232 



KNOWLEDGE 



[October 1, 1890. 



common, as, for example, in the very large order Umbelli- 

 fenv, to which the stately Cow-parsnip and the deadly 

 Hemlock belong, the inflorescence is generally an nmbel 

 of iimbels ; the Horse chestnut, Sd.ri/rni/d nmhrnsii, and 

 probably also Sd.nj'rai/ti iii/riiniidiilis, have a I'aceme of 

 scorpioid cymes ; in Veruonia centiflora there is a raceme 

 of capitula, while the wliite Dead Nettle (Lamiuni allium) 

 furnishes an example of a raceme of contracted scorpioid 

 cymes. Extreme aggregate beauty and enormous develop- 

 ment often characterise such inflorescences ; that of Sa.ri- 

 frri(fi( piiriDiiidiiUs appears like a huge cone, whose base is 

 the cluster of foliage leaves, while from amidst the group 

 of succulent leaves of the so-called American aloe {Aijair 

 americtinti) — which, by-the-bye, is not an aloe, but an 

 agave, as its botanical name shows — there spiings up a 

 huge compound indefinite inflorescence which often attains 

 a height of from 24 feet to 86 feet, and whose constituent 

 flowers may reach the enormous and almost incredible 

 number of four thousand. The plant is so exliausted by 

 this supreme eft'ort that it dies down to the ground, and 

 does not recover itself for some time. In its native 

 Mexico it does not flower luitil the sixth or seventh year 

 of its life, but when grown in greenhouses in this country 

 fi'om forty to sixty years generally elapse before it can 

 summon up enough energy to enable it sexually to propa- 

 gate its species ; hence the origin of the tale dear to gar- 

 deners, that the American aloe flowers only once in a 

 hundred years. 



ON THE CONSERVATION OF ENERGY. 



By -T. .T. Stewart, Thwimstrator of Phi/sirx nt rninisit;/ 

 <_'ollr//i', London. 



ONE of the most remarkable features in the intellec- 

 tual growth of the present century is the rapid 

 advance made in physical science ; and unques- 

 tionably one of the grandest of recent generaliza- 

 tions, which marks a distinct advance in philosophy 

 as well as in science, and gives us, as it were, a vantage 

 groimd from which to attack with greater power future 

 problems, is that which is known as the Principle of the 

 Conservation of Energy. This theory has been gradually 

 unfolded within the present century, chiefly through the 

 researches of such men as Dr. Joule and Sir William 

 Thomson in our own country and Helmholtz in Germany. 

 The law of conservation is that great principle which tells 

 us that the energy in the universe is not many but onr, 

 and though it can be neither added to nor diminished by 

 man, it can yet be changed by him from one into another 

 of its ever varying forms. As the fact that matter is in- 

 destructible forms the foundation of modern chemistry, so 

 the fact that the quantity of energy, or power of doing 

 work, implanted in the arrangements of matter around us, 

 can neither be diminished nor increased, is now the basis 

 of physical science. This pregnant truth has now become 

 a most powerful instrument of research, and has not only 

 led to numerous discoveries in the past, but is of the 

 utmost value in suggesting the right methods by which to 

 prosecute further inquiries into the mysteries of matter. 



The law connecting the manifestations of energy can- 

 not be proved Ijy any one experiment ; it has been slowly 

 led up to by the observation of many facts, and by the 

 reasonings founded on those observations. It is, more- 

 over, being confirmed every day through the observation 

 that no phenomenon in the physical world fails to com- 

 pletely accord with it when once it has been thoroughly 

 investigated by the instruments at our command. Its 

 ti-uth is also powerfully brought home to scientific men 



when it is found that theories assuming its existence are 

 able to jiri'dirt what will occur in given circumstances, this 

 prediction being fulfilled when an experiment is made 

 under the conditions supposed. On the whole, no physical 

 axiom has a more sure basis than this great law. 



Science owes much to those ingenious but deluded men 

 who gave so much of their time to the search for Per- 

 petual Motion. This quest was, of course, not for mere 

 contimtous motion, but for a machine which could do 

 useful work without the expenditure of power upon it ; in 

 other words, give out work, such as raising a weight or 

 pumping up water, without taking in energy from the out- 

 side. This the statement of the law of the Conservation 

 of Energy declares to be impossible. No truth in physics 

 reposes on a firmer or more extended basis, for none has 

 been investigated by a gi-eater number of the ablest and 

 most ingenious men, or with greater ardour and detenui- 

 nation. The negative result arrived at by them, though 

 disappointing to themselves, has led to the advancement 

 of science as represented to-day by modem physics. As 

 the search for the philosopher's stone and the elixir of life 

 by the old alchemists resulted in the discovery of many 

 useful and valuable facts incorporated now in our deve- 

 loped chemistry, so the pursuit of Perpetual Motion has 

 not been without permanent and important results. 



The word " energy," in its physical sense, was first em- 

 ployed by the philosopher. Dr. Young, and is a felicitous 

 adaptation of a word in ordinary use to express a definite 

 scientific idea. Energy may be defined as the power of 

 doing work. Thus a cannon-ball when it has been fired 

 from a gun possesses energy which is exhibited in its 

 capability of shattering obstacles. The amount of its 

 energy depends partly on its mass ; a ball weighing twice 

 as much as another has an energy twice as great if the 

 two move with the same speed. But the energy is not 

 simply proportional to the velocity. A ball moving at 

 twice the rate of another is capable of piercing four times 

 as many thin plates as the slower ball, that is, its energy 

 depends on the square of its velocity. If the speed be- 

 comes four times as great, the energj' is increased sixteen- 

 fold. The recoil of a gun is a familiar instance of equaUty 

 of action and reaction. The gun is driven backwards at 

 the same time that the bullet is projected forwards, and 

 the mass of the gun multiplied by its velocity of recoil is 

 equal to the mass of the shot multiplied by its velocity in 

 the opposite direction. But the bullet is capable of exert- 

 ing a very diflerent effect from that of the gun-stock, 

 owing to its rapid motion. There is another form of 

 energy due to position, like that of a head of water in a 

 mill-pond, which is capable of doing work by turning the 

 wheel during its fall to a lower level. A wound-up watch 

 spring, a bent bow, wind, compressed air, heat, electric 

 currents, are examples of energy associated with matter. 



The energy which a body has on account of its position 

 is called " potential energy," that which it possesses due 

 to its motion is named " kinetic energy." An example of 

 the change of potential energy into the kinetic form, and 

 of kinetic energy back again into potential energy, is given 

 by the pendulum. A\'hen it is at the highest point of its 

 swing it is for a moment at rest, and its energy is entirely 

 potential. By the action of gravity upon it, it is caused 

 to descend till it is at the lowest point of its path, when 

 it can fall no further and is then mo^-ing most rapidly — 

 its energy is at this point entirely kinetic. Its energy of 

 motion carries it through this point, and is changed into 

 potential energy again at the other extremity of its course ; 

 the energy the pendulum possesses at points intermediate 

 between its highest and lowest positions being partly of 

 one kind and partly of the other. 



