CHEMICAL PHILOSOPHY. 43 



latter produce no change in the actual constitution of the 

 metals themselves. 



38. Chemical Affinity. There is great variety in the range 

 and degrees of chemical attraction. Between some sub- 

 stances there appears to be no disposition to unite under 

 any circumstances: thus no compound of fluorine and oxy- 

 gen is yet known. We say yet known, for such a compound 

 may be discovered to-morrow, as we have no proof of the 

 absolute impossibility of the union taking place. In a gen- 

 eral way substances which are alike are not eager to unite ; 

 thus the metals form few definite compounds, their alloys 

 being mainly mixtures. Bodies belonging to the same 

 group, and therefore chemically similar, as chlorine, io- 

 dine, and bromine, are not very prone to form definite com- 

 pounds. 



The widest range of aflinity is possessed by oxygen ; it 

 unites with every known element except fluorine, as just 

 stated. Sulphur has a very wide range of attraction, unit- 

 ing with nearly all the metals, forming an important class 

 of bodies called sulphides. Oxygen and nitrogen show a 

 marked contrast in this respect, the latter having very lit- 

 tle tendency to enter into combination, particularly in its 

 free state, as it exists in the atmosphere. 



Chemical attraction varies much in its degrees, or power, 

 between different substances. This is seen strikingly in the 

 affinity of oxygen for the various metals. At one end we 

 have the so-called noble metals, gold, platinum, etc., uniting 

 with oxygen only under compulsion ; while at the other ex- 

 treme we have potassium, sodium, etc., so eager to unite 

 with oxygen that they are never found uncombined in nat- 

 ure. Between these two extremes at various points we 

 have iron, zinc, copper, lead, etc. The same variation in the 

 degrees of the attraction of oxygen is shown in relation to 

 other substances besides the metals ; we shall learn how 



