2i6 PICTURESQUE EXPERIMENTS 



as an authority in scientific matters. He pointed 

 out that the philosophers of Lagado, who were 

 extracting sun-beams from cucumbers, were not 

 doing anything absurd. On the contrary, since 

 the cucumbers had been built with the help of 

 sunshine, it was a reasonable expectation that 

 energy corresponding to the sunshine should be 

 obtainable. This indeed is what we do when we 

 drive a steam engine by burning coal which ages 

 ago was built by vegetable machinery driven by 

 sunlight. 



It is possible to show the existence of this 

 process by very simple experiments. The most 

 direct, but the least interesting, experiment is to 

 take two similar plants, and expose plant A to an 

 atmosphere containing COj while B is in air freed 

 from that gas. Both specimens are placed in 

 bright light, and after a sufficient interval of time 

 their leaves are tested for the presence of starch. 

 This is a simple matter ; the green colouring matter 

 is washed out of them by means of alcohol, and 

 they are then placed in a dilute solution of iodine, 

 which has the property of staining starch purple. 

 It is always pleasant to see the leaf that had 

 been supplied with COj turn blue, while the 

 starved leaf remains a hungry yellow. 



Some of the prettiest methods of demonstrating 

 this process depend, not on the manufacture of 

 starch in the leaf, but on the fact that an assimilating 

 plant sets free oxygen, by breaking up the molecule 

 CO2, building the carbon (C) into its own tissues, 

 and letting the oxygen (O) go free. A beautiful 

 method was discovered on these lines by Engel- 



