274 



THE FARMER'S MAGAZINE. 



form a round white spot when received on paper. 

 Now this white beam consists of three visible 

 coloured beams, which when mixed, or falhng on 

 the spot, make white, and of two invisible beams, 

 one of which produces heat, and the other a che- 

 mical influence called actinism, which produces 

 chemical changes, the most remarkable of which 

 are embodied in photographic pictures. The 

 whole sunbeam, therefore, contains luminous or 

 colour-making rays, heating rays, and chemical 

 rays. 



When white light, therefore, acts upon plants, 

 we require to know which of these rays produce 

 any of the remarkable changes that tike place; 

 and as it is not easy to insulate the different rays 

 and make them act separately, the inquiry is at- 

 tended with considerable dilficultj'. By using 

 coloured glasses and coloured fluids, which absorb 

 certain rays of white light, and allow others to pass, 

 Mr. Hunt made arrangements by which he could 

 submit jjjants to an excess of red, yellow, or blue 

 rays, or to an excess of the heating rays, or of the 

 chemical or actinic ones. In this wa}', he was not 

 able to study the pure influence of any of those 

 rays in a state of perfect insulation, but merely the 

 influence of a preponderance of one set of rays over 

 others, which is suflScient to indicate to a certain 

 extent their decided action. This will be better 

 understood from a few results obtained with differ- 

 ently coloured media. 



Lijrht. Heat. Chemical Rays. 

 White light contains . . 100 100 100 



Solution of bichromate 



of potash 87 92 27 



Solution of sulphate of 



chromium 85 92 7 



Series of blue grasses.. 40 72 90 



Solution of sulphate of 



copper 60 54 93 



Solution of ammoniate 



of copper 25 48 94 



It is very obvious that the action of the chemical 

 rays will be obtained from the three last of these 

 coloured media, and the action of the luminous 

 and heating rays from the two first, where the 

 chemical rays are comparatively feeble. In this 

 way Mr. Hunt obtained the following interesting 

 results : — 



1. Light prevents the germination of seeds. 



2. The germination of seeds is more rapid under 

 the influence of the chemical rays, separated from 

 the luminous ones, than it is under the combined 

 influence of all the rays, or in the dark.* 



3. Light acts in eflfecting the decomposition of 

 carbonic acid by the growing plant. 



4. The chemical rays and light (or all the rays 

 of the spectrum visible to a perfect eye) are essen- 

 tial to the formation of the colouring matter of 

 leaves. 



5. Light and the chemical rays, independent of 

 the rays of heat, prevent the development of the 

 reproductive organs of plants. 



* This important result has been confirmed by 

 the observations, on a large scale, of the Messrs. 

 Lawson and Sons, of Eilinburgh. See Hunt's 

 Poetry and Science, 3rd Edition, appendix, and 

 Researches on Light, p. 375. 



6. The radiations of heat, corresponding with 

 the extreme red rays of the spectrum, facilitate the 

 flowering of j)lants, and the perfecting of their re- 

 productive principles. 



In Spring, Mr. Hunt found that the chemical 

 rays were the most active, and in very considerable 

 excess, as compared with those of light and heat. 

 As the Summer advanced, the light and heat in- 

 creased in a very great degree relatively to the 

 chemical rays ; and in Autumn, the light and the 

 chemical rays both diminish relatively to the rays 

 of heat, which are by far the most extensive. 



'' In the spring," says Mr. Hunt, "when seeds 

 germinate and young vegetation awakes from the 

 repose of winter, we find an excess of that prin- 

 ciple which imparts the required stimulus ; in the 

 summer, this exciting agent is counterbalanced 

 by another possessing different powers, upon the 

 exercise of which the structural formation of the 

 ])lant depends; and in the autumnal season these 

 are checked by a mysterious agency which we can 

 scarcely recognize as heat, although connected with 

 calorific manifestations, upon which appears to 

 depend the development of the flower and the per- 

 fection of the seed." 



The very curious fact of plants bending towards 

 the light, as if to catch its influence, must have 

 been frequently observed. Mr. Hunt found that, 

 " under all ordinary circumstances, plants, in a very 

 decided manner, bent towards tlie light;" and, 

 what is exceedingly interesting, when the light 

 employed was red, from passing through red fluid 

 media, the plants as decidedly bent from it. The 

 property of bending towards the light is strikingly 

 exhibited by the potato ; and it has been found 

 that the yellow or most luminous rays are most 

 efficacious in producing this movement, while the 

 red rays, as before, produce a repulsive effect. 



If light, then, is so essential to the hfe of plants, 

 that they willeven exert alimited powerof locomotion 

 in order to reach it, it is not unreasonable to sup- 

 pose that it may be necessary, though to a less 

 extent, for the development and growth of animals. 

 When we look at the diflferent classes of the inferior 

 animals, we hardly observe any relations with light 

 excepting those of vision ; but, under the con- 

 viction that light does influence animal life, various 

 naturalists have devoted their attention to the sub- 

 ject. In his chapter " on the influence of light 

 upon the development of the body," Dr. W. F. 

 Edwards has given us some important information 

 on the efiect of light in the development of animals, 

 or in those changes of form which they undergo 

 in the interval between conception and fecundation, 

 and adult age — a process which, previously to 

 birth, is generally carried on in the dark. "There 

 are, however, animals," says Dr. Edwards, "whose 

 impregnated eggs are hatched, notwithstanding 

 their exposure to the rays of the sun. Of this 

 number are Batrachians (frogs). I wished to de- 

 termine what influence light independently of heat 

 might exercise upon this kind of development." 

 With this view he placed some frogs' sjtawn in 

 water, in a vessel rendered impervious to light, and 

 some in another vessel which was transparent. 

 They were exposed to the same temperature, but 

 the rays of the sun were admitted to the trans- 



