CH. VI] MICRO-SPECTROSCOPE AND POLARISCOPE 171 



bands, the light is simply less brilliant in certain regions and the 

 red rays so predominate that they give the prevailing color. 



§ 237. Nearly Colorless Bodies with Clearly Marked 

 Absorption Spectra. — In contradistinction to the brightly colored 

 objects with no distinct absorption bands are those nearly colorless 

 bodies and solutions which give as sharply defined absorption bands 

 as could be desired. The .best examples of this are afforded by 

 solutions of the rare earths, didymium, etc. These in solutions 

 that give hardly a trace of color to the eye give absorption bands 

 that almost rival the Fraunhofer lines in sharpness. 



§ 238. Absorption Spectra of Minerals. — As example take 

 some monazite sand on a slide and either mount it in balsam (see 

 Ch. IX), or cover and add a drop of water. The examination may 

 be made also with the dry sand, but it is less satisfactory. Light 

 well with transmitted light, and move the preparation slowly 

 around. Absorption bands will appear occasionally. Swing the 

 prism tube off the ocular, open the slit and. focus the sand. Get the 

 image of one or more grains directly in the slit, then narrow and 

 shorten the slit so that no light can reach the spectroscope that has 

 not traversed the grain of sand. The spectrum will be satisfactory 

 under such conditions. It is frequently of great service in deter- 

 mining the character of unknown mineral sands to compare the 

 spectra with known minerals. If the absorption bands are identical, 

 it is strong evidence in favor of the identity of the minerals. For 

 proper lighting see § 226. 



§ 239. While the study of absorption spectra gives one a 

 great deal of accurate information, great caution must be exercised 

 in drawing conclusions as to the identity or even the close relation- 

 ship of bodies giving approximately the same absorption spectra. 

 The rule followed by the best workers is to have a known body as 

 control and to treat the unknown body and known body with the 

 same reagents, and to dissolve them in the same medium. If all 

 the reactions are identical then the presumption is strong that the 

 bodies are identical or very closely related. For example, while 

 one might be in doubt between a solution of oxy- or CO-hemoglobin 

 and carmine, the addition of ammonium sulphide serves to change 

 the double to a single band in the O-hemoglobin, and glacial acetic 

 acid enables one to distinguish between the CO-blood and the car- 



