32 On the black rete mucosum of the Negro. 
the effect of the bites of insects; but I never satisfied myself upon 
that subject. 
The effect of the sun’s rays apon the fish under water, led me 
to suspect the mixture of light and heat to be’ the cause of this 
scorching effect. 2 
To ascertain the truth of this opinion, I made the following 
experiments. 
Exp. 1.—In August 1820, I exposed the back of my hand to 
the sun at twelve o ‘clock, with a thermometer attached to it, an- 
other thermometer being placed upon a table, with the same ex- 
posure, That on my hand stood at 90°, the other at 102°. In 
45 minutes blisters rose, and coagulable lymph was exuded, which 
became vascular under my eve: the pain was very severe. 
Exp. 2.—1 exposed my face, my eyelids, and the back of my 
hand to water heated to 120°: in a few minutes they became 
painful; and when the heat was further increased, I could not 
bear it. 
Exp.3. —I exposed the backs of my two hands to the sun’s rays, 
with a thermometer upon each; the one hand was uncovered ; 
the other had a covering of black cloth, under which the ball of 
the thermometer was placed. After ten minutes, the degree of 
heat of each thermometer was marked, and the appearance on 
the skin examined. This was repeated at three different times, 
The 
ist time the thermometer under the cloth 91°, the other 85° 
2d time SI Ucmsdeh et Niel ge ie, JOSS0 2 dO 
od time ainay copenicaeinn asic Wie Snlas eh OO Sn eae 
In every one of these trials the skin was scorched that was un- 
covered; the other had not suffered in the slightest degree; there 
was no appearatice of perspiration on either hand. 
Exp. 4.—The back of a Negro’s hand was exposed to the sun 
with a thermometer upon it, which stood at 100°; at the end of 
ten minutes the skin had not suffered in the least. 
Exp.5.—During the eclipse of the sun on September 7, 1820, 
1 exposed the back of my hand to the rays concentrated by a 
double lens of half an inch focus, at three different periods of the 
eclipse. When the heat to a thermometer was 75”, that is from 
47 to 57 minutes past one o’clock, the concentrated rays felt 
warm, but gave no pain, although applied for ten minutes. 
When the heat to a thermometer was 79", that is at 15 mi- 
nutes past two o *clock, the concentrated rays in four minutes gave 
pain; in five minutes blistered the skin, ‘and produced: dots of 
coagulable lymph, which became vascular under the eye. 
When the heat to a thermometer was $2°, that is at half past 
two o’clock, the concentrated rays in three minutes gave pain ; 
in four, the part was blistered, and the pain could not longer be 
endured. ® Exp. 
a 
