304 CHEMICAL COMBINATIONS 



greater portion of the oxygen which performs so important a 

 part in the numerous and diversified changes which are contin- 

 ually taking place in the interior of plants. Few changes are 

 really more wonderful, in chemical physiology, than the vast 

 variety of transmutations which are constantly going on through 

 the agency of the elements of water. 



It rarely, perhaps never, happens that we find the same 

 unhealthy and disagreeable smell in the external atmosphere, 

 which we frequently perceive in forcing-houses after a strong 

 fire has been kept up during the night. Sometimes this condi- 

 tion may occur in the confined streets of closely-built cities, and 

 in the vicinity of chemical works, where the heavier gases rise 

 into the air in a rarefied state, and, on cooling, fall again to the 

 surface of the earth, producing sometimes injurious conse- 

 quences. The combustion of fuel for the production of artificial 

 heat produces also carbonic acid gas in great abundance. And 

 to form this gas the oxygen is drawn from the plants to form 

 the combination ; and in this way the deficiency of oxygen, so 

 much felt in forcing-houses, may partly be accounted for. Oxy- 

 gen must exist ia the atmosphere to the amount of 21 per cent, 

 of its bulk to be capable of supporting animal and vegetable life 

 in a state of vigorous development ; and when this proportion is 

 reduced, the plants under its influence must suffer accordingly. 

 The most convenient method of supplying the atmosphere with 

 oxygen is by saturation with water, which latter element con- 

 tains a very large amount of this gas, every nine pounds of this 

 liquid containing no less than eight pounds of oxygen. In the 

 interior of plants, water undergoes continual decomposition and 

 recomposition. In its fluid state it finds its way and exists in 

 every vessel and in every tissue; and so slight, it would appear, 

 in such situations, is the hold which its component elements 

 have upon each other, or so strong their tendency to combine 

 with other substances, that they are ready to separate from each 

 other at every impulse, yielding now oxygen to one, now hydro- 

 gen to another, as the production of the several compounds 

 which each organ is destined to elaborate respectively demands. 

 Yet with the same readiness do they re-attach themselves, and 

 cling together, when new metamorphoses require it. 



6. In the constitution of the natural atmosphere we are at 



