846 SCIENCE OF AGRICULTURE. Part II. 



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*0 ; 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 to- 

 wards both extremities ; but the heating power, which is least at the violet end, increases 

 from that to the red extremity ; and when the thermometer 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 invisible rays was greatest at the distance 

 of half an inch beyond the red ray, but it was sensible at the distance of one inch and a 

 half. 



2259. The influence of the different solar rays on vegetation has not yet been stu- 

 died ; 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 colors ; 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. C/iem. p. 402.) 



Sect. II. Of Electricity. 



2260. 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 correctly estimated. It has been shown by experiments made by means of the 

 voltaic battery, 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 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 ex- 

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



2261. Respecting the nature of electricity A^erent 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 eflPects to be pro- 

 duced by two diflferent 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. 



2262. A profitable application of electricity y 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 atmo- 

 sphere. 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 J3ritai?i, 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. 



2263. Water is a compound of oxygene 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 

 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 

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



