146 



Report on the Progress of Chemistry, 



taken instead of hydrogen as a type. Eow in these typi- 

 cal compounds hydrogen is a common element, and it was 

 noticed that, in them, the number of atoms were respec- 

 tively one, two, and three; and, moreover, it was seen, that 

 the one, two and three atoms of hydrogen were respectively 

 combined with a single atom of another element. It seemed 

 clear, therefore, as Dr. Odling noticed, that the combining 

 powers of different elements were not alike. An atom of 

 chlorine could take only a single atom of hydrogen, while 

 an atom of oxygen could take two, and an atom of nitro- 

 gen could take three. 



The chemical attraction exerted by an atom of hydrogen 

 thus came to be the unit of measure, in terms of which the 

 combining powers of all the other elements could be des- 

 cribed. Chlorine, one of whose atoms can hold only a sin- 

 gle atom of hydrogen in combination, is described as being 

 uni-equivalent, or briefly, univalent. Many other substances, 

 like chlorine, able to hold one atom of hydrogen to one 

 of their own, are brought into a single class known as the 

 uinvalent group. All other substances, which, like oxygen, 

 have power to hold two atoms of hydrogen to one of their 

 own, constitute a second class known as the bivalent 

 group. In a similar way there is the trivalent group in which 

 the combining power of an atom is equivalent to that in 

 three atoms of hydrogen and others in which the equiva- 

 lence of atoms is higher still. 



This principle of classification has led to the theory of 

 compound radicals upon which the composition of organic 

 substances can be explained and the otherwise overwhelm- 

 ing mass of disconnected facts reduced to a manageable 

 system. Take, for example, the typical body marsh gas ; 

 its composition is one atom of carbon with four atoms of 

 hydrogen, and since an atom of carbon is equivalent to four 

 of hydrogen the attractions of both elements in this com- 

 pound are fully satisfied. But now let an atom of hydrogen 



