LIGHT AND VEGETATION 33 



infra-red, which constitute almost the whole of solar radiation. 

 Clean glass, receiving the rays of the sun perpendicularly, 

 allows about nine-tenths of their energy to pass through it; 

 we can warm ourselves in the sun as well or better behind 

 glass than in the open air. But the thermal radiation of bodies 

 at ordinary temperature, with wave-lengths longer than 5 /a, 

 principally around 10 /x, is totally absorbed by glass. Thus 

 the heat produced in a glasshouse by solar radiation, which 

 penetrates into it freely, is as it were imprisoned. We can say 

 that, at night, the thermal radiation that the glass receives 

 warms it. In its turn, it re-emits this absorbed energy, in the 

 form of thermal radiation, from its two faces ; half is therefore 

 lost and the other half is sent back to the interior of the 

 glasshouse. The protection of the glass has the effect of 

 reducing by half the heat losses by thermal radiation without 

 perceptibly reducing the contribution of energy from solar 

 radiation. To this action is obviously added the suppression 

 of cooUng by movements of the air and by evaporation. 



