156 HEAT. 



the constituents before combination. As a heated gas, which is emitting 

 a line spectrum, is compressed the character of the light sent out is 

 found gradually to change. On examining the spectrum it is found to 

 consist at first of a greater or less number of bright lines. As com- 

 pression goes on these first widen out into bands, and then gradually a 

 continuous spectrum appears, showing that light of every refrangibility 

 is being sent out. This is explained by the gradual shortening of the 

 free path. The molecules interfere more and more with each other, 

 their time for uninterrupted natural vibration becomes less and less, 

 while the times of clashing and general disturbance in collisions become 

 greater and greater, and this general disturbance gives rise to the 

 continuous spectrum. If, instead of a gas, we take a glowing liquid or 

 solid, we cease to have any appearance of bright lines or bands. There 

 is merely a continuous spectrum, showing, apparently, that the molecules 

 interfere with each other too much to allow the natural vibrations to 

 have play. We might, perhaps, give an illustration of this. If a 

 number of bells are hung in a room by strings from a ceiling, and are set 

 swinging, they will occasionally collide, and the energy of translation 

 will partly be converted into energy of vibration. If the bells are far 

 apart the notes given out will be the natural notes of the bells, for the 

 free paths will be long and the time of free vibration long, compared 

 with the time of clashing. But the closer the bells are together the 

 more prominent will the clashing be. If they are packed loosely in a 

 box and rattled about the natural vibrations will be quite overpowered 

 by the clashing. 



Recent researches on the emission of radiation by gases appear to 

 show that each gas only emits a line spectrum when chemical change is 

 proceeding.* Thus sodium vapour when raised to a high temperature 

 in a neutral gas, one with which it does not combine, does not give the 

 characteristic D lines, but probably a continuous spectrum. But if it 

 combines with the surrounding gas the D lines appear. This would 

 appear to show that the waves corresponding to the D lines are not 

 emitted by the molecules as a whole, but by the parts which are 

 changing places in the chemical actions proceeding. At present, how- 

 ever, our hypotheses as to the nature of these actions are hardly definite 

 enough to warrant us in giving an account of the origin of the radiations. 



* An account of our present knowledge will be found in the Reports of the 

 Congres International de Physique, 1900, vol. ii., p. 100, by E. Pringsheim. 



