Knowledge. 



With which is incorporated Hardwicke's Science Gossip, and the Illustrated Scientific News. 



A Monthly Record of Science. 



Conducted by Wilfred Mark Webb, F.L.S., and E. S. Grew, M.A. 



DECEMBER, 1913. 



STELLAR SPECTROSCOPY FOR BEGINNERS. II. 



By PROFESSOR A. W. BICKERTON, A.R.S.M. 



Sound and Light. 



We speak to one another by means of the vibrations 

 of air ; the stars tell us their story by the move- 

 ments of a medium we call ether, which is perhaps 

 best conceived of as a highly elastic jelly. We 

 cannot see the air, but we can feel it, and when it 

 moves swiftly enough it may knock us down or 

 hurl us over a house. We can neither see nor feel 

 the ether, yet able men tell us we have more proof 

 of the existence of ether than of matter, while some 

 think matter is ether in motion. Air and other 

 matter are made up of little moving particles of 

 extreme minuteness called " atoms." 



Atoms. 



A popular way of putting the size of atoms is to 

 say that there are more atoms in a drop of water 

 than drops of water in the Atlantic Ocean. 



Perhaps the best way of realising some of the 

 properties of atoms is by means of a little instrument 

 Sir William Crookes made and called the sphin- 

 thariscope. By means of this pretty toy one can 

 see the effect of swift atoms of helium hitting a 

 surface of sulphide of zinc, each blow producing a 

 flash of light. The point of a needle is coated with 

 a little radium and placed in front of the zinc 

 surface, which thus becomes luminous. Solid radium 

 is for ever breaking up into other elements and 

 becoming a heavy and a light gas, and it takes two 

 thousand years for radium to half become gas. In 

 thus changing, each heavy atom of radium loses a 

 light atom of helium and becomes the atom of 

 another heavy element called " niton," or " radium 

 emanation," which is a gas. The element helium is 

 extremely important in stellar spectroscopy, and we 



require to know a lot about it ; at present we want 

 to form an idea of the minuteness of the atom. 

 The zinc surface, with the radium-coated needle in 

 front, is put at the back of a little brass box with a 

 small convex lens in front. This instrument is 

 called a spinthariscope or scintilliscope. On looking 

 through the lens in a perfectly dark room, each 

 individual atom of helium as it strikes the sensitive 

 surface makes a flash of light. One sees hundreds 

 of such flashes each second. In some spinthari- 

 scopes this has been going on for years, and will do 

 so for thousands of years more without much 

 apparent diminution of the number of escaping 

 atoms of helium. Heat or cold makes no apparent 

 difference in the speed of escape of the helium 

 atoms, and no physical or chemical treatment seems 

 either to hurry up or in any way change the rate at 

 which these atoms — the so-called " alpha particles ' 

 — leave the radium. Such, then, is the minuteness 

 of the atom. 



An atom, although so small, is of wondrous com- 

 plexity, but for purposes of spectrum analysis we 

 had best think of them as little spherical bells flying 

 about in all directions, ringing for a little time after 

 they hit one another, and, like bells, ringing true to 

 tone. Like other bells also they alter the character 

 of the note according as to how hard and how often 

 they are struck. Heat makes atoms move faster, 

 and so when a gas is heated the particles hit harder, 

 and more overtones are produced. There is a 

 wonderful complexity in these atomic overtones ; the 

 atom of iron gives off thousands ; some atoms seem 

 to be much less complex. Many atoms seem to be 

 like compound bells, as though made up of many 

 bells of different tones fixed together. 



441 



