76 Introduction to the Study of Science 



substances ; and that this accounts for the increased weight of 

 the products of combustion above the weight of the original 

 visible substances that are burned. 



36. The active part of air, oxygen. It is necessary to study 

 specially the active part of the atmosphere. It has been learned 

 that, when separated from the active one fifth of the air, nitro- 

 gen not only does not support, but also promptly extinguishes 

 fire. Hence it may be inferred that nitrogen mixed with the 

 active part of the atmosphere dilutes it and consequently 

 retards combustion. The importance of this may be readily 

 imagined. If the air were entirely composed of the active 

 substance, a fire once started would burn up everything com- 

 bustible on earth. If the atmosphere were wholly nitrogen, 

 fire and life would be impossible. But mixed as they are, the 

 mixture supports safely both fire and life. 



The active part of the atmosphere is not easily separated 

 from it ; but it may be derived without difficulty from many 

 compounds which it helps to form. Yellow lead or litharge, 

 for example, when heated in an open crucible to a high tem- 

 perature, becomes quite red, or forms in burning a new com- 

 pound known as red lead. In burning, the active part of 

 the air combines with the yellow lead, yielding a product 

 of combustion, just as bright lead did in the experiment just 

 cited. 



Red lead is a familiar commercial article, commonly utilized 

 in making paints for ironwork and farming implements, and 

 even in the manufacture of finer paints for artists. It might 

 be used as a source of the active part of air, if it were not neces- 

 sary to employ high temperatures to make it give up this 

 constituent. If a small quantity of red lead is heated in the 

 air to a high temperature, at which the color changes rapidly, 

 it gives up a substance which causes a glowing match to burst 

 into flame. This indicates that it yields under the influence 

 of heat some of the active part of the air. The same result 

 can be obtained by vigorously heating iron rust, copper rust, 



