ANALYSIS OF COMPLEX MATTER 137 



determines what the changes shall be. The changes chiefly 

 associated with the life of animals are those in which oxygen, 

 after diffusing into the blood through the lungs, forms, with 

 a portion of the blood, a chemical compound, which then 

 diffuses outwards. Inspired air differs from that which is 

 expired, as may be shown by the use of a solution of calcium 

 oxide. 



In addition to the influence of the atmosphere in this 

 direction, it has an important bearing upon all thermal 

 changes in helping to bring about thermal equilibrium. 

 Whenever a change in the temperature of a body is produced 

 by any other change, there is soon a restoration of thermal 

 equilibrium between it and neighbouring bodies. In this 

 process the air is very active. By special appliances it may 

 be retarded sometimes. 



We may add to this the very important function of the 

 atmosphere in bearing away gaseous products from a region 

 of chemical change. The nature, as well as the extent, of 

 many chemical changes are determined by the dissipation of 

 some of the products of the change. By this means an action 

 may be extended, or even changed in character ; but probably 

 the latter result is likewise due to thermal equilibrium rapidly 

 occurring among gaseous particles. When a chemical change 

 takes place under altered pressure, we need to consider how 

 far it is likely to be affected by such conditions as those given 

 above 



Water- vapour, which is always necessarily present in the 

 air, is undoubtedly active in many natural changes. It has 

 been shown that in dry air fewer substances burn than in 

 moist air. Experiments, however, are difficult in this direc- 

 tion ; but the value of our present results is not diminished 

 so long as we remember that all the substances present in a 

 given change should be recorded, even when they appear 

 insignificant. It is of primary importance that we should 

 bring all these considerations of pressure, mass, temperature, 

 and diffusion, together with others, to bear upon the investi- 

 gation of all changes, even those which appear, or have been 

 regarded as, simple. 



