186 THE FORCE INCLUDED IN PLANTS. [MEMOIR XI. 



ber of vibrations a method now universally adopted in 

 spectrum analysis. 



By other experiments a narrative of which would be 

 too long for the present occasion I established this 

 result: that for any ray to produce a chemical effect, it 

 must be absorbed. For instance, when a ray has passed 

 through a mixture of chlorine and hydrogen gases, and 

 by causing them to unite has produced hydrochloric acid, 

 it can no longer produce the same effect if made to pass 

 through a second portion of the same mixture ; its act- 

 ing part has been detained or absorbed by the first. So, 

 too, the radiations which have fallen on a daguerreotype 

 plate, and impressed their image upon it, have lost the 

 quality of producing a similar effect on a second plate 

 that may be placed to receive them. Their active por- 

 tion has been taken up or absorbed by the first. The es- 

 sential preliminary of all chemical changes ly radiations 

 is absorption. 



But it must not be supposed that the rays thus ab- 

 sorbed are annihilated or lost. They are simply held in 

 reserve, ready to be surrendered again, undiminished and 

 unimpaired, if the conditions under which they were ab- 

 sorbed are reversed. They may appear under some other 

 form as heat, electricity, motion but their absolute 

 energy remains unchanged. This is a necessary conse- 

 quence of the theory of the Conservation and Correla- 

 tion of Force. 



From this point of view how interesting is that great 

 discovery made by Angstrom, that an ignited gas emits 

 the same rays it absorbs a discovery that explained 

 the Fraunhofer lines of the solar spectrum, and consti- 

 tuted an epoch in the history of spectrum analysis. 



I have now presented the facts that are requisite for 

 answering the question proposed on one of the foregoing 



