ACTINISM.] 



UNDULATORY FORCES. LIGHT. 



67 



that portion of the plant which is not exposed to the 

 external light, scarcely evinces any sign of growth. If, 

 however, the shaded portion be turned round, it will, 

 owing to the action of light, soon show active signs of 

 life, and will rapidly bud forth. The common hollyhock, 

 which has a long stem, may be thus twisted into a screw- 

 like shape by exposing its sides alternately to light. Its 

 tall stem gradually bends towards the sun during its 

 growth, and it may thus be cited as another and excellent 

 illustration of the effects of light on the growth of plants. 



Even by means of artificial light, plants may be grown 

 wliich will present nearly the same colours as they would 

 if freely exposed to the rays of the sun. A geranium 

 plaut may thus be brought up by confining it in a place 

 excluding the light of day, if a powerful light, such as 

 from a paraffin or gas lamp, be employed. This in- 

 teresting experiment may easily be repeated by the stu- 

 dent if a little care is exercised. To show the effect of 

 even artificial light, two equally healthy plants should be 

 chosen, and whilst one is exposed to the light of the 

 lamp, the other should, for an equal space of time, be 

 entirely secluded from light of any kind. The latter 

 will speedily fade, whilst the former will flourish pretty 

 well. 



Now, it is found that certain of the rays of light have 

 more effect on germination and growth than others. Air. 

 Hunt tried a very ingenious experiment for proving this. 

 He cast the rays of coloured light obtained in the usual 

 way from a prism upon some cress, and found that that 

 portion of the plant growing under the Uue rays grew 

 rigorously, whilst that under the red rays bent away 

 from them as from the face of an enemy. 



One of the simplest modes of trying this experiment, is 

 that of exposing cress seed to coloured light, produced 

 by passing white light through glasses of different colours. 

 Three portions of common cress may be placed behind a 

 piece of red, yellow, and blue glass respectively, and the 

 relative effect may thus be readily observed. Since the 

 discovery of the chemical effect of the blue rays of light, 

 the principles to which we have alluded have been put 

 into practice in the construction of greenhouses. In- 

 stead of using white glass, which permits the flow equally 

 of all the rays of light and heat, greenish or blue-coloured 

 glass is now much employed, and great advantage thus 

 arises in respect to the growth of the plant, and the pro- 

 u of its flowers and fruits. 



The spectrum, however, as we have already remarked, 

 presents rays having three characteristics the red being 

 associated with the heating, the yellow with the luminous, 

 and the blue with the chemical effects of light ; and each 

 of these emanations has a specific office in connexion 

 with vegetable life. It has been stated, as the result of 

 experiments by Dr. Draper, of New York, that the yel- 

 low rays assist the nutrition of plants, whilst the blue 

 rays are essential to their movements or expansion in 

 size or direction. 



Without entering, at present, into the department of 

 organic chemistry, we may state, that all plants thrive 

 through the chemical changes which their organisms 

 effect on the carbonic acid contained in the atmosphere 

 surrounding them. It may appear paradoxical to those 

 unacquainted with the many astonishing facts presented 

 to tlic philosopher, when we state, that the greater part 

 of plants, from the creeping moss on our garden walls, to 

 the stately oak tree of our forests, is obtained not from 

 the earth by their roots, but is actually received from 

 the air, by the decomposition of the carbonic acid, and 

 the assimilation of its carbon or charcoal to the body of 

 the vegetable. During the day-time, the rays of light 

 assist and give vital energy to the plant. Its leaves 

 absorb the carbon from the carbonic acid, and set free 

 the oxygen which is combined witli it. They thus lite- 

 rally live on air. In the daylight, all plants give off 

 oxygen ; whilst at night, their vital force being diminished 

 bi intensity, they are found to undergo a partial decom- 

 position, ai "if carbonic acid gas. 



To popularise still further these interesting principles, 

 we may state, that by excluding light entirely from a 

 growing vegetable, we as effectually destroy its chance of 

 VOL. i. 



vitality and growth, as we should do in the case of a 

 human being by sinking him in a pond of water. In- 

 deed, the analogy is quite complete, because the cause of 

 death is identical ; namely, the asphyxiating effect of an 

 undecomposable gas on the "lungs" of either object. 



We cannot omit here to mention a beautiful illustra- 

 tion which is found in this instance, of the universality 

 of design in nature. All animals expire from their lungs 

 carbonic acid gas, which is to them a deadly poison. The 

 lungs that is, the leaves of trees actually breathe this 

 poison, and, assisted by the rays of light, they decom- 

 pose it, returning to the air the oxygen, which is to us 

 its vital portion, and retaining the solid part, or carbon, 

 for their own subsistence. Thus, what we poison, the 

 plant purifies, and a grand compensating action is con- 

 tinually kept up between the products and requirements 

 of the animal and vegetable kingdoms. 



The cause of those peculiar effects produced on vege- 

 table and other objects by the blue rays of light, and to 

 which the chemical changes observed in the photographic 

 process are due, has been termed actinism ; and when we 

 refer to the actinic rays of light in our future pages, the 

 reader will understand that we speak of those which have 

 the powers to wliich we have here alluded, in contradis- 

 tinction to those of the luminous and calorific rays of 

 the spectrum, wliich do not possess them. 



To render our remarks better understood, we here in- 

 troduce a diagram, which will give a clear idea of the 

 relative position and space occupied by those three classes 

 of rays in the spectral image, as received on a screen ; as 

 also of those rays which extend beyond the visible spec- 

 trum, both at the red and blue apparent terminals 

 (See Fig. 33). 



Fig. 33. 



( Space of extra spec- 

 tral blue, olit:i:ii<-i! 

 by a solution of qui- 

 nine, and some of 



I the mineral uiU. 



ActinUm, or ctieml- ) p 

 cal radiant power, j 



Light. C 



In this figure we observe, on the right-hand side, at the 

 top, the place of the epipolic rays, to wliich we have already 

 alluded.* Below these we have the lavender, violet, in- 

 digo, and blue, which are the chief seat of the actinic, or 

 chemical rays of the spectrum. This is indicated at E, 

 on the left of our engraving. The green rays, which seem 

 to form the middle of the spectrum both in a physical 

 and in actual position, are succeeded by the yellow 

 and orange, wliich are those most luminous in their 

 character, as shown at C. Following these are the red 

 rays ; in which, as at D, the heating effects of the prism 

 are most manifest. The student will thus, by studying 

 the above engraving, be enabled to experimentalise directly 

 with any of these three classes of rays independently; 

 and of thus repeating some of the interesting effects of 

 coloured light on plants, <fec. , which we have already named. 

 With respect to the extension of the rays beyond the spec- 

 trum at B, it is found, that the actinic or chemical effect is 

 active, as are the calorific effects beyond A. These re- 

 present the position of rays which cannot be observed by 



See ante, p. M. 



