130 



THE REASON WHY. 



" The hay appeareth, and the tender grass showeth itself, and herbs of the 

 mountain are gathered." PBOV. xxvil. 



Because blue glass obstructs many of the luminous rays, but it 

 is perfectly transparent to actinism. 



558. Why do plants become scorched under the unclouded 

 sun? & - 



Because the heat rays are in excess. The clouds shut off the 

 scorching light; but, like the blue glass of the photographer's 

 studio, they transmit actinism. 



559. What effect has actinism upon vegetation ? 



It quickens the germination of seeds ; and assists in the forma- 

 tion of the colouring matter of leaves. Seeds and cuttings, which 

 are required to germinate quickly, will do so under the effect of blue 

 glass (which is equivalent to saying, the effect of an increased 

 proportion of actinism), in half the time they would otherwise 

 require. 



560. In what season of the year is the actinic power of 

 light the greatest ? 



In the spring, when the germination of plants demands its 

 vitalising aid. In summer, when the maturing process advances, 

 light and heat increase, and actinism relatively declines. In the 

 autumn, when the ripening period arrives, light and actinism 

 give way to a greater ratio of heat. 



We shall have frequently, in the progress of our lessons, to refer to light in 

 its connection with the chemistry of nature, and with organic life. But let us 

 now invite the student to pause, and for a moment contemplate the wonders 

 of a sunbeam. How great is its velocity how vast its power how varied its 

 parts yet how ethereal! First, let us contemplate it as a simple beam in 

 which light and heat are associated. How deep the darkness of the night, and 

 how that darkness clings to the recesses of the earth. But the day beams, and 

 darkness flies before it, until every atom that meets the face of day is lit up 

 with radiance. That which before lay buried in the shade of night is itself now 

 a radiator of the luminous fluid. Mark the genial warmth that comes as the 

 sister of light ; then stand by the side of the experimentalist and watch the 

 point on which he directs the shining focus, and in an instant see iron melt 

 and stones run like water, under the fervent heat ! Now look upward to the 

 heavens, where the falling drops of rain have formed a natural prism in the 

 rainbow, and shown that the beam of pure whiteness, refracted into various 

 rays, glows with all the tints that adorn the garden of nature. These are the 

 visible effects of light. But follow it into the crust of the earth, where it is, by 

 another power, which is neither light nor heat, quickening the seed into life; 

 watch it as the germ springs up, and the plant puts forth its tende 



