HEAT OF THE ROCKS. 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 are 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*7 ; during the 

 night the thermometer on the rock indicated 36, the air 

 being at 26. When the accumulation of heat in the stony 

 masses has reached a stationary degree, these masses be- 

 come at the same hours nearly of the same temperature. 

 What they have acquired more in the day they lose at night 

 by radiation, the force of which depends on the state of the 

 surface of the radiating body, the interior arrangement of 

 its particles, and, above all, on the clearness of the sky, that 

 is, on the transparency of the atmosphere and the absence 

 of clouds. When the declination of the sun varies very 

 little, this luminary adds daily nearly the same quantities 

 of heat, and the rocks are not hotter at the end than in 

 the middle of summer. There is a certain maximum which 

 they cannot pass, because they do not change the state of 

 their surface, their density, or their capacity for caloric. 

 On the shores of the Orinoco, on getting out of one's ham- 

 mock during the night, and touching with the bare feet the 

 rocky surface of the ground, the sensation of heat expe- 

 rienced is very remarkable. I observed pretty constantly, 

 in putting the bulb of the thermometer in contact with tlie 

 ledges of bare rocks, that the laxas negras are hotter during 

 the day than the reddish-white granites at a distance from 

 the river ; but the latter cori during the night less rapidly 

 than the former. It may be easily conceived that the 

 emission and loss of caloric is more rapid in masses with 



