183 



Glass which has been stained yellow with oxide or 

 chloride of silver allows a flood of light to pass through it, 

 but permits uo permeation of an Actinic ray. 



And on the contrary, if we use a glass colored deeply 

 blue, with the oxide of cobalt, though but very little light 

 can pass through it, experiment proves that it offers no 

 obstruction to the chemical rays, that is, it permits the 

 permeation of the Actinic ray. 



This fact of yellow glass interfering with and intercept- 

 ing the actinism of the sunbeam, has lately been taken 

 advantage of in photography. The photographer no longer 

 shuts himself and his prepared plate in a dark dungeon, for 

 now the old dark chamber, camera oscura, is beautifully 

 illumined by the sun's rays passing through yellow glass 

 which effectually excludes the actinic, the chemical rays 

 which alone the photographer dreads in this part of his pro- 

 cess, but which allows all illuminating rays to be trans- 

 mitted. 



The strength of evidence appears to be in favor of con- 

 sidering light, heat and actinism as three distinct principles 

 or powers, active in regulating the great phenomena of 

 nature. These agents are unceasingly at work. It is im- 

 possible to expose any body, however solid and persistent 

 it may appear, to the influence of sunshine, without its 

 undergoing a molecular or chemical change. In darkness, 

 all bodies appear to possess the power of restoring them- 

 selves to their normal state. Should the sun shine uninter- 

 ruptedly upon a granite monolith or a bronze statue, it 

 would perish independently of any other destructive in- 

 fluences. 



Night seems as necessary to secure the permanence of 

 the inorganic world as darkness and sleep are essential to 

 maintain in healthful life the organized creations. 



At the enormous distance of 95,000,000 miles from us is 

 the sun, a great orb having a diameter of 882,000 miles, 

 forming the centre of the solar system. Not only is the 



