284 THE ORIGIN OF LIFE 



molecules is maintained, and during this 

 state of things their structure is altered, but 

 as the effect is only pro tempore, chemical 

 change is not manifested and all is reduced to its 

 initial state shortly after the incident light is 

 removed, except in the case of phosphorescent 

 bodies, when the return to the initial state is not 

 rapid, but takes many minutes, or hours, or per- 

 haps days. The molecules are thus during the 

 period of excitation thrown into a new state of 

 molecular grouping or aggregation, from which 

 they gradually recover, when the light is shut 

 off, slowly in the case of phosphorescent bodies, 

 almost instantaneously in the case of fluorescent 

 bodies, although here also it lasts for a fraction of 

 a second. The agglomeration of molecules is 

 of greater or less stability according as the body 

 is phosphorescent or fluorescent, and this period 

 can be increased by fixing the molecules in 

 the case of liquids by means of glycerine or 

 gelatin. 



The addition of glycerine or gelatin to solutions 

 of fluorescent liquids, as we have seen, produces a 

 considerable increase in the intensity of the fluores- 

 cent light and also in its duration. This thickening 

 of the solutions diminishes the loss of energy by 

 collisions between the fluorescent molecules them- 

 selves, since in consequence of the lesser mobility 

 and greater viscosity, the molecules of the same 

 kind impinge much less frequently and therefore 

 exist for a longer time. This concentration, how- 



