Oxonium salts of fluoran and its derivatives, by Dr. 
Hewitt and Mr. ‘lervet. The authors have observed that 
fluoran and substances related to it, such as fluorescein, form 
salis with mineral acids, and ot these the nitrate and sulphate of 
fluoran, chloride and sulphate of fluorescein and others have 
been prepared, analysed and described.—Influence of substi- 
tutions on the reactivity of the aromatic diamines, by Dr. G, S. 
Morgan. The author has studied particularly the influence 
exerted by the introduction of alkyl groups in various positions 
into the molecule of aromatic diamines on the reactivity of 
these substances with methylating agents.—The influence of 
certain acidic oxides on the specific rotations of lactic acid and 
potassium lactate, by Drs. Henderson and Prentice. It was 
found that antimonious oxide exerts no action on lactic acid 
and its potassium salt, and consequently has no influence of 
their rotations in solution. On the other hand, arsenious and 
boron oxides produce a change in the rotation of these sub- 
stances which is greatest when they are present in quantity 
sufficient to form with the potassium salt compounds of the 
formule (AsO) C,H,O,;K and (BO) C,H,O;K respectively.— 
The amounts of *‘ammonia” and “nitric” nitrogen and of 
chlorine in rain water collected at Rothamsted, by Dr. Miller. 
This paper gives the amounts of ammonia, nitrates and chlorine 
contained in Rothamsted rain water for each month from 
September 1888 to August 1901. The results show that the 
total nitrogen available to the soil from this source varied during 
this period from 3°31 to 4°43 lb. per acre per annum, the 
average being 3°84 lb., of which 1°8 Ib. is secured during the 
winter and 2°03 lb. during the summer months. Of this total 
nitrogen, 70 per cent. is present as ammonia and 30 per cent. 
in the more easily available form of nitrates. Chlorine, on the 
other hand, is found in greatest quantity during the winter, the 
average content per annum for the period being 14°87 lb., of which 
10°12 lb. is obtained during the winter season,—The amounts 
of nitrogen as nitrates and chlorine in the drainage through 
uncropped and unmanured land, by Dr. Miller. During the 
last twenty-four years—September 1877 to August 1901—the 
loss of nitrates sin drainage water has been systematically in- 
vestigated at Rothamsted, and this paper gives the results 
obtained. The average loss of nitrogen in this way amounts 
to 30 lb. per annum per acre, but varies greatly with the 
amount of rain and distribution of drainage. There appears 
to be also a considerable loss of lime. The average yearly 
amount of chlorine per acre in the drainage is about the same 
as that found in the rain, but wide differences occur occasion- 
ally. Drain gauges at adepth of 20 inches have during the last 
twenty-four years received on an average 7 lb. more chlorine 
than they have lost in drainage; the vaiues for the 40-inch 
gauge are 17°5 lb. lost and 31°9 lb. received.— Benzylidene- 
camphoroxime, by Dr. M. O. Forster. The method of pre- 
paration, properties and behaviour towards reagents of this sub- 
stance have been studied as part of a proposed systematic 
examination of substituted camphoroximes. 
Linnean Society, April 3.—Prof. S. H. Vines, F.R.S., 
president, in the chair.—Mr. R. Morton Middleton exhibited 
two letters from Linnzus to Dr. David van Royen and Mr. 
Richard Warner, of Woodford, dated respectively April 18, 
1769, and September 29, 1758, and alsoa letter from Sir J. E. 
Smith to N. Wallich on Nepalese plants, written in 1819.—Mr. 
R. A. Rolfe, on behalf of the Director, Royal Gardens, Kew, 
exhibited a series of specimens of Pachzra aguatica, Aubl., and 
P. insignis, Savigny, from British Guiana, collected by the late 
G. S. Jenman, Government botanist, to illustrate the great 
variation which exists in the size and shape of the fruits. There 
was also a certainamount of variation in the leaves and flowers, 
though in the latter each species retained its own essential 
character. These trees were common over the great alluvial 
forest-region, extending also to Brazil, and were commonly 
cultivated for ornament.—On behalf of Mr. W. B. Hemsley, 
