304 ASSIMILATION. 



The first two substances occur in very nearly the same pro- 

 portions in free atmospheric air wherever found, 1 but the amounts 

 of the last two vary within narrow limits. 



Besides the foregoing substances, the following are also men- 

 tioned as having been found in dry air in minute traces : Nitric 

 acid, nitrous acid, ozone, marsh gas, carbonic oxide, sulphurous, 

 sulphydric, and hydrochloric acids, and hydrogen. 



812. Under ordinary circumstances the proportion of car- 

 bonic acid in the atmosphere does not increase much beyond the 

 amount stated above, namely, four one-hundredths, or one twenty- 

 fifth of one per cent. 2 Pettenkofer assigns one twentieth of one 

 per cent as the amount in the air of Munich (1,690 feet above 

 the level of the .sea) . 



In confined spaces, however, the accumulation of carbonic 

 acid (once known by the significant term fixed air) ma}' be- 

 come so great as to render the air irrespirable. It was the con- 

 sideration of the question how such air could be again rendered 

 fit for respiration that led to the first successful 8 investigation of 

 the action of plants upon the atmosphere. 



813. The amount of carbonic acid found in ordinary water 

 which has been exposed for a time to the air is sufficient for the 

 supply of this gas to water plants. The percentage of the gas 

 in the atmosphere under ordinary conditions is ample for all the 

 needs of land plants. The consideration of the effect of supply- 

 ing a larger amount than usual of this gas to water and laud 

 plants, in order thereby to influence the activity of the assimila- 

 tive process, must be deferred until all the conditions essential 

 to assimilation have been considered ; but it ma}- be said, in 

 passing, that any large excess of carbonic acid over the supply 

 furnished to plants in nature diminishes assimilative activity. 



1 For a very instructive summary of results of the examination of the air 

 in different localities, the reader should consult "Air and Rain, the Begin- 

 nings of a Chemical Climatology," by E. Angus Smith (London, 1872). 



8 Angus Smith gives the following results of his examination in 1864 of the 

 air of Manchester, England : 



Per cent of CO 2 in atmosphere. 



In the streets, usual weather 0403 



During fogs 0679 



Where the fields begin 0369 



In close buildings 1604 



Minimum amount found in suburbs 0291 



See also Ann. de Chimie et de Physique, 1883, for reports on the amount of 

 CO, in the atmosphere of different localities. 

 See the historical sketch, pp. 323, 324. 



