32 



growth of the plant was checked ; and its leaves, on 

 being analyzed, yielded more carbonic acid and wa- 

 ter, and less hydrogen and residual carbon, than an 

 equal weight of green leaves, results similar to 

 those obtained by Chaptal, Hassenfratz, and Senne- 

 bicr. The various colours of flowers are ascribed 

 also to the combination of light in different propor- 

 tions. Red rose trees, if carefully excluded from 

 light before their flowers begin to appear, will yield 

 flowers almost white: and marine plants are govern- 

 ed by the same laws with regard to colour as land 

 vegetables *. Scheele observed, that it is the violet 

 ray which acts most powerfully in reducing the o- 

 xide of silver t ' and Senncbier ascertained, that the 

 same ray has the greatest effect in producing the 

 green colour of plants J ; but according to Mr 

 Gough, the presence of oxygen gas is required to 

 aid its operation . The late Dr Hope found, that 

 the light produced by art effected changes in degree 

 similar to that which issues from the sun, which 

 has since been confirmed by others : and the light 

 of the moon has a similar effect. It follows, there- 

 fore, from these facts, that although light is not im- 

 mediately necessary to the growth of plants, yet that 

 many of their more peculiar and characteristic qua- 

 lities depend essentially upon its action. 



* Davy in Beddoes' Contributions to Science, p. 186. ct 

 seq. t On Fire, p. 98. 



J Mem. Phys. Chim. vol. iv. p. 72. 

 Manchester Memoirs, vol. iv. p. 501. 



