CHEMICAL RAYS. 247 



denly the beam darts through a luminous white cloud, which 

 has banished the preceding darkness. It has, in fact, shaken 

 asunder the molecules of the nitrite of amyl, and brought 

 down upon itself a shower of liquid particles which causes 

 it to flash forth in your presence like a solid luminous 

 spear. It is worth while to mark how this experiment 

 illustrates the fact that, however intense a luminous beam 

 may be, it remains invisible unless it has something to 

 shine upon. Space, though traversed by the rays from all 

 suns and all stars, is itself unseen. Not even the ether 

 which fills space, and whose motions are the light of the 

 universe, is itself visible. 



You notice that the end of the experimental tube most 

 distant from the lamp is free from cloud. Now the nitrite- 

 of-amyl vapor is there also, but it is unaffected by the 

 powerful beam passing through it. Let us make the trans- 

 mitted beam more concentrated by receiving it on a con- 

 cave silver mirror, and causing it to return by reflection into 

 the tube. It is still powerless. Though a cone of light of 

 extraordinary intensity now traverses the vapor, no pre- 

 cipitation occurs, no trace of cloud is formed. Why ? Be- 

 cause the very small portion of the beam competent to de- 

 compose the vapor is quite exhausted by its work in the 

 frontal portions of the tube. The great body of the light 

 which remains, after this sifting out of the few effectual 

 rays, has no power over the molecules of nitrite of amyl. 

 We have here, strikingly illustrated, what has been already 

 stated regarding the influence of period, as contrasted with 

 that of strength. For the portion of the beam which is here 

 ineffectual has probably more than a million times the ab- 

 solute energy of the effectual portion. It is energy specially 

 related to the atoms that we here need, which specially re- 

 lated energy being possessed by the feeble waves, invests 

 them with their extraordinary power. When the experi- 

 mental tube is reversed so as to bring the undecomposed 



