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KNOWLEDGE. 



December, 1913. 



Helium appears to be a very complex atom ; in fact 

 it seems to be an inseparable double atom, each part 

 being groups of bells. In it, however, one 

 characteristic tone rings out so clearly that midst the 

 complexity we can recognise helium by a line of a 

 pure yellow tint. 



Colour. 



The vibrations of a sheep bell we call sound ; 

 those produced by the atoms we call colour ; so it is 

 by colour the stars tell us their story. In the 

 spectrum the pure tints of singing atoms stand 

 apart, each in its proper place and so form the 

 cipher messages that reveal the mystery of the 

 stellar heavens. These ciphers are being gradually 

 deciphered and read by our industrious observers. 



But " when the mind is blind the eye does not 

 see." What are wanted are guiding hypotheses, and 

 on all sides workers are awakening to this need. 

 Now their want is felt, the keys to open the doors of 

 the mind to see the richness of the corridors of space 

 and the many mysteries of Creation will surely be 

 forthcoming. 



Some of these cosmic keys were found a long time 

 ago, but have only just begun to be used ; yet already 

 they have opened up many rich realms of Nature. 



Vibrations. 



Sound and light are both examples of propagation 

 of vibrations by wave motions. One gains a good 

 idea of wave motion by watching a field of ripening 

 wheat when moved by the wind. The waves travel 

 the length of the field, yet one knows the individual 

 heads remain each on its own stalk ; each con- 

 stituent of the wave only oscillates. Ear after ear 

 moves in succession, and so the motion looks to be 

 one of translation. 



If a stone be thrown into the middle of a still 

 lake, the disturbance starts a series of waves that 

 travel until they reach the margin of the water. Yet 

 although the waves travel outwards, a cork on the 

 troubled waters merely moves up and down. 



The speed of the wave depends on the elasticity 

 and density of the medium ; elasticity increases and 

 density diminishes speed. Elasticity in the air is 

 perhaps best understood by resistance to compres- 

 sion. Make the air in a football hot, and it resists 

 compression more, and is more elastic. The reason 

 is that the particles of air fly faster. The velocity 

 of a sound wave in a gas depends on the speed of 

 the moving molecules of the gas. Thus, at the 

 same temperature, hydrogen particles move four 

 times as fast as oxygen particles. So sound travels 

 four times as fast in hydrogen as it does in oxygen. 



A tuning fork strikes a gas particle as a bat does 

 a ball, and it bounces away a minute degree quicker 

 than it came. It hits its next particle more quickly 

 than normal, and so a wave is produced. Sound 

 waves are often many feet in length and travel one 

 thousand one hundred feet a second. Light waves 

 are many thousands to the inch, and travel one 

 hundred and eighty-six thousand miles a second. 



A tuning fork may only move one hundredth of 

 an inch. Yet it produces waves of sound that travel 

 thousands of times as fast as its own motion, and 

 the speeds of these waves differ whether they be 

 transmitted by gas, liquid or solid ; for example, if 

 they travel in air, in water, or in glass. 



The curious thing is that the wave travels fastest 

 of all in the densest of these three substances. Not, 

 however, because it is dense — density reduces the 

 speed of progression — but because it is more elastic; 

 it recovers from compression more energetically. 

 How are we to explain these phenomena? We may 

 do so as suggested by imagining the tuning fork to 

 be a bat, hitting swift molecular balls thrown by a 

 demon bowler. The bat causes the balls to bound 

 back quicker than they come. In the case of liquids 

 and solids potential energy of molecular and cohesive 

 attraction has been converted into molecular kinetic 

 energy, so the molecules oscillate quicker than in 

 gas. Molecular and cohesive attraction make the 

 liquid and solid more elastic. 



Light. 



A beam of common white light is one of the most 

 complex things in existence. The waves of sunlight 

 are of many lengths and move in many planes. In 

 this respect the motion of sunlight is more complex 

 than the motion of the air when stirred by an 

 orchestra. 



The vibrating particles of air, as the waves come 

 from a musical instrument, move in the line of 

 propagation towards and away from us ; not so 

 light. In the ether waves the motion is across 

 the direction from which the light is coming. As 

 the waves enter the eye the motion may be 

 horizontal, or vertical, or in any plane. In common 

 light they move in all planes— in all azimuths, as 

 astronomers say. When they are sorted out so as to 

 move in a single plane we say the light is polarised. 

 An instrument called the polariscope is used to so 

 sort the waves of light. A sound wave cannot be 

 polarised ; its sides do not differ. In plane-polarised 

 common white light the ether moves in one plane 

 only, but its waves are of many lengths. It is the 

 work of the spectroscope to sort the waves into their 

 different lengths. Light consisting of such separate 

 waves moving in one plane is said to be pure mono- 

 chromatic polarised light. A ray of polarised light 

 of any pure tint is the simplest conception we can 

 have of any beam of light. We picture it best by 

 imagining a tightly stretched skein of silk being 

 jerked, and watching the waves travel along the 

 length of the silk. Let us imagine an atom of 

 sodium to be vibrating, and picture the successive 

 motions in the ether as moved by the yellow beam 

 of light after it has passed through a polariscope. 

 Some transparent crystals have a structure like a 

 gridiron with very close bars. These minerals only 

 allow the vibrations of light to pass in one plane. 

 Such a crystal is a polariscope ; there are many more 

 effective kinds of polariscopes, but the action of 

 these crystals is easy to imagine. 



