Book III. ELECTRICITY.— WATER. 353 



mometer, and their heating power was observed. The heating power of the violet, green, 

 and red rays were found to be to each other as the following numbers : — Violet, 16-Q • 

 Green 22*4; Red, 55 -0. The heating power of the most refrangible rays was least, 

 and this power increases as the refrangibility diminishes. The red ray, therefore, has 

 the greatest heating power, and the violet, which is the most refrangible, the least. The 

 illuminating power, it has been already observed, is greatest in the middle of the 

 spectrum, and it diminishes towards both extremities ; but the heating power, which 

 is least at the violet end, increases from that to the red extremity ; and when the thermo- 

 meter was placed beyond the limit of the red ray, it rose still higher than in the red ray, 

 which has the greatest heating power in the spectrum. The heating power of these invi- 

 sible rays was greatest at the distance of half an inch beyond the red ray, but it was sen- 

 sible at the distance of one inch and a half. 



2326. The influence of the different solar rays on vegetation has not yet been studied ; 

 but it is certain that the rays exercise an influence independent of the heat they produce. 

 Thus plants kept in darkness, but supplied with heat, air, and moisture, grow for a short 

 time, but they never gain their natural colours ; their leaves are white and pale, and their 

 juices watery and peculiarly saccharine : according to Knight they merely expend the 

 sap previously generated under the influence of light. (Notes to Sir H. Davy's Agr. 

 Ckem. p. 402.) 



Sect. II. Of Electricity. 



2327. 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 on vegetation have not 

 been correctly estimated. It has been shown by experiments made by means of the vol- 

 taic battery, that compound bodies in general are capable of being decomposed by elec- 

 trical 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 instrument, 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 electricity, 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 experi- 

 ment is related by Dr. Darwin. ( P/iytologia, sect. xiii. 2, 3.) 



2328. Respecting the nature of electricity different opinions are entertained amongst 

 scientific 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. 



2329. 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 angling 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 eva- 

 porating surface, produced by the vegetation of improved culture, and especially from 

 the increase of pastures, hedges, and ornamental plantations. 



Sect. III. Of Water. 



2330. Water is a compound of oxygen and hydrogen 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 15 ; 

 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." 



A a 



