ARTIFICIAL NATURE OF OUR CLASSIFICATIONS. 205 



the truths separately enumerated, and thus enables us 

 to think of and remember as one instance an unlimited 

 number of similar facts at once. The number of scientific 

 truths already known is so vast that, without the aid of 

 classification, each person could only be able to master a 

 mere fraction of the knowledge which he is now enabled 

 to acquire. Memory acts largely by association ; if know- 

 ledge is classified and systematized, one idea, by being 

 intelligibly linked to another, is more effectually held fast 

 in the mind. The merest glance at a system of truths 

 in the memory would recall into conscious perception all 

 its details, as the remembrance of a leaf recalls the pano- 

 ramic idea of all the parts of a tree. In this way the 

 limits of classification control those of our power of 

 acquiring a knowledge of science. 



But we must remember that most of our systems of 

 classification are artificial, and without distinct lines of 

 demarcation. Being based upon limited knowledge, they 

 have been formed upon apparent rather than upon real 

 similarities and differences ; and they are to our minds 

 but artificial aids, like crutches to cripples. In conse- 

 quence of their conventional nature, nearly the whole of 

 them have been broken down and swept away by the 

 development of more extensive and accurate knowledge. 

 The division between solids and liquids was destroyed by 

 the discovery of the facts that some bodies were semi- 

 fluid, and that various solid substances pass through every 

 intermediate degree of fluidity, from solid to liquid, by 

 gradual rise of temperature. That between liquids and 

 gases was obliterated chiefly by the discovery of the con- 

 tinuity of the liquid and gaseous states, and that between 

 metals and non-metallic elementary substances, by the 

 discovery of selenium, tellurium, boron, silicon, arsenic, 

 osmium, &c. The division between conductors and non- 



