COLOURED HEAT-RAYS. 85 



There are many remarkable results dependent entirely 

 on the colours of bodies, Trhich are not explicable upon 

 the idea of difference in mechanical arrangement. We 

 know that different colours are regulated by the powers 

 which structures have of absorbing and reflecting light ; 

 consequently a blue surface must have a different order 

 of molecular arrangement from a red one. But there 

 are some physical peculiarities which also influence heat 

 radiation, quite independently of this surface condition. 

 If we take pieces of red, black, green, and yellow glass, 

 and expose them when the dew is condensing, we shall 

 find that moisture will show itself first on the yellow, 

 then on the green glass, and last of all upon the black 

 or red glasses. The same thing takes place if we expose 

 coloured fluids in white glass bottles or troughs, in which 

 case the surfaces are all alike. If against a sheet of 

 glass, upon which moisture has been slightly frozen, we 

 place glasses similarly coloured to those already described, 

 it will be found that the earliest heat-rays will so warm 

 the red and the black glasses, thut the ice will be melted 

 opposite to them, long before any change will be seen 

 upon the frozen film covered by the other colours. 



The order in which heat permeates coloured media, it 

 has already been shown, very nearly agrees with their 

 powers of radiation. 



These most curious results have engaged the attention 

 of Melloni, to whose investigations we owe so much ; 

 and from the peculiar order of radiations, which present 

 phenomena of an analogous character to those of the 

 coloured rays of light, obtained by him from dissimilarly 

 coloured bodies, he has been led to imagine the existence 



and he imagines that by shading grasslands with "boughs of trees, 

 or any light litter, a more abundant crop is produced. The sub- 

 ject has been discussed in the journals of the Royal Agricultural 

 Society. May not the apparent increase be due entirely to the 

 succulent condition in which a plant always grows in the shade? 



