732 THE PRINCIPLES OF SCIENCE. [chap. 



sounds. A simple sound undulation, if we could meet, 

 with such a sound, would admit of precise and exhaustive 

 classification as regards pitch, the length of wave, or the 

 number of waves reaching the ear per second being a suf- 

 ficient criterion. But almost all ordinary sounds, even 

 those of musical instruments, consist of complex aggregates 

 of undulations of different pitches, and in order to classify 

 the sound we should have to measure the intensities of 

 each of the constituent sounds, a work which has been 

 partially accomplished by Helmholtz, as regards the vowel 

 sounds. The different tones of voice distinctive of different 

 individuals must also be due to the intermixture of minute 

 waves of various pitch, which are yet quite beyond the 

 range of experimental investigation. We cannot, then, at 

 present attempt to classify the different kinds or timbres of 

 sound. 



The difficulties of classification are still greater when a 

 varying phenomenon cannot be shown to be a mixture of 

 simpler phenomena. If we attempt to classify tastes, we 

 may rudely group them according as they are sweet, bitter, 

 saline, alkaline, acid, astringent or fiery ; but it is evident 

 that these groups are bounded by no sharp lines of defini- 

 tion. Tastes of mixed or intermediate character may exist 

 almost ad infivitum, and what is still more troublesome, 

 the tastes clearly united within one class may differ more 

 or less from each other, without our being able to arrange 

 them in subordinate genera and species. The same remarks 

 riiay be made concerning the classification of odours, which 

 may be roughly grouped according to the arrangement of 

 Linnffius as, aromatic, fragrant, ambrosiac, alliaceous, fetid, 

 virulent, nauseous. Within each of these vague classes, 

 liowever, there would be infinite shades of variety, and 

 each class would graduate into other classes. The odours 

 which can be discriminated by an acute nose are infinite ; 

 every rock, stone, plant, or animal has some slight smell, 

 and it is well known that dogs, or even blind men, can 

 discriminate persons by a slight distinctive odour which 

 usually passes unnoticed. 



Similar remarks may be made concerning the feelings 

 of the human mind, called emotions. We know what is 

 anger, grief, fear, hatred, love ; and many systems for 

 classifying these feelings have been proposed. They may 



