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 ApRIL 21, 192 3] 


























It will be seen that the formula represents the 
viscosity of the liquid within an average error of 
2 parts in a thousand ; and that the constants A and 
B are in fair agreement with the values calculated 
from the data for the viscosity of the vapour. An 
empirical formula of the type Ae*/t is found to 
represent closely the variation of the viscosity of 
many liquids, especially at the higher temperatures. 
_As we have assumed that the ‘‘ vapour ’”’ molecules 
are identical with those actually found in the gaseous 
state, we cannot expect the experimental constants 
A and B to agree exactly with those indicated by the 
theory outlined in this note in all cases. Consider- 
able deviations actually occur in the case of ‘‘ as- 
sociated ”’ liquids, in which presumably the effect of 
the molecular fields of force cannot be handled so 
Baoly- 
_ The further discussion of this question and of 
the extension of the theory to the case of dense 
vapours on one hand, and to supercooled liquids 
and amorphous solids on the other hand, offers a most 
interesting field of research. The treatment suggested 
can obviously be improved in several directions, 
peccelly in the discussion of the dissociation equi- 
librium between the two types of molecules, and the 
effect of high pressures on the viscosity of liquids 
could probably be explained by a more exact in- 
vestigation. C. V. RAMAN. 
210 Bowbazaar Street, 
_ Calcutta, India, March 1. 

Colour Temperature and Brightness of Moonlight. 
_ Our more complete knowledge of full or black-body 
‘radiation embodied in Planck’s law makes it possible 
to speak of the temperature of radiation as well as the 
temperature of radiating bodies. Thus, the tempera- 
ure of any visible radiation is the temperature to 
which a black body must be raised to emit light as 
nearly as possible of the same integral colour or 
quality as that of the radiation in question. 
The necessary ‘‘colour matches’’ involved in 
‘comparisons of a given radiation with that of a black 
body at a known temperature may be easily and 
‘quite accurately made with a contrast photometer. 
Radiation temperatures thus determined are called 
* colour temperatures.’’ The colour temperature of 
the zenith sun as seen from the earth, according to 
Abbot’s bolometric data, which extend into the infra- 
ted spectrum, is 5600° abs. If correction is made for 
the absorption of the earth’s atmosphere, we get a 
value of 6500° abs. for the colour temperature of 
sunlight above atmospheric limits. When a contrast 
hotometer is used for making ‘‘ colour matches ’’ to 
termine colour temperature, a black-body source 
_at a corresponding temperature is necessary for com- 
ison. To avoid the necessity of a comparison 
k body at very high temperatures, advantage 
can be taken of Planck’s formula for black-body 
radiation for computing a distribution of intensities 
in the visible spectrum which will give the integral 
colour of the source under examination, as measured 
by an optical pyrometer with monochromatic 
screens. 
This procedure was followed in some observations 
made to determine the colour temperature of moon- 
light. The disappearing filament pyrometer with 
blue and red glass screens was focused on one of the 
brighter portions near the centre of the full September 
moon, 1916, when near the meridian. These readings 
were repeated under nearly the same conditions a 
year later. The colour temperature found for moon- 
light on the two evenings in question agreed to 
within 50°. 
NO, 2790, VOL. 111] 
- 
NATURE 
£272 
III 

With the same pyrometer data we can also deter- 
mine the brightness temperature of the moon for a 
given wave-length ; that is, determine the tempera- 
ture of a black body which has the same brightness 
or intensity for the same small wave-length interval 
chosen for comparison. Thus, with a red glass 
screen transmitting an average or effective wave- 
length of 0°665 », we may determine the brightness 
temperature of the moon for this wave-length. It 
is also possible, from the data thus obtained and the 
brightness of a black body, to calculate the brightness 
of the moon in candles per square centimetre. Thus, 
knowing the illumination due to the sun, the reflecting 
power of the moon for sunlight may be calculated. 
The data determined from these various observa- 
ne and calculations are shown in the following 
table : 
Colour temperature of moonlight .  4125° abs. 
Brightness temperature (\=0-6654) 1575° abs. 
Brightness for totallight . . 025 candles/cm.? 
Reflecting power for total light 0-07 ; 
t 
The difference in colour temperature between the 
sun and sunlight reflected from the moon, 5600° and 
4125° respectively, indicates that the observed area 
of the moon reflects selectively, the coefficient being 
about twice as large at the red end of the spectrum as 
at the blue. The greater difference in brightness 
temperature of these two is due to the low albedo or 
average reflecting power of the moon’s surface. 
W. E. FORSYTHE. 
Nela Research Laboratories, 
National Lamp Works, 
Cleveland, Ohio, 
March 21. 

Botanical Aspects of Wegener’s Hypothesis. 
In the account which appeared in Nature of 
January 27, p. 131, of the discussion on the distribu- 
tion of life in the southern hemisphere, which took 
place before the Royal Society of South Africa, I am 
said to regard the botanical evidence as completely 
opposed to Wegener’s theory. The remainder of 
the article generally followed the official report issued 
by the society. 
My point was that the ancient phyla, with 
excellent means and ample time for dispersal, 
are generally valueless as indicating former land con- 
nexions. On the other hand, the distribution of the 
modern groups, especially the Angiosperms, in the 
South Temperate sub-continents took place in the 
main after the disruption envisaged by the Wegener 
theory. Thus neither ancient nor recent groups 
give us any material assistance in criticising this 
suggestive hypothesis, so far as concerns the 
relationships between the South American, South 
African, and Australasian floras. The botanical 
evidence for the southern hemisphere is certainly 
not “‘ completely opposed ’’ to Wegener’s theory: it 
simply does not provide any critical test of that 
theory, so far as I can see at present. 
R. H. Compton. 
National Botanic Gardens, 
Kirstenbosch, Newlands, 
Cape Town, 
February 26. 

I accept Prof. Compton’s correction of the phrase 
“completely opposed ”’; it is perhaps too strong a 
term to have used. Prof. Compton’s letter, however, 
at least admits that the evidence from the botanical 
side is valueless as a critical test for or against 
Q2 
