BOOK II. ELECTRICITY. WATER. 253 



expend the sap previously generated under the influence of light. (Notes to Sir H. 

 Davy's Agr. Cliem. p. 402.) 



SECT. II. Of Electricity. 



1210. Electrical changes are constantly taking place in nature, on the surface of the earth, 

 and in the atmosphere; but as yet the effects of this power in vegetation have not been cor- 

 rectly estimated. It has been shown by experiments made by means of the voltaic bat- 

 tery, that compound bodies in general, are capable of being decomposed by electrical 

 powers, and it is probable that the various electrical phenomena occurring in our system, 

 must influence both the germination of seeds and the growth of plants. It has been found 

 that corn sprouted much more rapidly in water positively electrified by the voltaic instru- 

 ment, than in water negatively electrified ; and experiments made upon the atmosphere 

 show that clouds are usually negative ; and, as when a cloud is in one state of electri- 

 city, the surface of the earth beneath is brought into the opposite state, it is probable that 

 in common cases the surface of the earth is positive. A similar experiment is related 

 by Dr. Darwin. (Phytologia, sect. xiii. 2, 3.) 



1211. Respecting the nature of electricity different opinions are entertained amongst sci- 

 entific men ; by some, the phenomena are conceived to depend upon a single subtile fluid 

 in excess in the bodies said to be positively electrified, and in deficiency in the bodies said 

 to be negatively electrified. A second class suppose the effects to be produced by two 

 different fluids, called by them the vitreous fluid and the resinous fluid ; and others 

 regard them as affections or motions of matter, or an exhibition of attractive powers, 

 similar to those which produce chemical combination and decomposition ; but usually 

 exerting their action on masses. 



1212. A profitable application of electricity, Dr. Darwin observes, to promote the 

 growth of plants is not yet discovered ; it is nevertheless probable, that in dry seasons, 

 the erection of numerous metallic points on the surface of the ground, but a few feet 

 high, might, in the night-time, contribute to precipitate the dew by facilitating the 

 passage of electricity from the air into the earth ; and that an erection of such points 

 higher in the air by means of wires wrapped round tall rods, like angle rods, or elevated 

 on buildings, might frequently precipitate showers from the higher parts of the atmosphere. 

 Such points erected in gardens might promote a quicker vegetation of the plants in 

 their vicinity, by supplying them more abundantly with the electric ether. (Phytologia, 

 xiii. 4.) J. Williams (Climate of Great Britain, 348.), enlarging on this idea, proposes 

 to erect large electrical machines, to be driven by wind, over the general face of the 

 country, for the purpose of improving the climate, and especially for lessening that 

 superabundant moisture which he contends is yearly increasing from the increased 

 evaporating surface, produced by the vegetation of improved culture, and especially 

 from the increase of pastures, hedges, and ornamental plantations. 



SECT. III. Of Water. 



1213. Water is a compound of oxygens and hydrogene gas, though primarily reckoned a 

 simple or elementary substance. " If the metal called potassium be exposed in a 

 glass tube to a small quantity of water, it will act upon it with great violence ; elastic 

 fluid will be disengaged, which will be found to be hydrogen ; and the same effects will 

 be produced upon the potassium, as if it had absorbed a small quantity of oxygen ; and 

 the hydrogen disengaged, and the oxygen added to the potassium, are in weight as 2 to 

 1 5 ; and if two in volume of hydrogen, and one in volume of oxygen, which have the 

 weights of 2 and 15, be introduced into a close vessel, and an electrical spark passed 

 through them, they will inflame and condense into 17 parts of pure water." 



1214. Water is absolutely necessary to the economy of vegetation in its elastic and fluid 

 state ; and it is not devoid of use even in its solid form. Snow and ice are bad con- 

 ductors of heat ; and when the ground is covered with snow, or the surface of the soil or 

 of water is frozen, the roots or bulbs of the plants beneath are protected by the congealed 

 water from the influence of the atmosphere, the temperature of which, in northern win- 

 ters, is usually very much below the freezing point ; and this water becomes the first 

 nourishment of the plant in early spring. The expansion of water during its congelation, 

 at which time its volume increases one twelfth, and its contraction of bulk during a 

 thaw, tend to pulverise the soil, to separate its parts from each other, and to make it more 

 permeable to the influence of the air. 



