CH. VI] MICRO-SPECTROSCOPE AND POLARISCOPE 149 



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. 



§216. Absorption Spectra of Minerals. — As example take some 

 monazite sand on a slide and either mount it in balsam (see § 256), 

 or cover and add a drop of water. The examination maj^ be made also 

 with the dry sand, but it is less satisfactory. Light well with trans- 

 mitted 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 determining 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 § 204. 



§ 217. While the study of absorption spectra gives one a great 

 deal of accurate information, great caution must be exercised in draw- 

 ing conclusions as to the identity or even the close relationship 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 ident- 

 ical 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 carmine, although the ammonium sul- 

 phide would not enable one to make the distinction. Furthermore it 

 is unsafe to compare objects dissolved in different media. The same 

 objects as "cyanine and aniline blue dissolved in alcohol give a very 

 similar spectrum, but in water a totally different one." "Totally dif- 

 ferent bodies show absorption bands in exactly the same position (solid 

 nitrate of uranium and permanganate of potash in the blue)." (Mac- 

 Munnj. The rule given by MacMunn is a good one : "The recogni- 

 tion of a body becomes more certain if its spectrum consists of several 

 absorption bands, but even the coincidence of these bands with those 

 of another body is not sufficient to enable us to infer chemical identity; 



