CHEMICAL RAYS. 239 



This caused me a momentary anxiety, for I did not know 

 how far, in describing my previous experiments, actions 

 might have been ascribed to pure cloudless vapor, which 

 were really due to those newly-observed nebulas. Inter- 

 mittent discomfort, however, is the normal feeling of the 

 investigator ; for it drives him to closer scrutiny, to greater 

 accuracy, and often, as a consequence, to new discovery. 

 It was soon found that the nebulae revealed by the beam 

 were also generated by the beam, and the observation 

 opened a new door into that region inaccessible to sense, 

 which embraces so much of the intellectual life of the phys- 

 ical investigator. 



"What are those vapors of which we have been speak- 

 ing ? They are aggregates of molecules^ or small masses of 

 matter, and every molecule is itself an aggregate of smaller 

 parts called atoms. A molecule of aqueous vapor, for ex- 

 ample, consists of two atoms of hydrogen and one of oxy- 

 gen. A molecule of ammonia consists of three atoms of 

 hydrogen, and one of nitrogen, and so of other substances. 

 Thus the molecules, themselves inconceivably small, are 

 made up of distinct parts still smaller. When, therefore, 

 a compound vapor is spoken of, the corresponding mental 

 image is an aggregate of molecules separated from each 

 other, though still exceedingly near, each of these being 

 composed of a group of atoms still nearer to each other. 

 So much for the matter which enters into our conception of 

 a vapor. 1 To this must now be added the idea of motion. 

 The molecules have motions of their own as wholes y their 

 constituent atoms have also motions of their own, which 

 are executed independently of those of the molecules ; just 



1 Newton seemed to consider that the molecules might be rendered 

 visible by microscopes ; but of the atoms he appears to have entertained 

 a different opinion. He finely remarks : "It seems impossible to see 

 the more secret and noble works of Nature within the corpuscles, by 

 reason of their transparency." (Herschel, On Light, Art. 1145.) 



