540 MELLONI OX THE NOCTURNAL COOLING OF BODIES. 



observed at night in plants does not arise from their radiation, 

 hnt from the presence of a thin stratum of cold air, which at sun- 

 set suddenly appears near the terrestrial surface ; thus substi- 

 tuting the effect for the cause, and therefore falUng into one of 

 the greatest errors with which observers of nature can be re- 

 proached. The cold produced by the radiation of vegetable 

 leaves, of the soil, or of any other substance exposed to the noc- 

 turnal influence of a calm and clear sky, always precedes, as we 

 have said, the fall of dew. The condensation of the vapour at 

 first communicates to the radiating substance the heat disen- 

 gaged in passing from the aeriform to the liquid state ; but this 

 heat is soon destroyed by virtue of the great emissive power of 

 water ; so that the moistened body, always preserving a tempe- 

 rature lower than that of the surrounding medium, continues to 

 envelope itself with dew. All this may easily be verified in the 

 fields by means of observation and of our thermometer pre- 

 pared with the cork and metallic armature. 



It must nevertheless be remarked, that in certain cases the 

 nocturnal temperature of plants, under a clear sky, may equal, 

 and even surpass for a few instants, the temperature of the sur- 

 rounding air, when, in the midst of the calm, and the phaeno- 

 mena of cold and of dew which thence result, a sudden breeze 

 comes and carries off from the radiating body the air which sur- 

 rounded it, and substitutes in its place that of other bodies 

 placed in conditions more favourable to cooling. For instance, 

 the grass under a tree, enveloped suddenly in the air carried off 

 from the neighbouring meadow, will at first show itself warmer 

 than the surrounding medium, and will come at length to ac- 

 quire the same temperature, if the action of the wind be suffi- 

 ciently prolonged. But these anomalies are rare and easily 

 recognised, because of the wind which must necessarily precede 

 or accompany them. 



We have seen in the former memoir, that two of our thermo- 

 meters with metallic armature, one of which was polished and 

 the other covered with lamp-black, varnish, sawdust or leaves 

 of plants, marked the same temperature in the fi'ee air, when 

 care had been taken to shut them out from the aspect of the sky 

 by means of metallic vessels closed on all sides ; but that they 

 indicated different temperatures the instant that the covers of 

 the receptacles were removed ; for then the former remained 

 nearly immoveable, whilst the second descended 3° or 4° in a few 



