500 SPECIAL METHODS OF DISCOVERT. 



mentary solid bodies diminish in electric conductivity 

 by rise of temperature. These instances should be further 

 investigated. 



Exceptions would probably be found to nearly every 

 general statement in science, if all the instances were 

 known ; and in any research already published, if it includes 

 only a small proportion of the possible instances which 

 might have been examined, it is highly probable that by 

 extending the investigation to all the additional cases some 

 exceptional ones might be found, and a new and wider 

 conclusion be thus discovered. There appear, however, 

 to be some statements to which there are no known 

 exceptions ; for instance, the united bulk of several gases 

 or liquids when mixed is never greater than when they 

 are separate. Such statements are usually important 

 ones. 



e. By examining extreme cases and conspicuous 

 instances. By experiments with very highly rarefied 

 gases and extremely dilute solutions many valuable truths 

 have been discovered, and our knowledge of the molecular 

 constitution of bodies has been much extended, because 

 the molecules are then more isolated from each other's 

 influence and from other disturbing causes. The pheno- 

 mena of electric discharge and optic spectra in rarefied 

 gases, rotation of bodies in rarefied media by means of 

 heat, Graham's discovery of osmose, &c., are instances o: 

 this kind. 



Conspicuous instances also are extremely valuable as 

 sources of new discoveries, because they enable us to 

 examine not only the more prominent effects and features 

 of a phenomenon in all their phases, but also the effects o 

 the more recondite and feeble influences ; and those are 

 often the most important, because they disclose the mosi 

 general laws. It is probably because of the excessively 



