448 coblextz : REFLECTING power of various substances 



Bricks, granolith, dusty asphaltum pavements, and green leaves 

 fall in the same group with a reflecting power about 25 per cent 

 for green leaves. But there is a vast difference in their effect 

 upon the surrounding atmosphere and consequently upon the 

 comfort of the community, The building material continues 

 to reradiate energy long after the sun has set, while in the growing 

 leaf the absorbed energy is at once used in photo-chemical proc- 

 esses, with practically no rise in its temperature. It is evident 

 that, until traffic demands the full width of it, a great portion 

 of a street should be covered with grass and trees instead of 

 granolith and asphalt, thus adding to the comfort of the com- 

 munity as well as decreasing the cost of maintenance. Observa- 

 tories should be surrounded with grass and evergreen shrubbery 

 which will absorb the solar rays without warming the atmosphere 

 by reradiation. 



As a covering for an observator}- dome the problem is to find 

 a white paint that reflects a maximum amount of visible rays 

 and a minimum amount of infra-red rays of wave-lengths between 

 6 and 10/j.. In this manner a large portion of the solar rays will 

 be reflected into space; and the rise in temperature resulting 

 from the solar rays that are absorbed will be prevented by rera- 

 diation, which is facilitated by a high emissivity (low reflectivity) 

 at 6 to 10/i. Unfortunately the paint (zinc oxide) that fulfills 

 the latter condition cannot be used for exterior work. The next 

 best primary paint for exterior work is white lead. Radiomet- 

 ricalh r it would therefore be better to use a mixture composed 

 of white lead and as high a proportion of zinc oxide as is permis- 

 sible to form a good paint. 



The carbonates, sulfates, and silicates have bands of strong 

 metallic reflection in the region of 7 to 9^, which suppresses 

 reradiation of the absorbed solar rays. In binary mixtures, 

 sulfates, and silicates are less desirable than carbonates ; for their 

 infra-red reflecting power is higher and the bands do not lie close 

 to those of lead carbonate. This suppresses the emissivity over 

 a wider range of the infra-red spectrum. Of course a few (5 

 to 8) per cent of silica or china clay which is said to improve 

 white lead mechanically, cannot seriously affect the radiating 



