Ch. VIII] USING THE MICRO-SPECTROSCOPE 263 



rare earths, didymium, etc. These in solutions that give hardly a 

 trace of color to the eye give absorption bands that almost rival the 

 Fraunfoher lines in sharpness. 



§ 419. Absorption spectra of minerals. — As example take some 

 monazite sand on a slide and either mount it in balsam (see Ch. X), 

 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. Absorp- 

 tion 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 condi- 

 tions. It is frequently of great service in determining the char- 

 acter of unknown mineral sands to compare the spectra with known 

 minerals. If the absorption bands are identical, it is strong evi- 

 dence in favor of the identity of the minerals. For proper lighting 

 see § 407. 



§ 420. 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 identical 

 or very closely related. For example, while one might be in doubl be- 

 tween 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 

 sulphide would not enable one to make the distinction. Further- 

 more, it is unsafe to compare objects dissolved in different media. 

 Different objects as "cyanine and aniline blue dissolved in alcohol 

 give a very similar spectrum, but in water a totally different one." 

 "Totally different bodies show absorption bands in exactly the same 



