XXX.] CLASSIFICATION. 713 



" Synopsis tradit Divisiones arhitrarias, longiores aut brevi- 

 orcs, plures aut paiiciores : a Botanicis in geiiere nou 

 aguoscenda. Synopsis est diehotoniia arbitraria, quie 

 inslar vise ad Botanicem ducit. Liniites auteni non de- 

 tenninat." 



The rules and tables drawn out by chemists to facilitat(} 

 the discovery of the nature of a substance in qualitative 

 analysis are usually arranged on the biturcate method, 

 and form excellent examples of diagnostic classification, 

 the qualities of the substances produced in testing being 

 in most cases merely characteristic properties of little im- 

 ])ortance in other respects. The chemist does not detect 

 potassium by reducing it to the state of metallic potas- 

 sium, and then observing whether it has all the principal 

 qualities belonging to potassium. He selects from among 

 the whole number of compounds of potassium that salt, 

 namely the compound of platinum tetra-chloride, and 

 ]>otassium chloride, which has the most distinctive ap- 

 ])('nrance, as it is (xjmnaratively insoluble and produces 

 a peculiar yellow and highly crystalline precipitate. Ac- 

 cordingly, ywtassium is present whenever this precipitate 

 can be produced by adding platinum chloride to a solu- 

 tion. The fine purple or violet colour which potassium 

 salts communicate to the blowpipe tlamo, had long been 

 used as a characteristic mark. Some other elements were 

 readily detected by the colouring of the blowpipe flame, 

 liarium giving a pale yellowish green, and salts of stron- 

 tium a bright red. By the use of the spectroscope the 

 coloured light given off by an incandescent vapour is made 

 to give perfectly characteristic marks of the elements con- 

 tained in the vapour. 



Diagnosis seems to be identical with the process termed 

 by the ancient logicians ahscissio ivjiniti, the cutting olf 

 of the infinite or negative part of a genus when we dis- 

 cover by observation that an object possesses a particular 

 difference. At every step in a bifurcate division, somn 

 objects possessing the difference will I'ail into the adirma- 

 tive part or species ; all the remaining objects in the world 

 fall into the negative part, which will be infinite in extent. 

 Diagnosis consists in the successive rejection from I'uither 

 noticf! of those inlinite clas'scs with which the specimen lU 

 question does nut agree. 



