496 Plant Case for growi?ig Plants 



As the solar light consists of rays possessing very different 

 powers, M. Senebier endeavoured to discover to which species 

 of rays the coloration of the leaves of plants was to be speci- 

 ally ascribed. Scheele had remarked that the violet rays of 

 the prismatic spectrum acted soonest in blackening muriate of 

 silver, a fact confirmed by the experiments of Senebier, who 

 extended the same views to the action of light in the coloration 

 of plants. He caused young colourless plants to grow in dif- 

 ferent glass vessels, so constructed that the light which fell 

 upon them should first pass through fluids of different colours, 

 red, yellow, and violet. At the end of four or five weeks, 

 the leaves which had been exposed to red light had a 

 tinge of green ; those in the yellow light were at first green, 

 but afterwards became yellow ; and those in violet light were 

 quite green, and the depth of colour increased with their age. 

 {Mem. Phys. Chim., torn. ii. p. 55. et seq.) The subsequent 

 experiments of Ritter and Wollaston have shown that these 

 effects were produced, not by the coloured rays, but by certain 

 invisible rays associated with them ; and which exist in greatest 

 force at and beyond the boundary of the violet extremity of the 

 spectrum. To these rays have been assigned the names of the 

 chemical or deoxidating rays ; of their deoxidating power we 

 shall have abundant evidence in the next section. 



8. Condition of Plants, *with regard to Air, in close Cases and in 

 the free Atmosphere. 



In the last place, we have to treat of the state or condition 

 of the air which contributes to the support of vegetation in 

 these plant cases. Mr. Ward appears to think that the air 

 suffers no other change than that of " expansion by heat. With 

 every change of temperature, a corresponding change," says he, 

 "takes place in the volume of air ; and without such variation 

 the plants would soon perish." Besides a change of volume in the 

 way above mentioned, it is, however, certain, that the air, in 

 these cases, must also undergo a change of composition, which 

 gradually impairs, and would ultimately destroy, its power of 

 supporting vegetation. Unless, therefore, fresh air be supplied to 

 replace that which may have been injured by the vegetative 

 process, or means be found of restoring the deteriorated por- 

 tion to its former purity, vegetation cannot long continue. 

 Though the cases in which the plants are confined may not 

 be perfectly air-tight, yet they are made so close as to prevent 

 that amount of change in the air which is required for healthy 

 vegetation ; and we must, therefore, seek for other means by 

 which a wholesome state of the atmosphere may be maintained. 

 As the mode in which this object is accomplished is somewhat 

 perplexing, and opinions concerning it are much at variance, 



