76 GENERAL ABSORPTION OF HEAT. 



tively to the quantity poured from its source, more heat 

 penetrates the uncovered soil, than during the spring 

 or summer. 



There is a constant tendency to an equilibrium ; and, 

 during the night, the surface is robbed of more heat, by 

 the colder air, than by day ; as, when the earth is not re- 

 ceiving heat, it is constantly radiating it back into space. 

 Even in these processes of convection and radiation, a 

 similar law prevails to that which is discovered in examin- 

 ing into the rate of calorific absorption. 



Every tree spreading its green leaves to the sunshine, 

 or exposing its brown branches to the air every flower 

 which lends its beauty to the earth possesses different 

 absorbing and radiating powers. The chalice-like cup 

 of the pure white lily floating on the lake the variegated 

 tulip the brilliant anemony the delicate rose and the 

 intensely coloured peony or dahlia have each powers 

 peculiar to themselves for drinking in the warming life- 

 stream of the sun, and for radiating it back again to the 

 thirsting atmosphere. These are no conceits of a scien- 

 tific dreamer ; they are the truths of direct induction ; 

 and, by experiments of a simple character, they may be 

 put to a searching test.* 



A thermometric examination of the various coloured 

 flowers, by enclosing a delicate thermometer amongst 

 their leaves, will readily establish the correctness of the 

 one ; and by a discovery of recent date, connected with 

 calorific radiation, which must be particularly described 

 presently, we can, with equal ease and certainty, test the 



* By reference to the Treatise on Heat, in the Encyclopedia 

 Metropolitana, numerous suggestive experiments will be found, all 

 bearing on this subject. Peschel's Elements of Physics may also 

 be consulted with advantage. The fact is, however, simply proved, 

 as stated in the text, by placing the bulbs of delicate thermometers, 

 so as to be completely involved in the petals of flowers exposed to 

 sunshine, shading the upper portion of the stem of the instrument. 



