AGENCY OF LIGHT. 105 



grees. This limit is about 20° N. latitude. The southern 

 limit of barley is farthest north, after which wheat, then rye, 

 and farthest south, Indian corn. Near the southern limit, as 

 in Bengal, '' Wheat, barley and oats are sown in the autumn, 

 and harvested in March ; while rice and maize are sown in 

 May, to be harvested, as with us, in October." 



It is the same condition of temperature, which limits the 

 cultivation of the food-bearing plants on mountains. Thus, 

 in the Alps the grains cease growing at the following heights : 



Wheat at 3,400 feet, corresponding to latitude 64 degrees. 

 Oats 3,500 " " 65 " 



Rye 4,600 " « 67 " 



Barley 4,800 " " 70 " 



The above facts show the indispensable agency of heat up- 

 on the vital power, and should lead the farmer to adapt his 

 crops to its influence, as affected by the location of the farm 

 and the character of the soil. 



V. Light. Light has been shown to consist of three 

 kinds of rays. 1. Colorific, or those rays which give color 

 to the various objects in nature. 2. Calorific, or those rays 

 which produce heat, and are the cause of the sun's warmth. 

 3. Chemical rays, or those which produce effects upon chemical 

 combinations. These rays are easily separated by means of 

 a prism. Whether the effect upon vegetation results from a 

 combination of all the rays, or is produced by one kind, is 

 not certainly known ; but the probability is that the power, in 

 this respect, depends partly upon the chemical rays, and part- 

 ly upon the colorific rays. 



1. Light is a powerful stimulant to vegetation, changing 

 the color of the leaves and stalks, and giving the pungent pro- 

 perties peculiar to each species of plant. Those vegetables 

 which grow in the shade are always pale and sickly in growth, 

 and insipid in taste. But let a ray of light cross the stem or 

 leaf of such a plant, and it will turn green in that spot, and 

 become pungent, although all other parts remain as before. 

 9* 



