Dr. Tyndall on Molecular Influences, 135 



from within and the sudden accession of it from without. But 

 nature has gone further, and clothes the tree with a sheathing 

 of worse-conducting material than the wood itself, even in its 

 worst direction. The following are the deflections obtained by 

 submitting a number of cubes of bark of the same size as the 

 cubes of wood to the same conditions of experiment : — 



The direction of transmission, in these cases, was from the 

 interior surface of the bark outwards. 



The average deflection produced by a cube of wood, when the 

 flux is lateral, may be taken- at 



12°; 

 a cube of rock-crystal (pure silica) of the same size produces a 

 deflection of 



90°. 



This single experiment is sufficient to show how different must 

 be the meteorological effects of these two substances, when they 

 exist in sufficient quantity to exercise an influence upon climate. 

 Among the more prominent influences here, Humboldt mentions 

 the nature of the soil and of vegetation. The general influence 

 of an arid and exposed soil has been long known, but the part 

 played by this substance, silica, has hitherto had no particular 

 importance attached to it. Were gypsum, however, instead of 

 silica, the prevalent mineral in Sahara, a very different state of 

 things from the present would assuredly exist. A cube of the 

 latter substance examined in the usual manner produces a de- 

 flection of 



19° 

 only. It is scarcely superior to wood, while there is the strongest 

 experimental grounds for the belief that silica possesses a higher 

 conductive power than some of the metals. These grounds shall 

 be adduced in a future paper. 



Let us consider, for a moment, the process which takes place 

 jrom sunrise to the hour of maximum temperature in a region 

 overspread with forests, and compare it with that which must 

 take place in the African Desert. In the former case, the heat 

 slowly and with difficulty penetrates the masses of wood and 

 leaves on which it falls, and after the point of maximum tem- 

 perature is passed, the yielding up of the heat acquired is pro- 

 portionately slow. In the desert, however, the mass of silica 



