400 THE PRINCIPLES OF SCIENCE. 



slum, 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 whenever this precipitate can be produced by 

 adding platinum chloride to a solution potassium is pre- 

 sent. The fine purple or violet colour which potassium 

 salts usually 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 strontium a bright red. By the use of the 

 spectroscope the coloured light given off by any incan- 

 descent vapour is made to give perfectly characteristic 

 marks of the elements contained 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 classification when we 

 discover by observation that an object possesses a par- 

 ticular property. At every step in a bifurcate division, 

 some objects possessing the difference will fall into the 

 affirmative 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 almost infinite classes with 

 which the specimen in question does not agree. 



Index Classifications. 



Under the general subject of classification we may 

 certainly include all arrangements of objects or names, 

 which we make for the purpose of saving labour in the- 



