HEAT OF TIIE EOCES. 
247 
which cause the insalubrity of the atmosphere. Can it be 
admitted that, under the influence of excessive heat and of 
constant humidity, the black crusts of the granitic rocks are 
capable of acting upon the ambient air, and producing 
miasmata with a triple basis of carbon, azote, and hydrogen? 
This I doubt. The granites of the Orinoco, it is true, often 
contain hornblende ; and those who arc accustomed to 
practical labour in mines are not ignorant that. the most 
noxious exhalations rise from galleries wrought in syenitic 
and hornblende rocks : but in an atmosphere renewed every 
instant by the action of little currents of air, the effect can- 
not be the same as in a mine. 
It is probably dangerous to sleep on the laxas negras, 
only because these rocks retain a very elevated temperature 
during the night. I have found their temperature in the 
day at 48°, the air in the shade being at 29 'T ; during the 
night the thermometer on the rock indicated 36°, the air 
being at 20°. When the accumulation of heat in the stony 
Masses has reached a stationary degree, these masses be- 
come at the same hoars nearly of the same temperature. 
What they have acquired more in the day they lose at night 
V radiation, the force of which depends on the state of the 
surface of the radiating body, the interior arrangement of 
d® particles, and, above all, on the clearness of the sky, that 
s, on the transparency of the atmosphere and the absence 
of clouds. When the declination of the sun varies very 
nttle, this luminary adds daily nearly the same quantities 
°i' heat, and the rocks are not hotter at the end than in 
We middle of summer. There is a certain maximum which 
Wey cannot pass, because they do not change the state of 
Weir surface, their density, or their capacity for- caloric. 
,J n the shores of the Orinoco, on getting out of one’s ham- 
^leck during the night, and touching with the bare feet the 
r pcky surface of the ground, the sensation of heat expe- 
rienced is very remarkable. I observed pretty constantly, 
jn putting the’ bulb of the thermometer in contact with the 
edges of bare rocks, that the laxas negras are hotter dining 
j'e day than the reddish- white granites at a distance from 
ie river ; but the latter cool during the night less rapidly 
nan the former. It may be easily conceived that the 
^mission and loss of caloric is more rapid in masses with 
