NATURAL COLOURS . 445 



of vegetation is tinted in such rich variety, would tell, if it could 

 be completely and truly read, how much there is still to learn of 

 nature's secrets. The chemist is just beginning in the twentieth 

 century to find out the mere composition of a few of these dye- 

 stuffs contained in the minute cellular laboratories of the plant, 

 but the materials out of which they are formed and the process 

 by which they are elaborated are alike unknown. From remote 

 antiquity it has been known that solar energy is the source or, 

 at any rate, a condition of all life. It must seem strange, there^ 

 fore, that the agent which more than any other controls the 

 chemical changes which result in the production of vegetable 

 colours should be the last to engage systematic enquiry. 



The action of light in producing decomposition in certain 

 metallic compounds has been recognised, and the result is the 

 art of photography. But the student of organic chemical 

 changes has not until recently given much attention to the 

 changes which light brings about, and as yet there is little to 

 report. The field is wide and the enquiry will be difficult, for 

 it will be necessary to know the relative effects of light of different 

 colours, that is of different degrees of refrangibility, and in the 

 first instance it would be well to operate on pure substances 

 alone or two together. The greater part of the photo-chemical 

 observations hitherto recorded are oxidations or reductions which 

 have been obtained in other ways. 



A few cases of polymerisation, that is condensation of two or 

 more molecules into one, almost completes the list of known 

 changes induced by light, and apart from the extension of 

 photographic processes it is evident that a wide and fertile field 

 is open to the industry of future generations. Out of such 

 investigations may come, and probably will come, a more 

 intimate knowledge of the way in which the plant does its work. 



Before leaving the subject of natural colouring matters we 

 may return to a brief survey of some colouring matters of animal 

 origin. Of these the red substance in blood corpuscles is obviously 

 the most important. This exists in the blood in two conditions 

 according as it is taken from the arteries or from the veins, and 

 especially after asphyxiation. In the former it is called oxyhaemo- 

 globin, and consists of haemoglobin combined loosely with oxygen. 

 This power of entering into union with various gases is char- 

 acteristic of haemoglobin, and in at least one case, namely, car- 

 bonic oxide, this ought to be borne in mind as it serves to explain 



