EADIANT HEAT AND ITS RELATIONS. 87 



possess but little power. This might be illustrated by 

 a long array of facts ; and one of the most singular of 

 these is furnished by the deportment of that extremely 

 combustible substance, phosphorus, when placed at the 

 dark focus. It is impossible to ignite there a fragment 

 of amorphous phosphorus. But ordinary phosphorus is 

 a far quicker combustible, and its deportment towards 

 radiant heat is still more impressive. It may be ex- 

 posed to the intense radiation of an ordinary fire with- 

 out bursting into flame. It may also be exposed for 

 twenty or thirty seconds at an obscure focus, of suffi- 

 cient power to raise platinum to a red heat, without 

 ignition. Notwithstanding the energy of the sethereal 

 waves here concentrated, notwithstanding the extremely 

 inflammable character of the elementary body exposed 

 to their action, the atoms of that body refuse to partake 

 of the motion of the powerful waves of low refrangi- 

 bility, and consequently cannot be affected by their heat. 

 The knowledge we now possess will enable us to 

 analyse with profit a practical question. White dresses 

 are worn in summer, because they are found to be 

 cooler than dark ones. The celebrated Benjamin 

 Franklin placed bits of cloth of various colours upon 

 snow, exposed them to direct sunshine, and found that 

 they sank to different depths in the snow. The black 

 cloth sank deepest, the white did not sink at all. 

 Franklin inferred from this experiment that black 

 bodies are the best absorbers, and white ones the worst 

 absorbers, of radiant heat. Let us test the generality 

 of this conclusion. One of these two cards is coated 

 with a very dark powder, and the other with a perfectly, 

 white one. I place the powdered surfaces before a fire, 

 and leave them there until they have acquired as high 

 a temperature as they can attain in this position. 

 Which of the cards is then most highly heated ? It 



