However, at the close of the interview, Mr Sturgis hospitably pressed on his visitor a glass of old Madeira—‘Very rare, my lord, existing only in a few private cellars; the present, forty years since, of a ducal client of mine.’
After some further quiet conversation upon the mysterious subject in hand, the lawyer put into the possession of Lord Harrogate the half of a card torn in two, which had for two decades reposed peaceably in the recesses of his own desk; and told him that this card, picked up on the towing-path by one of the men employed in searching for the child’s body, was the only fragment of mute evidence that was now in exigence.
THE EFFECTS OF LIGHT ON PLANTS.
It is now an ascertained fact that as a rule, no organised being in the world subsists alone by the nourishment which it absorbs, either in the form of food or of atmospheric air; it has also need of heat and light. Light is the creator of the charming colours, the sweet perfumes, the exquisite flavours which we gain from the vegetable kingdom. But how these marvellous operations are accomplished, what are the rules of the dispersion of darkness and its multiplied refractions, are not yet thoroughly determined. Let us glance at what has been already determined.
Plants are nourished by absorbing through their roots certain substances in the soil, and by decomposing through their green parts the carbonic acid gas contained in the atmosphere. They decompose this gas into carbon, which is assimilated, and into oxygen, which they exhale, and return to the atmosphere for the use of animals. This, which may be called the respiration of plants, cannot be performed without the help of the solar rays. Charles Bonnet, the well-known philosopher of Geneva, was the first in the last century to verify this truth. He remarked that all plants grow vertically, and stretch towards the sun in whatever position the seed may have been planted. We have all noticed how plants in dark places direct their stems to the place whence a ray of light issues. He also discovered that when plunged into water they disengage bubbles or gas under the sun’s influence. Our own Dr Priestley took up the subject and gained another step; he burned a light in a closed space until it went out, shewing that the oxygen had been consumed, and that in consequence the air had become unfit for maintaining combustion. Into the space he introduced the green parts of a plant, and after ten days the air was so purified that the candle would burn once more. In other words he had proved that plants can substitute oxygen for carbonic acid gas. If some water-cress, for instance, be grown in water, and exposed to sunlight, the presence of the oxygen gas given off by the leaves may be demonstrated by the rekindling of a paper the lingering spark of which is introduced into the vessel in which the plant is contained.
Dr Ingenhousz further explained this interesting fact. He observed that plants have the power of correcting impure air in a few hours; and that this marvellous operation is due solely to the influence of the sun upon plants. This influence only begins when the sun has risen some little time above the horizon; the obscurity of night entirely suspends the operation, as do also high buildings or the shade of trees. Towards the close of day the production of oxygen relaxes, and entirely ceases at sunset.
When these facts had been established, the explanation was soon discovered: the impure gas which was absorbed and decomposed during the day was nothing but the carbonic acid which is freely given out from the lungs of every breathing animal, the pure gas resulting from the decomposition being oxygen. But the diurnal respiration of most plants is exactly the inverse of the nocturnal, for the gas which they emit during night is the unwholesome carbonic acid. It was discovered also that mere heat could not take the place of light in these operations. There was another point which required elucidation; this was, the relation that existed between the amount of carbonic acid absorbed and of oxygen exhaled. Another Genevese citizen, De Saussure, maintained that the latter is always the smaller quantity, and that at the same time a portion of the oxygen retained by the plant is replaced by nitrogen; whilst Boussingault shewed that the volume of carbonic acid decomposed was equal to that of the oxygen produced.
There is a wonderful rapidity and energy in the performance of these functions by the green parts of plants, as was proved by placing an earthen vessel in the sun filled with vine-leaves. Through this a current of carbonic acid was passed, and when it came out it was pure oxygen. It is calculated that one single leaf of the water-lily thus exhales during the summer about three hundred quarts of oxygen. Indeed there are some peculiarities about aquatic plants which make them more valuable in clearing the atmosphere than others, for during the night they are inactive and disengage no carbonic acid, whilst they act as others do in the daytime. It is easy to shew the direct action of the sun on vegetable respiration by placing some leaves of the nayas in a vessel filled with water saturated with carbonic gas; as soon as this is exposed to the sun, an infinite number of little bubbles of almost pure oxygen will be seen rising to the surface. The shadow of a cloud crossing the sky suffices to lessen this action, which is again resumed with sudden activity when it has passed. By intercepting the solar rays with a screen, the changes of quick or slow production of gas-bubbles may be clearly observed.
So far these remarks apply only to white light, that is the mixture of all the rays which the sun sends us; but this light is not simple; it is composed of seven prismatic groups of colours, the properties of which are quite distinct. This prismatic group further prolongs and extends itself by invisible radiations. Beyond the red there are radiations of heat; beyond the violet, chemical radiations. The first act on the thermometer; the second determine energetic reactions in chemical compositions. What is their influence on vegetation? Does the solar light affect plants through its colour, its chemical properties, or its heat?