A HISTORY OF SCIENCE 



temperature) than under others. But apparently no 

 substances at ordinary temperatures, and no tempera- 

 ture above the absolute zero, are absolutely free from 

 its disturbing influence. Hence it is that molecules 

 having all the valency of their atoms fully satisfied do 

 not lose their chemical activity since each atom is 

 momentarily free in the exchange of partners, and 

 may seize upon different atoms from its former part- 

 ners, if those it prefers are at hand. 



While, however, an appreciation of this ceaseless 

 activity of the atom is essential to a proper understand- 

 ing of its chemical efficiency, yet from another point of 

 view the "saturated" molecule that is, the molecule 

 whose atoms have their valency all satisfied may be 

 thought of as a relatively fixed or stable organism. 

 Even though it may presently be torn down, it is for 

 the time being a completed structure ; and a considera- 

 tion of the valency of its atoms gives the best clew that 

 has hitherto been obtainable as to the character of its 

 architecture. How important this matter of archi- 

 tecture of the molecule of space relations of the 

 atoms may be was demonstrated as long ago as 1823, 

 when Liebig and Wohler proved, to the utter bewilder- 

 ment of the chemical world, that two substances may 

 have precisely the same chemical constitution the 

 same number and kind of atoms and yet differ utterly 

 in physical properties. The word isoinerism was coined 

 by Berzelius to express this anomalous condition of 

 things, which seemed to negative the most funda- 

 mental truths of chemistry. Naming the condition by 

 no means explained it, but the fact was made clear that 

 something besides the mere number and kind of atoms 



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