Final Remarks on the Leaf Pigments. 49 



land plants to be a mixture of four substances, two green and two 

 yellow, all possessing highly distinctive optical properties. The 

 green substances yield solutions exhibiting a strong red fluorescence, 

 the yellow substances do not. The four substances are soluble in 

 the same solvents and three of them are extremely easily decomposed 

 by acids or even acid salts, such as bis-oxalate of potash, but by 

 proper treatment each may be obtained in a state of very approxi- 

 mate isolation so far at least as coloured substances are concerned." 



Although it is a matter of national pride that the discovery of 

 the four leaf pigments should have been made by a British worker, 

 yet on the other hand the almost complete neglect with which 

 later investigators in this country have treated Stokes' work is 

 certainly very discreditable. When the obsession for demonstrating 

 the presence of formaldehyde in the leaf (started by Baeyer's 

 hypothesis in J870, and first 'experimentally' investigated by 

 Pollacci in ti@&) began in this country with the work of Usher 

 and Priestley in 1906, these writers neglected the presence of two 

 green pigments and completely left out of consideration the yellow 

 pigments in their theory of carbon assimilation. How much of the 

 recent inconsequent work on the same subject might have been 

 avoided if all these later writers had been aware of, and taken notice 

 of, the work of Stokes. 



Again, the extraction and separation of the pigments without 

 the aid of chemical action is due to Stokes. In a paper in the 

 Journal of the Chemical Society for 1864, he says" For convenience 

 and rapidity of manipulation, especially in the examination of very 

 minute quantities, there is no method of separation equal to that 

 of partition between solvents which separate after agitation. 

 Bisulphide of carbon in conjunction with alcohol enabled the 

 lecturer to disentangle the coloured substances which are mixed 

 together in the green colouring matter of leaves." 



The use of nettle leaves for extraction of chlorophyll was also 

 recommended by Stokes in a paper published in Transactions of 

 the Royal Society in 1852. 



Considering that these observations were only side issues of 

 Stokes' work, it is very remarkable that they should have been so 

 correct. There can be no doubt that he did a great deal more 

 work on chlorophyll than appears from his published work. He 

 announced his intention of publishing work on chlorophyll, but it 

 never appeared, and apparently nothing has so far been found 

 among his papers referring in detail to these investigations. 



