196 



KNOWLEDGE. 



[October 2, 1898. 



for equilibrium to be maintained, the number leaving the 

 liquid, and therefore the number of dissolved molecules, 

 must change in this same proportion — i.e., the quantity of 

 gas dissolved is proportional to the pressure, which is 

 known experimentally to be the case. 



An interesting example of the behaviour of gaseous 

 molecules is furnished by the radiometer invented by Mr. 

 Crookes. This consists of a set of light fans or vanes 

 made of aluminium, and mounted so as to be able to rotate 

 easily, the whole being enclosed iu a globe of glass, in 

 which the air is highly rarefied. One side of each vline is 

 of polished aluminium, the other is covered with lamp- 

 black. When the arrangement is exposed to radiation, 

 the lamp-black surface absorbs more heat, and is thus at 

 a higher temperature than the bright surface of the vane. 

 Therefore, the particles of gas, when they strike the black 

 surface, rebound with a higher velocity, and produce a 

 greater reaction than when they meet the bright and cooler 

 surface. In this way the metal vanes are caused to revolve, 

 the bright polished faces foremost, and when the radiation 

 on them is strong they spin rapidly round. The gas 

 surrounding the vanes must be rarefied, and not at the 

 ordinary pressure, in order that the time when a particle 

 is on its free path may be great compared with that taken 

 up by collisions. The gaseous molecules move backwards 

 and forwards between the glass sides of the globe and the 

 vanes, rebounding at a relatively low velocity from the glass, 

 which is transparent to the rays falling upon it, and is 

 not heated. 



In some remarkable experiments in 1822 Cagniard de la 

 Tour heated various liquids, such as alcohol and ether, in 

 strong glass tubes in which the vapour formed was confined 

 within a limited space and was not allowed to occupy more 

 than from two to five times its oi-iginal volume. Thus the 

 contents of the tube existed under a high pressure, and as 

 the temperature was raised at a certain point the liquid 

 seemed suddenly to become completely changed into 

 vapour. 



In an extended series of experiments on carbonic acid 

 gas, Dr. Andrews showed that above a temperature of 

 30-92^0. no amount of pressure was able to liquefy it. At 

 high pressures above this temperature the substance may 

 be considered to be in an intermediate condition, and to be 

 neither a Uquid nor a gas. The temperature above which 

 a gas cannot be liquefied by means of pressure is called the 

 critical temperature of the gas. 



A gas can continuously be changed into a liquid, without 

 any abrupt alteration in its state, by compressing it while 

 above its critical temperature to the volume it would 

 occupy when liquid ; then on lowering the temperature 

 below the critical point, while maintaining the pressure, 

 the substance is found to be in the liquid state. 



From these phenomena Dr. Andrews concluded that the 

 liquid and gaseous states are merely widely separated forms 

 of the same condition of matter. 



There is a mutual attraction between the molecules of a 

 gas, which acts so as to assist the external pressure to 

 which the gas is exposed. As the volume of the gas 

 decreases, the molecules may come so near together that 

 their attraction towards each other may hold them together 

 or vibrating about a common centre ; then the kinetic 

 energy of the molecules will not separate them, and no 

 external pressure is necessary to maintain the volume the 

 same. The gas has become a liquid. When the gas is 

 above the critical temperature, the kinetic energy of the 

 molecules is so great that no diminution of volume will 

 enable the attraction of neighbouring molecules to link 

 them together and so balance the pressure due to the 

 energy of their motion ; the gas cannot then be liquefied. 



THE EARTH IN SPACE. 



By William Schooling, F.R.A.S. 



THERE is a curious fascination in putting side by 

 side the myth and science of astronomy. The 

 old legends of the sun and moon, of earth and 

 sky, of heaven and the stars, tell us of the self- 

 same objects whose place and size, whose weight 

 and nature astronomers are chronicling to-day. 



The difi^erence is great indeed between the guesses of 

 early times and the methods of modern science ; nowhere 

 else, perhaps, is the contrast seen so well between the 

 infancy and the maturity of the mind of man, and no part 

 of astronomy shows it so clearly as that which tells of 

 the earth's place in the universe. 



To the Greeks eight centuries before Christ the earth was 

 flat, surrounded by the sea, and covered by the canopy of 

 sky, which is the floor of heaven, the abode of the Olympian 

 gods. 



Greece was at the centre of the earth, and Delphi at the 

 central point of Greece, for Jupiter sent forth two eagles 

 from the east and west that met upon the spot where 

 stood the Delphian temple, thus proved to be the 

 centre of the world. The sun and stars climb up the 

 crystal dome of sky, and daily descend again through the 

 water into the shadow land beneath — the realm of forget- 

 fnlness and the region of death. The Ethiopians in the 

 east and west were closer to the sun than were the Greeks, 

 and by its nearness, as it rose and set, the Ethiopians were 

 scorched. 



The sun plunged in the ocean as it set, and the Greeks 

 record the hissing heard by the Iberians at sunset when 

 the burning sun fell into the sea. Tacitus tells the same 

 story of our northern ancestors, and in Polynesia the 

 notion lingers still that the natives of the western isles 

 have heard the hissing of the sunset ; while in Peru, the 

 legend was that the sun rushed to the sea and by its heat 

 dried up the greater part, then dived below the earth tha.t 

 was upon the water, and daily rose again by the gateways 

 of the east. 



These seem curious fancies to us who have learnt that 

 the volume of the sun is more than a million times greater 

 than the earth ; who know that though the ocean stretches 

 for thousands of miles, yet the sun is so remote that while a 

 flash of light could pass more than seven times round the 

 earth in a single second, it would take eight minutes to reach 

 us from the sun ; who know also that if, not once a day, but 

 once a minute for fifteen centuries, the whole sea could be 

 poured into an abyss the size of the sun, it would not fill 

 it up. Yet the thought is natural that the ocean stretches 

 so far that the sun must fall into it when it sets, and is so 

 vast that it can easily contain the sun. Had we no 

 teachers to tell us of their own or other men's discoveries, 

 there are not many of ourselves but would be satisfied 

 with thinking of things as they appear at first sight to be, 

 and the heavens would appear to us, as to others, near at 

 hand, and readily accessible. To us, as to the Greeks, the 

 rainbow would stretch down from heaven a sign of war and 

 tempest, or the pathway of a messenger from gods to 

 men; just as to the South Sea Islander it is the heaven- 

 ladder that the heroes climb. To the Scandinavians it 

 was Bifnist, the trembling bridge, timbered of three hues, 

 and stretched from sky to earth. In German folk-lore the 

 souls of the just are led by guardian angels along the 

 rainbow into paradise, and it is the bridge of souls with 

 Arabs and Chinese. 



P)Ut not alone by the rainbow do we pass to heaven : a 

 legend from Samoa tells of a mountain i-eaching to the sky, 

 and of a tree that when it fell was sixty miles in length, 



