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THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. [December u, 1 & 72 . 
lie told me that lie was then overwhelmed with work and 
could not undertake the inquiry. 
Happening to he in New York during the recent hot 
summer, I have performed a few additional experiments; 
and so far as they go, they certainly tend to confirm my 
belief in the existence of a hitherto unrecognized pro¬ 
perty or force in the solar rays. 
In attempting to ascertain the precise nature of this 
force, I could not rely on ordinary thermometers, for 
they are actuated gradually and slowly while the pain 
and sensation of heat are instantly felt. Neither would 
the usual thermo-electric apparatus meet the difficulty, 
as my observations must he made in water as well as air. 
I finally concluded to rely on Nature’s own instrument, 
the nerves of sensation as they exist in exquisite perfec¬ 
tion in the integuments of the finger. In the propriety 
of this course, I was confirmed by a remark of Professor 
Tyndall that the optic nerve is more sensitive to the heat 
rays present in light than any thermometer. 
In experimenting, I generally placed on the end of 
the forefinger of my left hand the substance under ob¬ 
servation, steadying it with the thumb, and dipping the 
hand when necessary into water so as to have a layer of 
that liquid at least an inch thick above the. skin. The 
lens employed was of seven inches focal distance; and 
in using it, I always threw the focus behind the object 
examined so that the rays should traverse it instead of 
being concentrated upon it. 
"Whenever in experiments in air there was the slightest 
appearance of singeing or other action of heat on the 
substance examined, the observation was rejected. Care 
was taken not to press the finger firmly against the sub¬ 
stance, but merely to maintain the slightest possible con¬ 
tact. Under these conditions, a burning painful sensa¬ 
tion was felt when the concentrated solar ray T s were 
transmitted through the following substances both in 
water and air, namely, two layers of blue glass, black 
leather (glazed and unglazed), green leaves, thick wffiite 
cardboard, the same covered with blue or red paper, six 
layers of pink paper, earthenware, oilcloth and common 
brown glue a quarter of an inch thick. 
Now taking only these substances into consideration, 
it is not easy to explain on the views now current how 
rays could pass through an opaque non-conducting sub¬ 
stance like .oil-cloth, so as to pain the finger placed be¬ 
neath it even when both were immersed in water. But 
my observations came still more into collision with re¬ 
ceived opinions. Thus it is generally stated in scientific 
works that a crystal of alum is athermanous, that while 
allowing the rays of light to freely traverse it, those of 
heat are arrested. But on directing the concentrated 
rays through a crystal of perfectly transparent alum, I 
found that it produced a burning sensation in the skin, 
both in air and water. It is evident, therefore, that 
Melloni’s conclusions with reference to athermanous 
bodies do not apply to the concentrated solar ray r s, if the 
pain were produced by heat. Another still more curious 
and unexpected result was obtained. It is generally 
believed that metallic surfaces reflect heat rays of all 
degrees of refrangibility, and are consequently impene¬ 
trable to them and absolutely athermanous. But I 
found that a burning heat was felt in the finger when 
the concentrated rays were transmitted through double 
tin-foil and thin sheet-iron. The following experiment 
illustrates this point very clearly: I took a mirror 
formed of plate-glass a quarter of an inch thick, silvered 
in the usual way with tin amalgam, on the back of which 
was a thick layer of red paint, well dried. I let the 
painted back part of the mirror rest upon my finger 
both in air and water; and on concentrating the rays 
upon the glass, throwing the focus as usual behind the 
mirror, the burning pain in the finger was instantly felt 
by myself and others. The mirror itself was in no way 
affected by the experiment. 
Here the light rays were of course excluded, and those 
of radiant heat were, according to the generally accepted 
laws of physics, prevented from penetrating to the finger 
by the intervening metallic coating of the mirror, to say 
nothing of the layer of red paint; and yet rays capable 
of producing pain and inflammation in the integuments 
of the finger undoubtedly passed through, 1, the water; 
2, the thick plate-glass; 3, the layer of tin amalgam; 
and, 4, the coat of red paint. 
Now what were these rays ? 
They were very refrangible, they possessed great 
penetrating power, they acted instantaneonsly, and ener¬ 
getically upon the tissues of the living animal body. 
These are obvious and palpable conclusions, but without 
additional facts we cannot go much further in reasoning 
on the subject. 
That the concentration or mere convergence of the 
sun’s rays does really increase their penetrating power 
is, I think, highly probable, and this may partly explain 
some of the facts observed. But I still cling to the 
belief that the conditions present, for instance, in the 
last-mentioned experiment, rather point to the presence 
in the sun’s rays of a force acting specially and as a power¬ 
ful stimulant or irritant on living matter. It may be 
that the more refrangible heat rays thus exercise on 
vital structures a special influence analogous to the ac¬ 
tinic or chemical power of the more refrangible rays of 
light. I append other experiments, which also tend to 
render it probable that the irritation and pain in the 
living tissues, observed under these circumstances, are 
not induced by common heat. Ordinary albumen being 
coagulated at a temperature of about 150° F. might, I 
thought, serve as a test of the presence of common or 
thermometric heat in the sun’s rays, and so assist in de¬ 
termining the question whether the burning pain was 
really due to heat or to some other force. 
Some perfectly transparent egg albumen, placed on 
glass or on wffiite earthenware, was not at all changed by 
the concentrated sun’s rays ; on any dark surface, how¬ 
ever, it was instantly coagulated, and this effect occurred 
both in air and water. Thus wffien poured on dark pur¬ 
ple paper and the rays concentrated upon it in the 
slightest degree, the albumen presented at once an 
opaque clot. 
I took some of this purple thick glazed paper, and 
wrapped it round my finger. On the purple surface I 
placed some albumen, and on the latter a second layer of 
the purple paper. I then carefully concentrated the 
sun’s rays so as to avoid burning or injuring in any way 
the paper, throwing the rays through it and the albumen ; 
I instantly felt the burning pain in the finger, and then 
withdrew the lens and examined the albumen placed be¬ 
tween the two layers of pimple paper. It teas not at all 
coagulated. Here the irritating rays passed through two 
layers of purple paper and a film of albumen wdthout 
producing any effect on those substances, but instantly 
caused pain in the skin beneath. A little of the same 
albumen placed upon the same paper was at once co¬ 
agulated by the same condensation of the sun’s rays, so 
that, if any rays of heat had passed through the first 
layer of paper, they ought to have produced coagulation 
in the albumen resting upon the second layer of purple 
paper. As they did not do so, the probability is that the 
pain was not occasioned by ordinary heat. 
In another experiment the mirror above-mentioned 
was placed upon the finger, a layer of egg albumen in¬ 
tervening between the skin and the back painted surface 
of the mirror. The rays were then gradually concen¬ 
trated upon the upper glass front of the mirror until 
a burning pain was felt in the finger beneath the latter. 
The albumen, being then examined, was not coagulated. 
In leaving this subject for the present, I shall merely 
remark that the phenomena of “sun stroke” are pro¬ 
bably due to the peculiar rays producing the pain and 
other physiological and pathological effects noticed in 
my experiments. This inquiry may therefore not be 
without some practical interest to the members of the 
medical profession. 
