The Evolution of Chemistry. 137 



such way as the atoms themselves are grouped within their 

 molecules. He is thus able to foresee possible compounds 

 not y\3t discovered or made, and gains cues concerning the 

 proper method whereby to discover them. In constructing 

 such pictures, or " graphic formulas," as they are designated, 

 what has been called by Hoffman quantivalence is of great 

 importance. Molecules do not, like the deacon's one-horse 

 chaise, go to pieces all at once. Their bonds of union are of 

 such a character that what breaks one does not break all. By 

 studying the way they break, and how certain elements or 

 groups of elements may be substituted for each other within 

 them, valuable information concerning their structures can 

 be obtained. 



In no chemical change has hydrogen ever been known to 

 fill the place of another atom with more than a single atom 

 of itself. There are, on the other hand, innumerable cases 

 of other elements filling the place of two, three, and four 

 atoms of hydrogen with one atom of itself. If hydrogen be 

 figured as having but one bond of attraction, then such ele- 

 ments as can only saturate that one bond are with hydrogen 

 itself called monads. An element that ican saturate two 

 bonds of hydrogen or replace two atoms of hydrogen in a 

 compound is called a dyad. One that represents three hy- 

 drogen bonds, a triad. Beyond these we have tetrads, pen- 

 tads, and hexads. Chlorine saturates but one bond of hy- 

 drogen, and is therefore a monad. One volume of chlorine 

 gas unites with an equal volume of hydrogen gas to form a 

 volume of muriatic acid. The saturating powers of chlorine 

 and hydrogen atoms are seen to be equal. Using some of 

 the muriatic acid to precipitate a solution of nitrate of silver 

 as chloride, we will find our hydrogen replace the silver as 

 its equivalent, and our chlorine saturate it. Chlorine, hy- 

 drogen, and silver are thus shown to be monads. In water 

 we find that it takes two volumes of hydrogen to saturate 

 one of oxygen. When the oxygen is replaced by chlorine, 

 we find that two volumes of chlorine are required to take the 

 place of one of oxygen. This proves oxygen to be a dyad. 

 If we take oxide of zinc and act upon it with muriatic 

 acid, we will discover that two equivalents of the chlorine 

 from the acid are needed to replace the one of oxygen, and 

 two of hydrogen to replace the one of zinc. The quantiva- 

 lence of oxygen and zinc is therefore the same. They are 

 dyads. In this way all the elements are found to arrange 

 themselves in separate groups, according to their attractive 



