192 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1930 



And for thousands of years he so lived. But a few centuries ago a 

 marvelous transparent substance was discovered, namely glass, and 

 this discovery has in a marked degree changed the human habits of 

 life and of work, for the cold months are now spent largely indoors, 

 behind this glass, which shuts out the wintry winds, but transmits 

 sunlight, and with it some of the warmth of winter sunshine. Un- 

 fortunately, however, and this was unknown until quite recently, 

 glass does not transmit all the kinds of sun energy which we need, 

 for the rays of shorter wave lengths than those of light are in large 

 measure intercepted by glass and so these rays now fail to reach us. 

 Thus we receive in our homes, and even on our sun porch, a vitiated 

 and abnormal sun radiation — a radiation which has lost something, 

 something imperceptible to our organs of vision, but which we re- 

 quire for our physical well-being. 



EFFECTS OF SUN ACTIVITY 



The sun is about 93,000,000 miles distant. Its enormous mass ex- 

 hibits tremen<Jous chemical activity, and it is radiating its sub- 

 stance — so scientists inform us — at the rate of several million tons 

 per second. The sun radiation thus engendered streams into space 

 and only a relatively small per cent of it reaches the planet on which 

 we live. Yet, as this small portion of the total varies, we experience 

 winter or summer, great heat or extreme cold ; our vegetation flour- 

 ishes or dies ; our hills are green, or they are white with a covering 

 of water crystals from the skies ; our continent experiences a glacial 

 period, or an age of tropical warmth. 



The sun radiation vaporizes the waters on the earth and forms 

 clouds, fog and rain. It stirs up the atmospheric mantle which sur- 

 rounds our planet and causes winds to blow — gentle spring zephyrs, 

 or devastating tornadoes. It develops the green and the yellow pig- 

 ments in leaves and the riot of colors in flowers. It also makes the 

 vegetable cell function as a chemical laboratory in which water and 

 the carbon dioxide of the air form plant substance, and thus our 

 plants grow, producing our food, our shelter, and our raiment. Even 

 our fuels, gas, coal, and oil, have a sun-energy origin, and the modern 

 cliff dweller in his steam-heated apartment, reading his newspaper 

 under the incandescent Madza, gets light which originated in the 

 sun, no matter whether the electric current he uses is generated from 

 coal or by water power. We are warmed by the sun, fed by the sun, 

 and clothed b}^ it, just as was primitive man, ages before us. 



WHAT IS SUN ENERGY? 



Now what is it that the sun sends to us through 93,000,000 miles 

 of space? Is it a stream of electrons? So it would appear. But 



