CHEMISTRY. 171 



under the 45th, at 1300 ; and under the 62d degree, 

 at 900 fathoms above the level of the sea.* 



Polarized Light. 



When polarized light is transmitted through rock 

 crystal, it is depolarized, or converted into common 

 light. 



Under certain circumstances light can penetrate 

 through glass when in one position, but not in another. 



Phosphorescence. 



Bodies which become luminous without undergo- 

 ing combustion, are said to be phosphorescent. 



Solar phosphori become luminous when removed 

 into a dark room, after having been exposed to sun- 

 shine. 



Some are spontaneously phosphorescent, such as 

 the flesh of salt-water fish just before it putrifies, and 

 also decayed wood. 



When fish putrify, the light is not occasioned by 

 the flesh of the animal, but by numerous animalculae, 

 whose growth the putrefaction has promoted. 



The minute species of medusa are very common in 

 the sea, and produce those sparks and globes of light 

 constituting the luminousness and phosphorescence 

 of the sea, so visible during a dark night in the wake 

 of a ship, or when the water has been struck by an 

 oar. 



* Such are the ratios, according to theory, but they have not 

 been found to correspond practically at these elevations on the 

 Himalaya mountains. Lieut. Gerard says, " The highest vege- 

 tation we saw was a plant with the leaves like sage, but without 

 smell, and brown from the dryness of the atmosphere. It grows 

 at the height of 17,000 feet, beyond which we found no soil." 

 According to theory, the boundary of congelation traverses the 

 parallel of latitude 30 degrees, (that of the Himalaya,) at the al- 

 titude of 11,484 feet. 



