224 PRINCIPLES OF GARDENING. [CH. VII. 



surface absorbs them all. Blue absorbs all but the 

 blue rays — red all but the red — green and yellow, 

 all but those of their own name, and white reflects 

 all the rays. The lightest coloured rays are the 

 most heating, therefore light coloured walls, but 

 especially white, are the worst for fruit trees. The 

 thermometer against a wall rendered black by 

 coal tar rises 5° higher in the sunshine than the 

 same insti-ument suspended against a red brick 

 structure of the same thickness ; nor will it cool 

 lower at night, though its racUating power is increased 

 by the increased darkness of its colour, if a proper 

 screen be then employed. The elevation of the 

 temperature of a dark coloured fniit compared 

 with that of a lighter coloured of the same kind is 

 often remarkable, as in the instance of the muscle 

 plum and greengage, growing on standard trees. But 

 there are other causes than colour for fmit often 

 remaining of a cool temperature m the hottest 

 weather, and among these causes is then- covering. 

 Ever}^ one must have noticed the delicious coolness 

 of the peach s flesh compared with that of the nec- 

 tarine grovm on the same wall and in the same 

 bright sunshine ; and the reason of this is, that the 

 dense woolly cuticle of the first, hke all other dowiy 

 coveiings, is one of the worst conductors of heat. 

 Similar coverings are found on Mexican and Cretan 

 plants which have to endure exposui*e to a torrid 

 temperature. 



