March 1st, 1SS7.] 



SCIENTIFIC NEWS. 



viously noted, viz., that some of the rays emitted from an 

 arc light cause the leaves to shrivel, and the plants to be 

 otherwise injuriously affected. After this, the lamps were 

 surrounded with clear glass, as recommended by Sir W. 

 Siemens, and the general results are thus recorded : — 



1 . The electric light was unable to develop young 

 germinating plants. 



2. Vigorous plants, in full vegetation, were able to live 

 in the darkened portion of the house, in which they re- 

 ceived the reflected rays of the electric light both day and 

 night. 



3. None of the plants experimented on, except barley, 

 performed its normal functions; no flower opened. 



Healthy plants, under the influence of sunlight, emit a 

 very large quantity ot water (according to Professor 

 Deherain, a young blade of corn yields its own weight of 

 water in one hour, and maize still more) ; but with an arc 

 light this is very much less, and this he attributes to the 

 want of red or heat rays. 



In a third set of experiments, the plants were placed 

 much nearer the electric lights. Professor Deherain gives 

 the results in detail, but his general conclusions were as 

 follows : — 



1. The arc light has rays which are injurious to vegetation. 



2. The larger portion of the injurious rays can be arrested 

 by transparent glass. 



3. The arc light was proved to have rays favourable to 

 vegetation, in sufficient quantity to enable plants to vegetate 

 under its sole influence for two-and-a-half months. 



These results confirm the early experiments of Sir 

 William Siemens, but they are not nearly so good as those 

 last obtained by him, and this is probably due to the 

 following causes. In the first place. Professor Deherain 

 used lamps of about 1,500 candles' power each, whereas 

 those last used by Sir William Siemens had each a power 

 of about 5,000 candles, and the raj'S of the latter would, 

 therefore, have greater energy and a much greater influence 

 on the plants. In the next place. Professor Deherain had no 

 heating apparatus, whereas Sir W. Siemens employed the 

 usual circulation of hot water in pipes, and there is little 

 doubt that if the hours are increased during which a plant is 

 stimulated with light, it is important that the temperature 

 of the house should also be carefully regulated. Again, in 

 the case of the plants subjected to sunlight in the daytime, 

 and to arc lights at night, Professor Deherain's results were 

 doubtless influenced by the fact that he did not have the 

 sun's rays direct. It is true that he placed some of the 

 plants in the open air in the gardens, during the daytime, 

 but they were then subjected to even greater variations of 

 temperature than in the Exhibition building. 



Under the influence of the sun's rays the leaves of a 

 plant absorb and decompose carbonic acid gas, and this 

 effect is chiefly promoted by the red or heat rays. Professor 

 Deherain has proved that this effect can also be produced 

 by the rays emitted from a piece of platinum gauze highly 

 heated. The arc light, however, differs from the sunlight, 

 and from the usual artificial sources of light; it is much 

 more violet and much less j'ellow, and h^s fewer heat rays. 

 Professor Deherain is therefore sceptical as to its being used 

 with advantage for plants, and he recommends trials being 

 made with electric glow lamps, or with the " lampe soleil" 

 in which a piece of lime is heated to a white heat by an 

 electric arc. Possibly he may be right, but it may be 

 pointed out that Professor Deherain himself proved that an 

 arc light of 1,500 candles' power was capable of making 

 leaves absorb and decompose carbonic acid more readily 

 than the diffused sunlight in the Exhibition ; in fact, this 

 may be taken as a measure ot its efficiency. It is true that 

 this operation was performed much more slowly than with 



bright sunlight ; no one however would think of using the 

 arc light as a substitute for the sun's rays, but only as a 

 source of light when the sun's rays are not available. It is 

 certainly to be regretted that these trials were not made 

 under more favourable circumstances, as no real advance 

 has been made since the last experiments of Sir W. Siemens. 

 If, therefore, some one who has the time and means at his 

 disposal will pursue these investigations, he will doubtless 

 find a very interesting, although possibly not a profitable 

 field open to him. 



CONSERVATION OF ENERGY. 



FORMERLY it was supposed that much of the energy 

 or capacity to do work, or produce physical change, 

 was destroyed in doing that work or producing that change. 

 It is, however, much more in harmony with the other funda- 

 mental laws of Nature, to suppose that energy cannot be 

 created or destroyed, any more than matter tan be created 

 or destroyed. Happily it has since been proved beyond 

 doubt that there is no such thing as destruction of energy, 

 but that one kind of energy is merely transformed into 

 another kind. This, in fact, is the great principle of the 

 Conservation of Energy, which is one of the most interest- 

 ing, and at the same time one of the most important, 

 generalisations ever made. It helps to explain the greatest 

 questions not only in mechanics, but in chemical action, 

 in heat and electricity, and in all subjects treated of by 

 physical science. 



Energy is now divided into two classes — the energy of 

 motion, and the energy of position, and what this means 

 will be understood from the following example. When a man 

 raises a weight from the ground, or throws it upwards in 

 the air, he does so against the force of gravity tending to 

 keep it on the ground. The man must therefore impart so 

 much energy of motion to the weight as will cause it to be 

 raised, and formerly it was supposed that if the weight so 

 raised were lodged on the top of a house or other resting- 

 place, all the energy would have been spent. This, how- 

 ever, is only partly true ; for although there is no longer 

 energy of motion, seeing that the weight is at rest, it still 

 has the power to fall to the ground with a force depending 

 on its mass and the height of its fall. In falling, it is evi- 

 dent that there is again energy of motion, and the storage 

 of this energy on the top of the house, while the weight is 

 at rest, is called the energy of position. Again, in the case 

 of a pendulum, when it swings from side to side, it is for 

 an instant at rest at the end of each stroke, and then pos- 

 sesses the energy of position ; but this again is converted 

 into energy of motion as soon as it begins to fall. The 

 same may be said of a spring wound up or compressed, 

 which then has a store of energy of position capable 

 of being transformed into energy of motion. So far 

 it is clear that there is merely a transference of energy, 

 and that none is destroyed ; but we now have to consider 

 other cases, a little more complicated. For instance, when 

 a piece of iron is struck with a hammer it is heated, and 

 when a metal button is rubbed hard it is also heated. 

 What, then, becomes of the energy of motion in both these 

 cases, and whence comes the heat ? We may say that in 

 the one case it is the effect of a blow or impact, and in the 

 other of friction ; but this does not help us to answer the 

 question. Count Rumford and Sir Humphrey Davy were 

 among the first to show that heat could be obtained by 

 purely mechanical means, but it was left for Dr. Joule to 

 prove by the most careful experiments that the heat thus 

 developed was an exact equivalent of the energy expended 

 in producing it. The apparatus he used was very simple, 

 and consisted of blades made to revolve in a closed cyhn- 



