ATOM. 477 



ordinary bodies, and it follows that the impact of the corpuscules would raise 

 all bodies to an enormous temperature. We may also observe that according 

 to this theory the habitable universe, which we are accustomed to regard as 

 the scene of a magnificent illustration of the conservation of energy as the 

 fundamental principle of all nature, is in reality maintained in working order 

 only by an enormous expenditure of external power, which would be nothing 

 less than ruinous if the supply were drawn from anywhere else than from the 

 infinitude of space, and which, if the contrivances of the most eminent mathe- 

 maticians should be found in any respect defective, might at any moment tear 

 the whole universe atom from atom. 



We must now leave these speculations about the nature of molecules and 

 the cause of gravitation, and contemplate the material universe as made up of 

 molecules. Every molecule, so far as we know, belongs to one of a definite 

 number of species. The list of chemical elements may be taken as repre- 

 senting the known species which have been examined in the laboratories. 

 Several of these have been discovered by means of the spectroscope, and more 

 may yet remain to be discovered in the same way. The spectroscope has also 

 been applied to analyse the light of the sun, the brighter stars, and some of 

 the nebulae and comets, and has shewn that the character of the light 

 emitted by these bodies is similar in some cases to that emitted by terres- 

 trial molecules, and in others to light from which the molecules have absorbed 

 certain rays. In this way a considerable number of coincidences have been 

 traced between the systems of lines belonging to particular terrestrial sub- 

 stances and corresponding lines in the spectra of the heavenly bodies. 



The value of the evidence furnished by such coincidences may be estimated 

 by considering the degree of accuracy with which one such coincidence may be 

 observed. The interval between the two lines which form Fraunhofer's line D 

 is about the five hundredth part of the interval between B and G on Kirch- 

 hoff's scale. A discordance between the positions of two lines amounting to 

 the tenth part of this interval, that is to say, the five thousandth part of 

 the length of the bright part of the spectrum, would be very perceptible in 

 a spectroscope of moderate power. We may define the power of the spectro- 

 scope to be the number of times which the smallest measurable interval is 

 contained in the length of the visible spectrum. Let us denote this by p. In 

 the case we have supposed p will be about 5000. 



If the spectrum of the sun contains n lines of a certain degree of inten- 



