576 
, in New York in June-July, and the maximum number 
occur in March. Dr. Hess now informs me that the 
calcium content of the blood follows the same curve as 
the phosphorus content. Among earlier noted seasonal 
effects of sunlight, quoted by Hess in his latest paper, 
are the presence of increased iodine in the thyroid of 
cattle from June to November, and the greater resistance 
of guinea-pigs to aceto-nitrile poisoning in summer. 
Hess and his workers have also begun the study of 
various clothing materials in this connexion, and find 
that they vary in their power of permitting or obstruct- 
ing the action of light. Specimens of a mercerised 
cotton, one white and the other black, otherwise 
identical, the former allowing light to act and the 
latter interfering with it, have been examined by me, 
and I find no difference, due to the black dye, in the 
spacing between the fibres of the material. But I 
understand that the Department of Applied Physio- 
logy of the Medical Research Council has found, in a 
series of observations as yet unpublished, that the 
biological action of light can be graded by temperature. 
I am in hope that these specimens of material may 
be studied by the delicate methods associated with 
the name of Prof. Leonard Hill, and that it may be 
found that the black material produces a higher tempera- 
ture than the white of the subjacent skin, thus pre- 
judicing those unknown and _ beneficent chemical 
reactions which appear to need light and cold for their 
development. 
The belief grows upon me that the asserted futility 
of heliotherapy in phthisis is due to the overheating of 
the patients in the sun. I think that a new chapter will 
open in the treatment of that disease when practitioners 
acquaint themselves with the principles and practice of 
heliotherapy before exposing their patients to the sun. 
The power of sunlight and of cod-liver oil in rickets 
has suggested to Prof. Harden that the light may 
cause the skin to produce vitamin A for itsel{—though 
no instance of the synthesis of a vitamin by the animal 
body is known. The most recent work at the Lister 
Institute shows that light is unable to replace vitamin 
A completely, but appears to make a small quantity 
more effective. Miss Coward’s work shows that vitamin 
A is present in the parts of flowers which contain 
carotin. Sir William Bayliss has suggested to me 
that the production of this vitamin in green plants 
is a function of the carotin rather than of chlorophyll, 
and that probably the carotin acts as a sensitiser for 
ultra-violet rays. In this connexion we must remember 
that pigmentation of the skin is a marked feature of 
the sun-cure, and that patients who do not pigment 
well do not progress well. No one who has seen and 
touched the typical pigmented skin of a heliothera- 
peutic patient can doubt that very active chemical 

NATURE 

[Aprix 28, 1923 
processes are there occurring. Perhaps we should 
regard the skin less as a mere integument than as an 
organ of internal secretion. The pigmented skin under 
the sunlight is surely that ; and we may ask whether 
it contributes, as Sheridan Delépine suggested,® to 
the making of hemoglobin. I owe also to Sir William 
Bayliss the information that Dr. H. H. Dale, a member 
of his committee, has shown that smooth muscle can 
be made to contract by ultra-violet rays. 
Aerial and other photographs of Manchester, and 
the Potteries, and of Sheffield, taken at successive 
hours on Sunday and Monday, demonstrate the 
obstruction of sunlight by our urban smoke, the 
industrial and the domestic chimney being both 
responsible : but while Sheffield deprives itself of more 
than half its sunlight, Essen is absolutely smokeless, 
and Pittsburg, which I have visited for the purposes 
of this inquiry, has abolished 85 per cent. of its smoke. 
Sections of the lungs of an agricultural labourer and 
a typical urban inhabitant of our country, the latter 
being heavily infiltrated with smoke, illustrate a 
cognate aspect of our subject. 
Yet another point is illustrated by recent work of 
Hess, which shows that the milk of cows fed on pasture 
in the sunlight maintains the growth and health of 
young animals, whereas the milk of cows fed in shadow 
and on vitamin-free fodder will not maintain life. 
Our children are thus disadvantaged in winter by 
light-starvation, and by the defect of the milk of light- 
starved cows.® 
Photographic study of houses and housing on both 
sides of the Atlantic illustrates the problem of urban 
light-starvation. Finding New York smokeless in 
1919, I later made investigations with the aid of 
Dr. Royal S. Copeland, the Health Commissioner of 
that city, and found that the death-rate from pul- 
monary tuberculosis had been reduced by one-half 
in the period, 1905-1919, of the operation of the — 
sanitary regulation against smoke.? The restoration 
of sunlight to our urban lives is the next great task 
of public health in this country. 
“There is no darkness but ignorance,” as Shake- 
speare said. In every sense we need “ more light.” 
Then we must apply our knowledge, less for helio- 
therapy than heliohygiene, until we have banished 
what I call the diseases of darkness, and it may be said 
of us that “‘ The people that walked in darkness have 
seen a great light, and they that dwell in the land of the 
shadow of death, upon them hath the light shined.” 
5 Journal of Physiology, vol. xii., 1891, p. 27. 
* To some extent, Antipodean sunlight, in the form of dried milk from 
New Zealand, comes to the rescue. 
7 The smoke prohibited in New York, or in Winnipeg, where I found 
similar regulations, need not, as in our futile Public Health Act, be “ black.’” 
See ‘“‘ The Eugenic Prospect" (Part II., ‘‘ Let There Be Light ’’), by Dr. 
Cc. Ws Saleeby. (Fisher Unwin, London; and Dodd Mead and Co., New 
York, 1921.) 
Domestic Animals in Relation to Diphtheria. 
ee perennial alarm of the possible transmission of 
diphtheria from diseased animals to man is again 
occupying the attention of the British daily press. This 
time it arose out of the death of a little girl who was 
thought by her mother to have contracted diphtheria 
from certain chickens which were kept in the house. 
The mother’s view was supported by a medical man, 
who said that birds are subject to the germs of diph- 
NO. 2791, VOL. 111] 

theria and die of the disease. He had no doubt also 
that dogs and cats could have diphtheria, and he knew 
of instances of pigeons which had it. 
The present writer has recently made an exhaustive 
critical analysis of the literature on this subject, and 
can state definitely that this bird, cat, and dog story is 
a pure myth. Diphtheria bacilli have been found on 
three occasions in cows (cases of Dean and Todd (1902), 

