51 



boiled water, and observed that they retained their yellow 

 colour in perfectly closed vessels. Other sections were then 

 exposed to the direct influence of various gases and isolated 

 under quicksilver : the result of these experiments was, that 

 oxygen and some degree of moisture must be looked upon as 

 the causes producing the change of the yellow substance into 

 red ; oxygen alone, i. e. without moisture, produced no change 

 in the colour. The red colour of the Madder is therefore, as 

 M. Decaisne observes, a chemical product, entirely independ- 

 ent of the vitality of the plant, to which, however, belongs the 

 formation of the yellow sap. It results at least from these in- 

 teresting observations, that our knowledge of the bases of ve- 

 getable colours is not yet very far advanced. We have in this 

 case an instance of the transition of a perfect yellow colour 

 into the most beautiful red, and this produced by the influ- 

 ence of oxygen and the presence of water. 



The roots of young madder plants afford but little colouring 

 matter; from the age of eighteen months they may be used with 

 advantage, but their colouring substance keeps on increasing 

 to an age of three years. 



M. Decaisne also performed a series of experiments to in- 

 vestigate the influence of light on the production of the colour- 

 ing substance in madder plants ; for this purpose he grew 

 young plants exposed to various degrees of light. They were 

 placed in glass cases which were covered with cotton, others 

 in coloured glasses, as under red, yellow, or green glass. 

 Another plant was grown under white glass ; a third remained 

 exposed to the open air, and a fourth was buried in the earth 

 and covered to the height of 3 feet with loose soil. The plants 

 which had been covered with cotton had in eight days lost so 

 much of their colour as to appear quite pale ; the plants under 

 red glass exhibited this change of colour in a still greater de- 

 gree, and those under other glass were in a similar condition. 

 In general, these experiments on the growth of plants under 

 coloured glasses afforded as few results as those enumerated 

 in my first year's report (1835). According to Osann's obser- 

 vations, only 34 rays of light pass through red glass, while 

 966 are held back ; light green glass transmits 630 and re- 

 tains 340. We hence see how varied is the quantity of light 

 which arrives at the plants grown under such coloured glasses ; 



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