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THE AMERICAN NATURALIST. [Vol. XXXVllI. 



When experiments are instituted for the purpose of eliminat- 

 ing these substances, they fail, for it is out of the question to 

 remove all nitrogen, oxygen, and carbon-dioxide even if all N, O, 

 and Co., can be excluded at the beginning of an experiment, for 

 N and O will be contained in the organism and C02 will be formed 

 by it. Nevertheless, the living organism which is dependent 

 upon oxygen for respiration, upon nitrogen to dilute the other- 

 wise too destructive oxygen, and upon carbon-dioxide from which 

 to manufacture food, will succumb in any experiment of more 

 than brief duration in which the proportions of these gases are 

 greatly different from those in normal air. In other words the 

 physical properties of the atmosphere, its components, their com- 

 position and proportions, being and having for ages been what 

 they are, living organisms represent reactions to these qualities 

 and will not bear sudden change, whatever might be or may have 

 been the result of gradual change, if there has been any. 



Water is composed of hydrogen and oxygen in the propor- 

 tions of two to one. It always has had the same chemical com- 

 position, structure, and properties, the same physical qualities. 

 It is an indispensable constituent of living organisms and of 

 many lifeless things. It is a weak acid and the most universal 

 solvent known. Molecules and atoms of dissolved substances 

 move about in it with considerable freedom, and where two 

 volumes of water are separated from one another only by 

 permeable or semi-permeable membranes, there is molecular or 

 atomic movement both of water and of dissolved substance from 

 the one volume to the other through the membrane. Water is 

 then a medium in which ampler molecular and atomic move- 

 ments are possible at ordinary temperatures, etc., than in many 

 other substances. But the movement of the molecules and 

 atoms of the solutes are independent of, though taking place 

 among, the water molecules. The water remains the same, 

 physically and chemically, whatever substances may be dissolved 

 in it. The temperature of a solution of small volume may be 

 different while solution is taking place, but ultimately the tem- 

 perature of the solution, or of water holding nothing in solution, 

 will be approximately that of its surroundings, other things being 

 equal. The specific gravity of the solution will differ from that 



