lxviii 



the highest altitudes, that the face and neck are soon blistered when ex- 

 posed to them, although the mercury in the shade is at the time near the 

 freezing point. Immense volumes of vapor are daily raised from the 

 equatorial regions of the Atlantic by the action of that portion of the heat 

 rays which are not intercepted. Hence the air in those regions is con- 

 stantly being charged with vapor as it comes from the East deprived 

 of it by the African continent. Being thus charged in its passage across 

 the Atlantic, it is swept over the South American Continent by the action 

 of the earth's rotation, and gives to the countries east of the Andes a de- 

 lightful climate of great evenness of temperature, in which vegetation, 

 protected by this humid screen from the scorching rays of the sun, is con- 

 tinually reproducing itself in never-ending cycles of marvelous exuber- 

 ance. Having reached the high and cold ridges of the Andes, the trade 

 winds thus burdened with vapor give up their load at the eastern slopes 

 of the mountains, 'and the resultant rains fill the channels of the mightiest 

 system of rivers on the earth. On the Pacific slope of the same range of 

 mountains, the rivers are insignificant, and rains are almost unknown; 

 whilst Africa, unblest with vapor-laden breezes to intercept the heat waves 

 which the sun is incessantly sending forth, possesses, in the same latitude, 

 the most terrible climate endured by man. 



Iodine, bromine and lamp-black intercept the rays of light, but per- 

 mit the obscure heat rays to pass freely. This quality is called diather- 

 mancy. A lens of rock-salt coated so thickly with smoke or carbon as to 

 cut off every ray of light, will yet transmit the heat rays so freely as to 

 create a high degree of temperature in its focus. 



Certain substances have the power of reducing the rate of the wave 

 periods of the luminiferous ether. Thus the chemical rays which are too 

 rapid to excite vision, if passed through gla -s alloyed with uranium, are 

 reduced in their periods so that they become visible to us as green rays. 

 This phenomenon was demonstrated to you in the beautiful and instruc- 

 tive lecture of Dr. Curtman last January. When these rays fall upon cer- 

 tain substances, distilphate of quinine, for example, the body is made lumi- 

 nous. This is because the vibrations of the atoms of the quinine, being 

 of slower periods, the rays have their periods correspondingly lessened 

 and thus become visible. This phenomenon is called fluorescence. 



On the other hand, the obscure heat rays may be made visible by concen- 

 trating them through a lens upon a refractory substance; the molecular 

 vibrations become so much increased thereby as to produce luminosity. 

 This is called calorescence. 



If a hollow glass lens be filled with a solution of iodine and placed in 

 the sun, the few rays of light passing through the glass edge of the lens 

 will converge and indicate its focus. Let the lens be fixed, and the loca- 

 tion of the focus marked, and when this is done paste around the glass 

 edge of the lens black paper or cloth, so that no ray of light can possibly 

 pass through it. Within the dark shadow of this obscure lens, in its in- 

 visible focus, the heat rays will have such power that refractor)' metals may 

 be raised to whiteness, fusible metals melted, and gunpowder exploded. 



