xxx.] CLASSIFICATION. 713 



" Synopsis tradit Divisiones arbitrarias, longiores aut brevi- 

 ores, plures aut pauciores : a Botanicis in genere non 

 agnoscenda. Synopsis est dichotomia arbitraria, quge 

 instar viae ad Botanicem ducit. Limites autem non de- 

 terminat." 



The rules and tables drawn out by chemists to facilitate 

 the discovery of the nature of a substance in qualitative 

 analysis are usually arranged on the bifurcate 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- 

 portance 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 

 potassium chloride, which has the most distinctive ap- 

 pearance, as it is comparatively insoluble and produces 

 a peculiar yellow and highly crystalline precipitate. Ac- 

 cordingly, potassium 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 flame, had long been 

 used as a characteristic mark. Some other elements were 

 readily detected by the colouring of the blowpipe flame, 

 barium 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 abscissio infiniti, the cutting off 

 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, some 

 objects possessing the difference will fall into the affirma- 

 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 further 

 notice of those infinite classes with which the specimen in 

 question does not agree. 



