138 The Evolution of Chemistry. 



powers. Hydrogen and the other monads are like little 

 magnets having a single pole of attraction. Oxygen and 

 the other dyads are like magnets with two poles. Nitrogen 

 and the other triads are like triple-poled magnets. Carbon 

 and the other tetrads are like crossed magnets with four 

 poles. To represent these facts we can write the symbols 

 with strokes drawn from them representing the number of 

 bonds, thus : 



Cl- -0- X N X -C- H- 

 \ i 



Saturating these bonds with hydrogen, we have the follow- 

 ing symbols of common substances : 



Cl-H H-O-H N II-C-H 



Muriatic acid. Water. Ammonia. Marsh gas. 



As oxygen has two bonds and carbon four, we can take two 

 of oxygen and they will satisfy one of carbon. Because of 

 these bonds the elements named are unable to exist as single, 

 free atoms. Chlorine is not found as Cl- except during the 

 brief instant of a change. As known to us in its elemental 

 condition it is C1-C1, or C1 8 . The same is true of most ele- 

 ments. Hydrogen is H-H, or H 2 , and oxygen 0-0, or 8 . 

 The last named exists sometimes as ozone, and then it is 

 ^ 



0-0, or 3 . Many groups of atoms cohere through multi- 

 tudinous changes, and in such group form they simulate 

 more or less perfectly the atoms themselves. Sulphuric 

 acid is always written by chemists as H 2 S0 4 , an d never as 

 SH g 4 . The -S0 4 - part is known to maintain itself intact 

 through many changes, and to act like a dyad element. The 

 two hydrogen atoms leave it together to" let a dyad metal 

 take their places. One of them will go at a time to make 

 room for a monad metal. The group NH 4 - acts in a simi- 

 lar manner by non-metals. Two such groups will satisfy 

 the vacant bonds of -S0 4 - just as readily as two monad 

 metals or one dyad. NH 4 - differs from a metal in that we 

 know its composition, can pull it asunder, and have not yet 

 been able to make two such groups intermarry. -S0 4 - dif- 

 fers from a non-metallic element in the same manner. In 

 their chemical behavior both act like elements. There are 



