238 
matching the bichromate colour with the yellow (complementary 
to the violet) and the pure red colour sensation. From this 
equation and from the sensation equation of the bichromate 
colour already found, the sensation composition of the yellow 
was determined. By matching white with a mixture of the 
yellow and the violet, the sensation equation to white was deter- 
mined. The other colours of the spectrum were then used in 
forming white, and from their luminosity equations their per- 
centage composition in sensations were calculated. The per- 
centage curves are shown. The results so obtained were applied 
to various spectrum luminosity curves, and the sensation curves 
obtained. The areas of these curves were found, and the 
ordinates of the green and violet curves increased, so that both 
their areas were respectively equal tothat of the red. This gave 
three new curves in which the sensations to form white were 
shown by equal ordinates. 
A comparison of the points in the spectrum where the curves 
cut one another, and of those found by the red and green blind 
as matching white, show that the two sets are identical, as they 
should be. The curves of Koenig, drawn on the same sup- 
position, are mentioned, and the difference between his and the 
new determination pointed out. 
The red below the red lithium line, as already pointed out, 
excites but one (the red) sensation, whilst the green sensation is 
felt in greatest purity at A 5140, and the blue at A 4580, as at 
these points they are mixed only with the sensation of white, 
the white being of that whiteness which is seen outside the colour 
fields. 
“A Comparison of Platinum and Gas Thermometers, 
including a Determination of the Boiling Point of Sulphur 
on the Nitrogen Scale: an Account of Experiments made 
in the Laboratory of the Bureau International des Poids et 
Mesures, at Sevres.” By Drs. J. A. Harker and P. Chappuis. 
Communicated by the Kew Observatory Committee. 
The present paper is the outcome of the co-operation of the 
Kew Observatory Committee and the authorities of the Inter- 
national Bureau of Weights and Measures at Sevres, for the 
purpose of carrying out a comparison of some platinum thermo- 
meters with the recognised international standards. 
A new resistance-box, designed for the work, and special 
platinum thermometers together with the other accessories 
needed were constructed for the Kew Committee, and, after 
their working had been tested at Kew, were set up at the 
laboratory at Sévres in August 1897. The comparisons ex- 
ecuted between these instruments and the standards of the 
Bureau may be divided into several groups. The first group 
of experiments covers the range —23° to 80°, and consists of 
direct comparisons between each platinum thermometer and 
the primary mercury standards of the Bureau. Above 80° the 
mercury thermometers were replaced by a gas-thermometer, 
constructed for measurements up to high temperatures. The 
comparisons between 80° and 200° were made in a vertical bath 
of stirred oil, heated by different liquids boiling under varying 
pressures. For work above 200° a bath of mixed nitrates of 
potash and soda was substituted for the oil tank. In this bath 
comparisons of the two principal platinum thermometers with 
the gas-thermometer were made up to 460°; and with a third 
thermometer, which was provided with a porcelain tube, we 
were able to go up to 590°. Comparisons of the platinum and 
gas-scales were carried out at over 150 different points, each 
comparison consisting of either ten or twenty readings of the 
different instruments. 
By the intermediary of the platinum thermometers a determin- 
ation of the boiling point of sulphur on the nitrogen scale was 
also made. The mean of three very concordant sets of determin- 
ations with the different thermometers gave 445°°27 as the 
boiling point on the scale of the constant volume nitrogen ther- 
mometer, a value differing only about 0°*7 from that found by 
Callendar and Griffiths for the same temperature expressed on 
the constant pressure air scale. 
If for the reduction of the platinum temperatures in our com- 
parisons we adopt the parabolic formula proposed by Callendar, 
and the value of 6 obtained by assuming our new number for the 
sulphur-point, we find that below 100° the differences between 
the observed values on the nitrogen scale and those deduced 
from the platinum thermometer are exceedingly small, and that 
even at the highest temperatures the differences only amount toa 
few tenths of a degree, 
Full details as to the instruments employed and the methods 
adopted are given in the paper, 
NO. 1549, VOL. 60] 
NATURE 
[JuLy 6, 1899 
“On the Comparative Efficieny as Condensation Nuclei of 
positively and negatively charged Ions.” By C. T. R. Wilson, 
M.A. Communicated by the Meteorological Council. 
When moist air is ionised, a greater degree of supersaturation 
is required to cause water to condense on the positively charged 
ions than on the negatively charged ones. The experiments 
consisted in measurements of the expansion required to cause 
condensation in the form of drops in air initially saturated and con- 
taining ions alternately nearly all positive and nearly all negative 
The ratio of the final to the initial volume being indicated by 
Zo/%,, then to cause water to condense on negatively charged 
ions, the supersaturation must reach the limit corresponding to 
the expansion 7,/7;=1°25 (approximately a fourfold super- 
saturation). To make water condense on positively charged 
ions, the supersaturation must reach the much higher limit cor- 
responding to the expansion v)/v,=1°31 (the supersaturation 
being then nearly sixfold). Thus, if ions ever act on condens- 
ation nuclei in the atmosphere, it must be mainly or solely the 
negative ones which do so, and thus a preponderance of negative 
electricity will be carried down by precipitation to the earth’s 
surface. Experiments were carried out which appear to prove 
that the difference in the condensing power of positive and 
negative ions is not to be explained by supposing the charge of 
each negative ion to be, for example, twice as great as that of 
each positive ion. Experiments were also tried to test whether 
the rainlike condensation, which always takes place in moist air 
when the expansion 7/v=1'25 is exceeded, is due to slight 
ionisation of the moist air. These experiments led to the con- 
clusion that this is not a case of condensation on ions ; unless 
the process of producing the supersaturation itself gives rise to 
ionisation. i 
Mineralogical Society, June 20.—Prof. A. H. Church, 
F.R.S., President, in the chair.—Mr. E. G. J. Hartley gave the 
results of analyses of so-called plumbogummite from Roughten 
Gill, Georgia, and Huelgoat. The blue mineral from Roughten 
Gill, usually regarded as a silicate or carbonate of zinc, proved to 
be identical with the hitchcockite from Georgia. Both minerals 
have been analysed by Mr. Hartley, and shown to contain about 
19 per cent. of water and 3 percent. of carbonic acid. In a 
note on the optical characters, Prof. Miers finds that these two 
minerals present absolutely the same appearance under the micro- 
scope, and differ somewhat from the only other known hydrated 
lead aluminium phosphate, viz. the plumbogummite from 
Huelgoat in Brittany. Mr. Hartley’s analyses of this mineral 
differ from those of Damour, and shows that it has by no 
means the same composition as hitchcockite, and it is therefore 
considered to be a distinct species. —Mr. H. L. Bowman gave a 
detailed description of the optical crystallographic and chemical 
characters of a clear green rhombic pyroxene from the diamond- 
washings of South Africa.—Messrs. G. T. Prior and L. J. 
Spencer contributed a paper on the chemical composition of 
tetrahedrite. In a previous investigation proving the specific 
identity of the rare mineral binnite with tennantite, the numbers 
obtained in the analysis, like those of several older analyses of 
tennantite, agreed much more closely with the formula 
3Cu,S.As,S3 than with the ordinary text-book formula 
4Cu,S.As,S3, originally proposed by Rose. In the present 
communication the authors describe the physical and chemical 
characters of three specimens of tetrahedrite. The result of the 
analyses made by Mr. Prior is to confirm the idea that the true 
formula for tetrahedrite proper is 3Cu,S.Sb,S3, and also to 
show that when iron and zinc are present they enter into the 
composition of the crystals not as 3(Fe,Zn)S.Sb,S3, but as 
6(Fe,Zn)S.Sb,S3, in which 6(Fe,Zn)S isomorphously replaces 
3Cu,S. The proposed general formula for fahlerz (tetrahedrite 
and tennantite) is accordingly 
3R'gS. RS, + #[6R’S.R'.S5] 
where R’ = Cu, Ag; R’” = Sb, As, Bi; R” = Fe,Zn, and x is 
generally a small fraction, rising, however, to 4 in the case of 
the highly ferriferous tetrahedrite ‘‘ coppite.”—Mr. L. Fletcher 
described the chemical analysis of a constituent of the meteoric 
iron of Youndegin, Western Australia, and gave an account of 
the fall of meteoric stones at Mount Zomba, British Central 
Africa, on January 25, 1899.—Mr. Herbert Smith pointed 
out the specific identity of the new oxychloride of lead para- 
laurionite, described by him in April 1898, with the new 
mineral rafaélite, a description of which by the late Dr. Arzruni 
has just been published. 
a 
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