94 



Proceedings of Indiana Academy of Science 



corresponds to the increase in volume on cooling the liquid below 4°. 

 These breaks in the curves indicate points where marked changes occur 

 in the character of the liquid. The compressibility curve shows a 

 similar change. 



Fig. 2. The methane molecule consists of a tetrahedral carbon atom in combination 

 with four hydrogen atoms. The dots represent the valence electrons which take part in 

 the combination. This is probably the most non-polar molecule known. 



Fig. 3. Ammonia, NH3, is distinguished by a pair of electrons on the surface of 

 the nitrogen which gives to that part of the molecule a negative character. The hydro- 

 gens are slightly positive. 



Fig. 4. .Ammonium chloride is formed by the action of hydrogen chloride on am- 

 monia gas. The positively charged H from HCl adds onto the extra pair of electrons 

 shown in figure 3 to form the ammonium which is held to the chloride ion by electro- 

 static forces. . iil^TlB! 



In order to understand the peculiar properties of water it is neces- 

 sary to understand its molecular structure and for that we must know 

 the structure of the atoms of which water is built up. Although there 

 are a great many details about atomic structure which are in an un- 

 satisfactory state, the theory has developed enough to show the great 

 value, not only in giving us understanding of the properties of most 

 compounds, but also in predicting the new properties which a substance 



