86 EVOLUTIO:^ AND GROWTH. 



their constitution. Percival ascertained by direct experiment tht 

 accuracy of this inference by placing plants in a current of atmo 

 spheric air, mixed with a pretty large proportion of this gas. By 

 means of a comparative experiment, he saw that a plant in such 

 circumstances made much greater progress than one subjected to a 

 current of ordinary air.* The researches of Saussure, in confirm- 

 ing in all respects those of his predecessors, added this farther very 

 important fact : that to act beneficially upon vegetables the carbonic 

 acid must be mixed with oxygen. 



Under a bell glass of the capacity of 398 cubic inches, placed over 

 mercury, with a delicate film of water swimming on its surface, he 

 introduced three young peas, which displaced about y|y of the in- 

 cluded air. The atmosphere was composed of common air and 

 carbonic acid gas in different proportions. 



The experiments were conducted successively in the sunshine and 

 in the shade. In the sun, the apparatus received daily the direct 

 action of the light during five or six hours : when the light was loo 

 vivid it was somewhat lessened by shading. In the sunlight the 

 plants lived for several days in an atmosphere composed of equal 

 parts of air and carbonic acid ; they then faded. But they died 

 much more speedily in atmospheres which contained two-thirds, or 

 three-fourths, or a fortiori which consisted entirely of carbonic acid. 

 The young plants throve decidedly when the atmosphere contained 

 about ^th of carbonic acid ; their growth was evidently more vigor- 

 ous here than it was in simple air ; and at the conclusion of one ex- 

 periment which extended over ten days, almost the whole of the 

 carbonic acid was found replaced by, or clianged into oxygen : the 

 peas had assimilated the carbon. 



The smallest quantity of carbonic acid added to the air, was 

 found injurious to the plants when they were kept in the shade. 

 Young peas lived only six days under sucii circumstances, when 

 the atmosphere annind them consisted of a quarter of its volume of 

 carbonic acid. They lived ten days when the proportion of this 

 gas did not exceed a twelt'th ; but then they scarcely grew at all in 

 the mixture ; they certainly n»adc much less progress than they 

 would have done in common air. Saussure concluded, from these 

 experiments, that cari)onic acid was useful to growing vegetables 

 only when present along with oxygen, and that it ceases to be 

 60 wiien the atmosphere contains more than ,'.jth of its volume of 

 the gas. 



To determine the proportion of oxygen set at liberty during the 

 decomposition of carbonic acid by plants, Saussure composed an 

 atmosphere of common air and carbonic acid, the latter in the pro- 

 portion of 0.075 ; the mixture was confined under a bell-glass of 

 the capacity of 5.7i6 litres or lO.lIC jiints, stantling over mercury 

 as in tlie former experiments. Seven plants of the periwinkle were 

 introduced into the apparatus, their roots dipping into 15 cub. centim. 

 or 5.895 cub. in. of water — the water was limited as much as possible, 



♦ MuncliONtor Mrmoirx, vol. li 



