26 



late these facts, as they came to my knowledge, 

 because we are almost wholly ignorant what 

 the gaseous mixtures are, that cause the insa- 

 lubrity 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, azot, and hydrogen ? Of 

 this I doubt. The granites of the Oroonoko, it 

 is true, often contain hornblende; and those who 

 are accustomed to practical labour in the mines 

 are not ignorant, that the most noxious exhala- 

 tions 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°, 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 become at the same hours 

 nearly of the same temperature. What they 



* For instance at Scharfenberg, near Misnia, in Saxony. 

 See Lampadius, Samml, pract, chem. Abhaudh B. p. 181. 



