90 



LIGHT, CONSIDERED WITH 



air and the sun, they consist only of a colourless, inodorous, insipid mass ; 

 so much so, that when they are dried and burned they do not give out flame. 

 The carbon contained in all plants, and which of course is in greatest abun- 

 dance in such as have woody stems, is entirely the result of the action of 

 light on the leaves, by which plants are enabled to decompose carbonic acid, 

 and thus to fix its carbon in their structure and expel its oxygen. {Dec. 

 Phys. vol. i. p. 47.) Fruits before they are ripe are acid ; that is, their hy- 

 drogen and carbon are combined with an excess of oxygen ; but they are 

 rendered saccharine by the action of light, which occasions the evolution of 

 the oxygen, and the fixation of carbon, by which the vegetable acid is con- 

 verted into sugar. In a word, no plant, nor any part of a plant, can be 

 brought to perfection without light ; but it deserves also to be remarked, that, 

 in the cultivation of plants for the use of man, it is sometimes not desirable 

 to bring all the parts of a plant to perfection ; and, in these cases, the absence 

 of light is as necessary as its presence is in others. For example, in the 

 case of the Celery and other plants, the stalks of w^hich, when rendered 

 green by light, are disagreeable to the taste and even poisonous ; but which, 

 by excluding the light, are rendered wholesome and agreeable : the same 

 may be said of the tubers of the Potato, and of the stalks and leaves of 

 Cardoons, Endive, &c. 



279. Lights to a certain extent, follows the same laws as heat. It is re- 

 ceived by radiation from the sun, reflected by smooth surfaces, transmitted 

 and refracted by transparent substances, such as water and glass ; concen- 

 trated by reflection from concave surfaces, and dispersed by reflection from 

 surfaces which are convex. All these properties of light are rendered more 

 or less available in horticulture. Light, however, differs from heat in the 

 impossibility of retaining it after the absence of the sun ; whereas heat can 

 be retained by enclosing heated bodies in non-conducting mediums, and by 

 reflecting it back to the surfaces from which it is radiated (218). 



280. The radiation of light is greatest when the radiating rays strike the 

 surface at a right angle, and least when the angle is most oblique : because, 

 in the former case, the rays are reflected on every side, and consequently 

 the surrounding objects are illuminated proportionately ; and in the latter 

 case the greater number of rays pass off at one side, and illuminate less 

 efl^&ctively the surrounding medium. The reflected rays are always returned 

 from the surface on which they radiate, at an angle equal to the angle 

 of incidence ; if the reflecting surface be a plane, the reflected rays will 

 be parallel to each other : if the surface be convex, they will be diver- 

 gent, and consequently dispersed ; and, if it is concave, they w 'lW be con- 

 vergent, and hence concentrated. Smooth and shining surfaces reflect most 

 light, and rongh and dark surfaces least ; and, with respect to colour, white 

 reflects almost all the rays of light which fall on it, and black absorbs them all. 



281. When light falls on a transparent medium, a portion of the rays is 

 transmitted through it, and a portion is reflected from its surface. The 

 latter portion follows the same laws as the light which is reflected from 

 opaque surfaces ; and the portion which passes through it is refracted — that 

 is, it leaves the transparent medium at a different angle from that in which 

 it fell upon it ; and by this change the light is also weakened, so as at a 

 very short distance from the surface of the transmitting medium, as of 

 glass for example, to be dispersed and transfused in the atmosphere, in 

 which state, in hothouses, it has no longer the same power on the vital energies 



