NATURAL PHILOSOPHY. 213 



subordinate part in qualitative analysis. This the author believed to have 

 arisen from the fact, that chemists have been content with observing: the 



O 



color as it appears to the unaided eye. The color of any object, however, is 

 the resultant of the various rays of the spectrum which it reflects or trans- 

 mits ; and by examining such objects with a prism strange peculiarities are 

 frequently made manifest among substances of similar color, and strange 

 analogies are often detected among substances which appear very different in 

 color when their light is not thus analyzed. 



Various methods of using the prism were described ; that preferred from 

 its easy application and great delicacy, was to view the light entering by a 

 slit in the window shutters by means of a prism, the liquid to be examined 

 being interposed in a wedge-shaped glass vessel, held in such a position that 

 the line of light is seen through the varying thicknesses of the solution. In 

 this way spectra of very characteristic configuration are obtained, all the 

 rays being usually permitted to pass through the thinnest stratum of the 

 liquid, but as the stratum gradually increases some of these rays are entire- 

 Iv absorbed, others are rendered faint in color, while others are transmitted 



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with almost undiminished brilliancy. These spectra may be easily copied ; 

 indeed, the paper was illustrated by a number of diagrams done in colored 

 crayons on black paper. 



It has been partially recognized hitherto that all the compounds of a par- 

 ticular base or acid have the same effect on the rays of light. Thus, the 

 various salts of nickel are green, those of zinc colorless. But the prism con- 

 firms the truth of this generalization in a very remarkable manner, and re- 

 veals not, indeed, an identity, but a distinct analogy in cases which were be- 

 fore thought to offer an exception. Many of these apparently exceptional 

 instances were investigated. Thus there are two modifications of chromium 

 salts in solution, the green and the blue, while other salts of this base always 

 appear red ; yet when these solutions are examined by the prism they all ex- 

 hibit the same configuration of spectrum, the maxima of luminosity being 

 always in the extreme red, and about the junction of the green and blue 

 marked by the fixed line F ; the differences of apparent color depend on 

 slight variations in the relative quantity of these rays that are transmitted. 

 Observations were likewise made on the blue and red salts of cobalt, and 

 the green and blue salts of copper, and it was found that these changes de- 

 pended solely on the quantity of water, while there were certain analogies 

 even between the spectra afforded by these differently colored combinations 

 of the same metal. Ferric salts all transmit the red ray with great facility ; 

 they were divided into several groups, the members of each group possessing 

 other analogies likewise. All soluble chromates, though differing considci*- 

 ably to the unaided eye, give almost the same prismatic appearance. The 

 changes that take place in a solution of litmus by the addition of an alkali, 

 boracic acid, or a common acid were likewise examined, and the spectra 

 transmitted in the different cases were shown to belong to the same type. 

 When two substances combine, each of which imparts a particular color to 

 its combinations, the resultant is not the compound of the two colors, but is 



