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CHAPTER II. 



THEOEY. 



IT will be easily seen, by examples that have been 

 brought forward in preceding chapters, how diffi- 

 cult and how ineffectual have been the attempts 

 to give a satisfactory explanation of phospho- 

 rescence. The opinions on this subject published by 

 Dessaignes and Becquerel certainly merit the most 

 consideration. The theory to which they lead may 

 be seen condensed in the following paragraph : 

 "It is perfectly demonstrated, at the present 

 day, that an evolution of electricity takes place 

 in bodies whenever the equilibrium of their mole- 

 cules undergoes a change of any sort, either in 

 their chemical constitution or in their physical ag- 

 gregation. If these molecules are not separated 

 thereby, we observe a recomposition, more or 

 less rapid, of the two electricities that, for an 

 instant, were put in liberty ; and this may deter- 

 mine, according to the nature of the body and 

 the tension of the electricity, a production of 



