Introduction 7 



When an atom of hydrogen is attached to not one but two other 

 atoms and thus acts as a tie between them, the union involves the 

 hydrogen bond. In such a case, one of the points of attachment is a 

 firm covalent bond, and the other a weaker one, essentially ionic in 

 character. The latter is the hydrogen bond. 



In general, the hydrogen bond is part of a situation in which 

 hydrogen appears to have two valences, one stronger with some 

 ionic character, the other weaker with more ionic character. The 

 strength of the latter weaker bond ranges from one-tenth to one- 

 hundredth that of the former, according to the atoms involved and 

 their distance apart. Because it is feebly attached, the hydrogen 

 bond shifts easily from one atom to another. 



The use to which the hydrogen bond may be put in interpreting 

 protoplasmic behavior in respect to structural continuity is illustrated 

 in a simple system like water. For some time it has been known that 

 the water molecule is polar, i. e., it has an electric moment. It 

 possesses, therefore, the characteristics necessary for molecular 

 orientation. The two hydrogen atoms are positively charged, and 

 the oxygen atom is negatively charged. At a distance, such a mole- 

 cule is electrically neutral, but viewed nearby, it is positive or nega- 

 tive depending on one's position. The negative oxygen atom of a 

 water molecule will attract and hold the two positive hydrogen 

 atoms of another water molecule, and with a force which may equal 

 the hold on its own two hydrogen atoms. The valence bonds within 

 a water molecule may be regarded as chemical and those between 

 adjoining molecules as physical, but it amounts to essentially the 

 same thing in the end, for the hydrogen atoms and the oxygen atoms 

 exert forces on the surrounding molecules which are no less chemical 

 in nature than those holding the atoms within the molecule together. 

 Fluidity depends upon the shift that takes place between one mole- 

 cule and another. Water molecules are always in partnership, but 

 always changing partners. We thus see how polarity leads to orienta- 

 tion and it in turn to continuity in structure in liquid systems. That 

 this is true is indicated by the diffraction patterns which certain 

 solutions yield, indicative of some kind of arrangement. 



In a somewhat similar manner, Pauling interprets the structural 

 continuity of water in terms of the hydrogen bond. Instead of the 

 classical H^O, it is presumed that each oxygen in water is surrounded 

 by four hydrogens, two of which, close to the oxygen atoms, are 



