130 ATOMS AND SUNBEAMS. 



At tbe ordinary teiii])eratiire of the air, liydrojieii is of course iuvisi- 

 ble; this iiieaus that the vibrations in the interior of the molecules are 

 uot sufficiently vehement to im})art pulses to tliciether with the energy 

 that would be required to produce visual effects. Now, let us su[)pose 

 that the hydrogen is heated. The effect of heating is to imi^art addi- 

 tional speed to the molecules of the gas, and consequently when the 

 molecules happen to come together their encounter is more violent. 

 The effect of such an occurrence on one of these little elastic bodies is 

 to set it quivering with greater vehemence in those particular modes 

 of vibration for which it is tuned. If the temperature of the gas has 

 been raised sufficiently higli, as it can be by the aid of electricity, 

 then the internal energy acquired by the molecules, in consequence of 

 the increased vehemence of their collisions, has become so great that 

 they are able to !mi)art pulses to the a'ther with sufficient intensity to 

 affect our nerves of vision; thereui)on we declare that the hydrogen is 

 now so hot as to ha v«' become luminous. Suppose we employ a spectro- 

 scope for the i)urpose of studying the particular character of the light 

 which the glowing hydrogen dispenses. It will appear that the spec- 

 trum consists of a definite number of bright lines. We know that each 

 one of these lines corresponds to a i>articulnr period of vibration of the 

 tether, and hence we see that the light emitted by the hydrogen does 

 not consist of vibrations of all periods indiscriminately, but onh' of cer- 

 tain particular waves which are in unison with the oscillations to which 

 the internal parts of the molecule of hydrogen are adopted. Had we 

 examined the spectrum of some other gas in a state of iiu-andesceuce 

 Ave should have found a wholly different system of hues from those per- 

 taining to hydrogen. This demonstrates that the molecules of one gas 

 differ essentially from those of another in respect to the character of 

 the internal vibrations which they are adajited to perform. The extra- 

 ordinary activity of the movements which take place within the mole- 

 cules may be appreciated from the following facts. We know tliat the 

 wave corresponding to one of the hydrogen lines has a length of about 

 the forty-thousanth of an inch; we also know that in a single second of 

 time light travels over a space of 186,000 iniles; a simple calculation 

 will, therefore, assure us that certain vibrations in the molecules of 

 hydrogen corresponding to this particular undulation must take place 

 with such an extraordinary fiequency that about 4()0 millions of millions 

 of them are performed in each second of time. 



Provided with these conceptions we shall now, I think, be able to 

 see without difficulty how it is that the sun's heat is sustained. We 

 may, for our present purpose, think of the great luminary as a mass 

 of glowing gas. It is quite true that the physical condition of the 

 matter in the interior of the tremendous globe can hardly be that 

 which we ordinarily consider as gaseous. But this need not affect our 

 argument. It is undoubtedly true that those portions of the solar 

 atmosphere from which the light and heat are mainly dispensed are 



