250 



HARDWICKE'S SCIENCE-GOSSIP. 



with oxygeu absorbed from the air, forming sugar, 

 and carbonic dioxide is liberated'; it is therefore an 

 action of oxidation to which light, if it has a cleoxi- 

 dizitig influence, ought to be injurious. To satisfy 

 myself that this is the case, I have made several 

 experiments with light, and various seeds placed on 

 moist cotton wool, in all of which those kept in 

 darkness, although their temperature was lower, 

 grew more quickly than those exposed to light. In 

 one of these experiments I carefully measured, when 

 at a certam stage of development, both the plumule 

 or stem and the radicles or roots of the sprouting 

 plant. The average of those placed in the dark 

 was, — the plumule | inch, the radicles 1\ inch ; of 

 those exposed to light the result was, that the plu- 

 mule was not visible and the radicles only just 

 emerged from the seed. 



The growth of fungi depends on their power of 

 oxidizing the organic substances on which they 

 grow and exhaling carbonic dioxide. Light, there- 

 fore, ought to be injurious to them, and we know 

 that they "prefer the shade to the sunshine, and 

 always, I believe, come out of the ground in the 

 night. If two similar pieces of pasty matter be 

 placed in two cups, and one exposed to light, the 

 other placed in the dark, after a few weeks the 

 latter will be found to be much more thickly covered 

 with mould fungi than the other. 



The effect of light on dyed fabrics is another 

 illustration of its deoxidizing influence. Almost all 

 the darker dyed materials are changed to a lighter 

 colour by exposure to sun-light : — Black becomes 

 blue ; blue, green ; and green is changed to yellow. 

 The same result is obtained by the action of an acid. 

 In the case of one important dye, viz. indigo, this 

 change has been carefully investigated. This sub- 

 stance is'obtaiued from the almost colourless sap of 

 several species of the Indigofera, a genus of plants 

 which grow principally in warm climates. The 

 leaves of these plants are placed in water and 

 allowed to ferment ; a yellow substance is dissolved 

 out, which, combining with the oxygen of the air, 

 becomes deep blue, and under the influence of a 

 deoxidizing agent is again converted into the 

 nearly colourless form. These two substances have 

 the composition represented by the following for- 

 mulae : — 



Blue indigo .... C16. HIO. N2. O. O. 

 White indigo .... 016. HIO. N2. 0. H2. O. 

 Thus we see that the change of dyed fabrics from 

 blue to yellow and white is one of hydration or 

 deoxidation, and as it is produced by light, it appears 

 that in this case, also it exercises a deoxidizing 

 influence. 



But the most familiar example of the combination 

 of oxygen with carbon is ordinary combustion. 

 Combustion, whether it be of coal, wood, gas, tallow, 

 &c., consists of the oxidation of the carbon and 

 hydrogen they contain ; therefore, if mv view be 



correct, light ought to interfere with this process. 

 We know that it is asserted, by those whose fire- 

 places are so situated that the sun can shine ou them, 

 that its rays do put the fire out ; but as some 

 attribute this phenomenon to ocular illusion, I have 

 attempted to decide the question by some experi- 

 ments oji candles. The first two experiments were 

 conducted rather carelessly, but, as in both these 

 the candle burnt in darkness consumed more tallow 

 than that exposed to light, I was induced to make 

 some trials with greater care. Por this I employed 

 night-lights, for, as they burn more slowly, I thought 

 there would be a greater opportunity for light to 

 influence the result. These, after being carefully 

 weighed, were placed, one in each of two equal-sized 

 boxes, the lid of one of these being substituded by a 

 glass plate, and equal-sized ventilation-holes being 

 bored in each. The boxes so arranged were placed 

 at a window, and after four hours' exposure the 

 lights were extinguished and again weighed. 



In the first experiment there was scarcely any 

 sunshine; the res alt, therefore, as far as it was affected 

 by light, was produced by diffused day-light. In 

 these circumstances the night-light in the dark 

 burnt 20 per cent, faster than that exposed. 



In the second experiment there was occasionally 

 a gleam of sunshine, which apparently influenced the 

 result, for in this instance that in the dark consumed 

 25 per cent, more tallow than that in the light, which 

 is an increase of 5 per cent., supposed to be due to 

 the injurious effect of the occasional sunshine to 

 which that under the glass was exposed. I regret 

 that, owing to the late cloudy weather, I have been 

 unable to repeat these expeiiments ; for although in 

 five experiments I have obtained a similar result, yet 

 I cannot be confident that it is not due to some ex- 

 traneous cause of which I am not aware; but if they 

 are reliable, they prove unmistakably that in combus- 

 tion also light exercises a deoxidizing influence. I 

 have endeavoured to account for this by supposing 

 it to be due to vibration communicated to the carbon 

 by the particles of luminous ether, which convey 

 light. To illustrate this let us imagine two balls, 

 one to represent carbon, the other oxygen. The 

 light impinging upon the carbon ball puts it in a 

 state of vibration, thereby hindering its combination 

 with the oxygen. 



Having then shown that the action of Jight is one 

 of deoxidation, we will proceed to examine its effect 

 on Chlorophyll. 



We know that in autumn, when the flow of sap to 

 the leaves is arrested, their colour is changed to 

 y.ellow. That this is the result of the action of light 

 may be proved by two simple experiments. If two 

 green leaves be pressed, one in the dark, the other 

 under glass, that in the dark will remain almost the 

 same colour, whilst that exposed to the influence of 

 light will be turned yellow. If an alcoholic solution 

 of Chlorophyll (produced by placing bruised leaves 



