ATOMS AND MOLECULES S5 



the hot molecules in the sun that causes the ether 

 waves which we know as light and heat ; and we 

 get some idea of the rapidity of the throbbing 

 when we remember that the ether-ripples beat on 

 the eye at the rate of from 450 to 750 million 

 millions per second. 



A very interesting fact may be mentioned in 

 connection with this. It has been found that 

 r molecules of different elements throb in different 

 ways, and thus produce different light waves, which, 

 when analysed by a spectroscope (an instrument 

 that spreads out and separates waves of light on 

 the principle of a prism), give definite characteristic 

 colours. ^ 



Each substance has its own " spectrum," or alterna- 

 tions of light and shade ; and "s^ can discover what 

 any substance is simply by heating it so as to agitate 

 its molecules, and then analysing, by a spectro- 

 scope, the ripples of light caused by the throbbing 

 molecules. In this way we may be said to be 

 able to tell any substance by feeling its pulse^ or by 

 listening to its heart, and the spectroscope may be 

 compared to a stethoscope. But a spectroscope is 

 far more wonderful than a stethoscope ; it is more 

 delicate, and it reaches farther. Sir Henry Roscoe, 

 in his Elementary Chemistry^ states that a portion 

 of sodium salt less than ^§0, 000. 000 P^^^ °^ ^ 

 grain, and that a portion of lithium ^o'ooTo^ P^^^ 



