[ 319 ] 



XXIV. JFote on the Theory of the Greenhouse. 

 By Professor R. W.' Wood *. 



THERE appears to be a widespread belief that the com- 

 paratively high temperature produced within a closed 

 space covered with glass, and exposed to solar radiation r 

 results from a transformation of wave-length, that is, that 

 the heat waves from the sun, which are able to penetrate the 

 glass, fall upon the walls of the enclosure and raise its 

 Temperature : the heat energy is re-emitted by the walls in 

 the form of much longer waves, which are unable to pene- 

 trate the glass, the greenhouse acting as a radiation trap. 



I have always felt some doubt as to whether this action 

 played any very large part in the elevation of temperature. 

 It appeared much more probable that the part played by the 

 glass was the prevention of the escape of the warm air 

 heated by the ground within the enclosure. If we open the 

 doors of a greenhouse on a cold and windy day, the trapping 

 of radiation appears to lose much of its efficacy. As a 

 matter of fact; I am of the opinion that a greenhouse made 

 of a glass transparent to waves of every possible length would 

 show a temperature nearly, if not quite, as high as that 

 observed in a glass house. The transparent screen allows the 

 solar radiation to warm the ground, and the ground in turn 

 warms the air, but only the limited amount within the 

 enclosure. In the "open," the ground is continually brought 

 into contact with cold air by convection currents. 



To test the matter I constructed two enclosures of dead 

 black cardboard, one covered with a glass plate, the other 

 with a plate of rock-salt of equal thickness. The bulb of a 

 thermometer was inserted in each enclosure and the whole 

 packed in cotton, with the exception of the transparent plates 

 which were exposed. "When exposed to sunlight the 

 temperature rose gradually to 65° C., the enclosure covered 

 with the salt plate keeping a little ahead of the other, owing 

 to the fact that it transmitted the longer waves from the sun, 

 which were stopped by the glass. In order to eliminate this 

 action the sunlight was first passed through a glass plate. 



There was now scarcely a difference of one degree between 

 the temperatures of the two enclosures. The maximum 

 temperature reached was about 55° C. From what we know 

 about the distribution of energy in the spectrum of the 

 radiation emitted by a body at 55°, it is clear that the rock- 

 salt plate is capable of transmitting practically all of it, 

 while the glass plate stops it entirely. This shows us that 



* Communicated by the Author. 



