RESIDUAL PHENOMENA. 201 



of a phenomenon such as would only be obtained in feeble 

 degree with ordinary ones. Our views of chemistry would 

 be somewhat modified, if instead of so frequently operating 

 upon common materials, we more frequently worked with 

 rare ones, and an opening for discovery exists in this 

 direction. 



Some of the greatest of discoveries have originated in 

 the detection of what is termed ' residual phenomena,' or 

 minute fractions of substances and forces unaccounted for. 

 It is clear that as we can neither create nor destroy matter, 

 out of 100 parts of substances only 100 parts can we 

 obtain ; and that if we get only 95 parts, 5 remain to be 

 accounted for. In a similar way, if the doctrine of con- 

 servation of energy be true, from 100 parts of cause we 

 ought to secure 100 parts of effect, and if we obtain 

 only 95, five remain to be found. The 5 parts in these cases 

 constitute what may be called residual substances and resi- 

 dual phenomena. By the progress of scientific discovery and 

 exactitude of research, this residual amount in both cases 

 becomes gradually reduced, first to I'O per cent., then to 

 1 per cent., -01 per cent. ; *001 per cent., or some 

 other diminished proportion ; and at each step we attain a 

 knowledge of new substances and new phenomena, which 

 are usually of a wider and wider character, and more and 

 more remote from common observation, and only become 

 apparent by the gradually increased precision of our know- 

 ledge. Residual phenomena are often so small that their 

 very existence is doubtful ; and, in the above manner, 

 small residual differences are more intrinsically important 

 than large ones. 



A residual effect led to the discovery of the planet 

 Neptune. It was found, both by Adams and Le Verrier, 

 that after having made all allowance for the perturba- 

 tion of Uranus by known bodies near it, a certain, but 



