514 ORGANOLOGY. 



or smaller district may frequently have its origin in the facility with 

 which the soil may be heated. A well-known expression amongst 

 gardeners and farmers for a certain injurious peculiarity of the soil is 

 "cold" ("kaltgrundig"). The colour of the earth has a considerable 

 influence on its power of absorbing heat. In Graziosa, one of the 

 Canary Islands, Humboldt found close together some basaltic sands, 

 which were coloured white and black ; whilst the first had a temperature 

 of 40 C., the last reached 54-2 C. In the experiments of Schiibler, 

 a mixture of various kinds of earth, in an atmosphere of 20 R., when 

 covered with a white surface, afforded a temperature of from 33 

 to 34-8 R., and when with a black one, from 39'1 to 41 R. The 

 same earths in their various natural colours, whilst in a dry state, 

 varied in temperature from 28-1 to 31*8 R., and in a moist state, 

 from 34*1 to 37 '9 R. From these experiments it would appear that 

 the chemical nature of the soil has but an extremely small influence 

 upon its power of absorbing heat. As the dark colour of the soil almost 

 entirely depends upon the mixture of organic matter, so we can see that 

 the humus may thus exert an important influence on vegetation, without 

 affording plants any of their nutriment. When we put together all the 

 foregoing facts, in relation to humus, with this one, we can explain the 

 strikingly favourable results of the action of humus, without in any 

 manner regarding it as a nourishing substance. 



203. II. Heat, light, and electricity must be mentioned in con- 

 nection with the assimilation of the absorbed nutritive matters. 

 Without heat and light none of the important chemical processes 

 in the plant can go on. A similar statement may, perhaps, be 

 made concerning electricity ; we are here, however, without posi- 

 tive proof from experiment. 



Heat has been already spoken of; and in reference to light, only 

 matters of fact should be spoken of, as an explanation of the phenomena 

 is impossible, since we are deficient in a knowledge of light, or rather 

 of the source from whence light proceeds the rays of the sun. The 

 chemical effects of the sunbeams on inorganic matter is seen in the 

 decomposition and combination of various elements, affording evidence 

 of a powerful agency, which can least of all be doubted in the organised 

 world, where isomerism, polymerism, and similar relations, make the 

 transition of one combination into another dependant on the slightest 

 collision. The facts are too evident, and too generally known, for any 

 doubt to be raised. The pale watery appearance which darkness pro- 

 duces in plants, and the quickness with which they become green by the 

 operation of light*; the great difference in the matter which is formed 

 in the plant during the presence or absence of light, as seen in the cauli- 

 flower, endive, and other cultivated plants, are well-known facts. In 

 following the analysis of the general appearances into separate chemical 

 operations we are not always very fortunate, and this is because we have 

 been satisfied with guessing at random instead of observing and investiga- 

 ting. Until recently, it was generally allowed that chlorophyll was a sub- 



* During this summer I allowed some Oats to germinate under a vessel of zinc till 

 the buds were four inches long, when they appeared of a pale yellow colour. They 

 were then cut down, carefully washed, dried in blotting-paper, and then placed upon 

 white paper to be further dried in the sun. In six hours the plants were almost 

 perfectly dried, but they had all become of a green colour. 



