46 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



that the carbon is attached to elements which are equivalent 

 in every case but one to four atoms of hydrogen ; the exception 

 is carbon monoxide, where only one atom of oxygen, equivalent, 

 that is, to two atoms of hydrogen, is attached to the carbon. 

 Carbon monoxide, however, as is commonly known, is a com- 

 bustible gas, burning with a blue flame to form COj, a com- 

 pound in which again the, carbon is attached to two other 

 atoms, together equivalent to four atoms of hydrogen. More- 

 over carbon monoxide can combine with two atoms of chlorine 

 to form a compound known as carbonyl chloride, COClj, 

 where again the carbon is combined with atoms which are 

 together equivalent to four atoms of hydrogen. Such examples 

 might be multiplied, with the result that it can be shown that 

 one atom of carbon is always capable of combining with four 

 atoms of hydrogen or their equivalent. Incidentally we have 

 learnt also that one atom of chlorine is capable of taking the 

 place of one atom of hydrogen ; one atom of oxygen is capable 

 of taking the place of two atoms of hydrogen ; one atom of 

 nitrogen is capable of taking the place of three atoms of hydro- 

 gen. This atom-replacing power of the elements is known 

 as their valency. We speak of chlorine and hydrogen as 

 being monovalent, of oxygen as divalent, of nitrogen as tri- 

 valent, and of carbon as tetravalent. Where the atom of an 

 element does not exercise its full valency, an unsaturated, com- 

 pound results, such, e.g., as carbon monoxide. 



Throughout the vast range of organic chemistry the carbon 

 atom is always tetravalent ; where it apparently is not tetra- 

 valent, further atoms can always be taken into combination 

 till saturation results. Victor Meyer indeed was accustomed 

 to define organic chemistry as ' the chemistry of constant 

 valency,' because such constancy is not so apparent among the 

 elements which build up the mineral kingdom. 



We must now consider the second very important property 

 of the carbon atom. Not only will the carbon atom, as we 

 have seen, combine with hydrogen, with chlorine, etc., it will 



