378 CHLOROPHYLL -GRANULES AND THE SUN'S RAYS. 



at disposal, but only that the sugar is isolated which has been formed synthetically 

 from these substances in those tiny chemical laboratories, the vegetable cells. 

 Consequently it is really incorrect to say that sugar is " made" in our manu- 

 factories; we should only say that there the sugar manufactured by the plants is 

 separated from other substances and prepared for further use. 



Although it is not possible to represent the processes concerned in the synthesis 

 of organic materials in plant cells as a matter beyond all doubt, one is justified in 

 taking refuge in hypotheses. And it must be looked upon as an hypothesis when 

 we consider the movement by which the atoms of the food-gases and food-salts are 

 displaced by the sun's rays in the vegetable cells as a transmission of the vital force 

 of the sun. The atoms have arranged themselves by this movement in a difierent 

 order, they hold and support one another, they are stable, and a condition of 

 mutual tension has been set up. The vital force of the sun has become the hidden 

 spring of all these changes. The now stable organic material formed by synthesis 

 is thus equipped with an adequate supply of energy, designated in other words as 

 latent heat. If the atoms of the organic material from whatever cause again break 

 loose, abandoning their combination and arrangement, they perhaps so displace and 

 rearrange themselves that those groups which previously existed are formed again, 

 and thus the potential energy is changed to vital force, the latent heat to sensible 

 warmth. When a tree-trunk is consumed, the vital force of the sun, which had 

 been changed by the formation of cellulose and the other organic materials 

 composing the wood of that time into latent force, is again transformed into 

 active energy; and when we burn coals, the sun's rays, which thousands of years 

 ago brought about the formation of organic vegetable substances and were 

 imprisoned in the coal, will again be set free, will warm our rooms, drive our 

 machines, or propel our steamships and locomotives. Keeping this idea in view, it 

 is at least possible to imagine the mechanical significance of the sun's rays in the 

 formation of organic substances in plants, and it may be reckoned that the 

 quantity of organic substance produced stands in a fixed proportion (which may be 

 expressed in figures) to the store of energy in the same. 



One circumstance on which particular stress must here be laid is that the 

 various rays of which sunlight is composed, the rays with various wave-lengths 

 and refrangibility, which, some of them at least, appear to our eyes as the diflferent 

 coloured bands in rainbows, play each a very distinct part in the formation of 

 organic materials in plant cells. Under the influence of the blue and violet rays, 

 i.e. of those which are most highly refrangible and have the smallest wave-lengths, 

 the oxidation of the organic materials called carbohydrates is assisted, that is to 

 say, not the formation but the decomposition and transformation of these 

 compounds are favoured. The red, orange, and yellow, i.e. those rays which are 

 less refrangible and have a greater wave-length, behave quite otherwise. These 

 favour the reduction of carbonic acid, assist the formation of carbohydrates from 

 raw materials, and are therefore chiefly concerned in the originating of such 

 organic substances. When a sunbeam passes through a colourless glass prism a 



