I Merely State These Facts As They Were Related To Me, Because We Are Almost Wholly Ignorant Of The Nature Of The Gaseous Mixtures 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 cannot 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 degrees, the air in the shade being at 29.7 degrees; during the night the thermometer on the rock indicated 36 degrees, the air being at 26 degrees. When the accumulation of heat in the stony masses has reached a stationary degree, these masses become 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.

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