96 



next comes plumbago, commonly called " Black Lead," which 

 contains one p.c. of impurity, while anthracite and common coals 

 leave considerable amounts of ash when burnt. Jet is regarded 

 as fossilized wood, while peat is the starting point of change from 

 living vegetation. 



Not one of these, however, supplies vegetables with their 

 carbon as all are solids, and roots can take up no minerals unless 

 they be in solution with water. The carbon of plants is solely 

 derived from the carbonic acid gas of the atmosphere, which is 

 absorbed by all their green parts. The air is composed of four- 

 fifths Nitrogen (Njj'j, one-fifth Oxygen (0 2 ) and four ten-thous- 

 andths Carbonic Acid Gas (C.0 2 ). This is derived from all kinds 

 of combustion of organic substances or by breathing, for both 

 animals and plants respire in the same way. Much is also given 

 off from volcanoes as many minerals are " carbonates," i.e., com- 

 bined with carbonic acid gas, as all chalk and limestone, as well 

 as certain ores of iron and other metals. To understand how 

 carbonic acid is taken in by the leaves, we must examine their 

 structure. A leaf-blade is covered by the skin or epidermis com- 

 posed of flat cells united together so that it can often be easily 

 peeled off. Below the upper epidermis are elongated cells, upon 

 the upper ends of which the skin rests. This is called from their 

 appearance the " palisade " layer. The lower half of the leaf 

 has roundish cells more loosely cohering. The cells of both these 

 layers abound with green balls of living substance called proto- 

 plasm. They are named " Chlorophyll," i.e., " leaf-green." The 

 chlorophyll cannot absorb C0 2 in the dark, nor can it be formed 

 itself without light, as it is certain rays of the solar spectrum, 

 such as nature displays in the rainbow, which are chiefly concerned 

 in the absorption and decomposition of CO.,. 



A spectrum can be made by a three-sided prism of glass, by 

 which the normally colourless light is split up into red, orange, 

 yellow, green, blue, indigo and violet, which we can see, but other 

 rays are invisible, such as heat-rays beyond the red at one end and 

 ultra-violet at the other. By growing plants, such as lettuces, 

 under coloured glasses it will be found that they grow very differ- 

 ently ; the height is greatest under red, and is shorter and shorter 

 as we pass from red to blue, and shortest of all when grown in 

 the open ground. They will all be green, but when they are dried 

 and weighed then we can ascertain the different amounts of solid 

 carbonaceous matter in each. The maxima are always under the 

 yellow and the blue glasses ; smaller amounts occur under red and 

 violet glasses.* 



When the coloured glasses were examined with the spectro- 

 scope it was found that they were rarely pure, the red and the 

 green being the best. Now the red is found brightest at the 

 extreme end, but continued fainter up to the usual position of the 

 green. On the other hand, with the green glass, the colour spreads 



* I could not get a good "indigo" glass. 



