FREEZING OF WATER. 303 



easily effected in a thin glass flask ; but the moment it 

 is agitated, it becomes a firm mass. Here we have the 

 indication of another cause aiding in producing crystals 

 of ice on the surface of water, under the influence of the 

 disturbance produced by the wind, which does not ex- 

 tend to any depth. 



As oxygen and hydrogen gases enter largely into 

 other chemical compounds besides water, it is impor- 

 tant to consider some of the forms of matter into the 

 composition of which these elements enter. To examine 

 this thoroughly, a complete essay on chemical philoso- 

 phy would be necessary ; we must, therefore, be con- 

 tent with referring to a few of the more remarkable 

 instances. 



The waters of the ocean are salt : this arises from 

 their holding, in solution chloride of sodium (muriate 

 of soda common culinary salt] and other saline bodies. 

 Water being present, this becomes muriate of soda, 

 that is, a compound of muriatic acid and soda : muriatic 

 acid is hydrogen, combined with a most remarkable 

 gaseous body, called, from its yellow colour, chlorine ; 

 and soda, oxygen in union with the metal sodium, 

 therefore, when anhydrous, culinary salt is truly a 

 chloride of sodium. Chlorine in some respects re- 

 sembles oxygen; it attacks metallic bodies with great 

 energy; and, in many cases, produces the most vivid 

 incandescence, during the process of combination. 

 It is a powerful bleaching agent, is destructive to 

 animal life, and rapidly changes all organic tissues. 

 There are two other bodies in many respects so similar 

 to chlorine, although one is at the ordinary tem- 

 peratures solid, and the other fluid, and which are 

 also discovered in sea-water, or in the plants grow- 

 ing in it, that it is difficult to consider them other- 

 wise than as different forms of the same principle. 

 These are iodine and bromine, and they both unite 

 with hydrogen to form acids. The part which 



