THE GROWTH OF PLANTS. 215 Researches on the Influence of the Solar Rays on the Growth of Plants. (376.) The experiments connected with this investigation have been extended over a period of seven years; they have been made at every season of the year; and the localities in which they have been carried on has been changed from the south-western extremity of the kingdom to the neighbourhood of the metropolis. Although there are many important points which remain open for investigation, and others, which although examined, require, from the complexity of their phenomena, still more minute research, I believe I am enabled to lay before this meeting of the British Association a series of important facts connected with the processes of germination and vegetable growth, as affected by solar radiations. The title heading to each of my former reports has been the "Influence of Light," &c. I have now changed that form of expression and adopted the above. My reasons for this are, that much confusion has arisen from our habit of referring all the effects observed in the processes of vegetation to the agency of Light, whereas it appears that some agencies which are not luminous materially influence the phenomena of vegetable vitality. Without entering into any discussion in this place on the probable existence, or otherwise, of a principle distinct from Light and Heat in the sun's rays, to which we refer the curious chemical changes produced by solar influence, it will be sufficient to admit the existence of three distinct classes of phenomena, which cannot, I think, be disputed. These are luminous influence, LIGHT;-calorific power, HEAT;- and chemical excitation, ACTINISM. The problem which these researches were directed to solve was the proportion and kind of influence exerted by Light, Heat, and Actinism—as the principle supposed to be active in producing the chemical phenomena of the solar rays has been called—in the various stages of vegetable growth. (377.) The means we have of separating these phenomena from each other are not very perfect; indeed, in the present state of our knowledge, it is impossible to have evidence of the operations of either Light, Heat, or Actinism absolutely separated from each other. If we use the prismatic spectrum, we have over every portion of it a mixture of effects. Even in the mean yellow, or most luminous ray, we have a considerable amount of thermic action, and, under some circumstances, evidence of chemical power. In the violet rays, which have been particularly distinguished as chemical rays, we have light and heat; and in the calorific rays we have decided proof of both luminous and actinic power. In the experiments which have been made with the prismatic spectrum, we have in fact no certainty that the results stated to be due to a particular ray that ray being regarded as the representative of a particular phenomenon are not the combined effect of the three forces. The same objections apply to absorbent media, but the amount of each influence is readily determined; and we are therefore enabled to refer any particular result to a tolerably well-defined agency. (378.) All the experiments recorded in this report have been made under the action of those radiations which have permeated variously coloured media, such as tinted glass and coloured transparent fluids. It is not sufficient to state that a yellow, red, or blue glass or fluid was employed, as it by no means follows that these media are permeated only by the rays corresponding in colour, or by the influences due to a given order of refrangibility. The difficulties which oppose themselves to experiments made with coloured media have been strongly felt by other observers. Dr. Daubeny says, in his memoir “On THE GROWTH OF PLANTS. 217 the Action of Light upon Plants," &c., Philosophical Transactions, vol. cxxvii. 1836, “The difficulty, however, of comparing the relative intensity of the light transmitted by the variously coloured media, which were employed in my experiments, induced me to content myself with showing that the effect of Light upon plants corresponds with its illuminating, rather than with its chemical or calorific influence; and to waive the more difficult inquiry, whether its operation upon the vegetable kingdom exactly keeps pace with the increase of its own intensity." (379.) In my previous reports to the British Association in 1842 and 1844, I have stated the kind of examination to which I then subjected each coloured screen. Many effects which have from time to time presented themselves, have convinced me of the necessity of a still more close examination of the order in which radiant principles permeate the media employed. I have therefore in every case examined with all care the illuminating, calorific and chemical effects of the solar rays which have passed the media employed. The amount of Light has been determined by measuring off, in parts of an inch, the prismatic rays which pass the screen. This is preferable to any system of measuring which depends upon the power of the eye to appreciate either light or shadow. Having formed a well-defined spectrum on a white tablet, and carefully marked off the centre of the yellow ray as being the point of maximum Light, and the limits of each of the other rays, the transparent coloured medium was interposed, and the amount of absorption observed. These examinations, many times repeated, were made with reference to the luminous rays only; and in the description of my experiments, I shall, considering the unabsorbed ray as being represented by 100, express the amount of light actually effective by such a number as may give the sum of the rays measured off after per meation. The calorific influences which escape absorption, and which have been determined by the expansion of the mercury in a thermometer with a blackened bulb placed behind the coloured glass or fluid, and by the evaporation of ether from a sheet of blackened paper, as recommended by Sir John Herschel, will be expressed numerically in the same way as light, without reference to the colour of any ray. I am far from considering the thermic influences of the solar rays as quite independent of the colour of the ray with which they may be associated; but in these experiments on plants, it appears to me, we can only deal satisfactorily with the total amount of radiant heat which is active under the conditions of the experiment, the ter restrial heat being in all comparative experiments the same. It has indeed been shown by Dr. James Stark (Philosophical Transactions, vol. cxxiv. 1833.), by direct experiment, and indirectly by other observers, that colour exerts a very powerful influence on the conduction, radiation, and permeation of heat. (380.) The determination of the chemical principle of the solar rays, or actinism, permeating the media employed, required more exact attention than the other phenomena. Dr. Daubeny ascertained this by placing paper washed with nitrate of silver behind the coloured screen; and in my early experiments I was satisfied with using tolerably sensitive chloridated photographic papers for the same purpose. The experience of many years enables me now to state that we are not acquainted with any transparent medium which is absolutely opake to actinism. Although nitrate of silver, or indeed any of the salts of silver remain unchanged behind yellow glasses and fluids, yet chlorophyl is deoxidized and turned yellow by the chemical principle which is enabled to permeate them. Upon all those bodies on which Light exerts a direct and determinate influence, as upon the organized compounds (particularly the sensitive photographic preparations now employed — 1854), LIGHT AND ACTINISM. 219 we find that the changes due to actinic power are but slightly interfered with, whereas upon all those inorganic bodies which undergo a change when exposed to the solar chemical radiations, that change being entirely due to actinism, Light acts as a powerfully interfering agent. The conditions under which these antagonistic forcesLight and Actinism — operate upon each other are unknown to us; but it is certain that every combination of an inorganic salt with an organic body presents a dif ferent scale of action. (381.) Nitrate of silver uncombined with organic matter undergoes no change by the influence of any portion of the solar spectrum, or of white light; spread it on paper, or combine it with gum or gelatine, and all that portion of the spectrum above the green ray blackens it; and if we combine this salt with unstable organic compounds, the blackening is found to take place, eventually, under every spectral ray. The other salts of silver, and metallic salts in general, are affected in precisely the same manner. From a knowledge of these facts it became evident that some means must be devised for ascertaining, as correctly as possible, the entire quantity of this chemical principle passing any particular media, without which knowledge any result would be almost valueless. In every instance I have therefore determined the influence of the modified radiations: first, upon the most sensitive silver salts; secondly, upon organic bodies, as the coloured juices of leaves and flowers, and on chlorophyl; and thirdly, upon combinations of the organic and inorganic materials. In this way I have arrived at a degree of correctness which has not been hitherto reached, and the results of the experiments have consequently a higher value. (382.) I have repeatedly stated that seeds would not germinate under the influence of Light deprived of that principle on which chemical change depends. This statement has been objected to by Dr. Gardner (American Journal of Science, Jan. 1844, vol. xlvi.), on the results of |
