450 COBLENTZ : REFLECTING POWER OF VARIOUS SUBSTANCES 



were kept in ice. The aluminum sheets were mounted side by- 

 side upon a board of yellow poplar, 20 by 19 by 0.18 cm. and 

 exposed to direct sunlight. The temperature of the pairs of 

 sheets, A and B, and the difference in temperature was measured 

 by means of a potentiometer. 



Sheets No. II of both A and B were given two coats of pure 

 white lead paint and when dry they were exposed to sunlight. 

 The sky was perfectly clear (10-7-1912) with only a slight breeze 

 blowing. Without glass coverings, which is the test of most 

 interest, the bare aluminum plate was the hotter. For A-I, 

 the temperature of the unpainted aluminum sheet was 3?0 to 

 3?1 higher than the exactly similar sheet having two coats of 

 white lead paint. The actual temperature of A-I was about 

 44? 7. Similarly, for B-I the temperature excess of the unpainted 

 aluminum sheet was 2? 3 to 2? 4, the actual temperature being 

 about 43? 7. The intensity of the solar radiation, Q, was about 

 1.16 gr. cal. cm 2 , min. ( 



These tests were repeated using three coats of paint. The 

 weather was perfectly clear (10-10-1912) and no breeze blowing. 

 The intensity of the solar radiation was 1.157 gr. cal. cm 2 , min. 

 The plates were exposed directly to the sun, i.e., no glass inter- 

 vening. For A-I the temperature excess was 4?1 to 4?3 the 

 actual temperature being about 45°C. Similarly for B-I (un- 

 painted aluminum) the temperature excess was 3?0 to 3?2, the 

 actual temperature being about 42? 7. 



From these tests it is evident that a metal roof painted with 

 several coats of white paint is far superior to one of bright metal. 

 As already mentioned this is owing to the fact that white paint 

 has a higher emissivity than metals in the infra-red. 



The unpainted sheets of aluminum (A-I, B-I) were then given 

 three coats of zinc oxide paint. When thoroly dry, the sheets 

 containing the zinc oxide paint appeared to be at about the same 

 (+ 0?1) temperature as obtained in the white lead. Another coat 

 of paint was then applied (making four coats in all) to each sheet. 

 When exposed directly to solar radiation, the temperature excess 

 of the zinc oxide paint for the thin sheets, B, was ±0?1 and for 

 the thick sheets it was about +0?5. From this appears that, in 



