394 BLOOD AND LYMPH. 



cases also as to the form that the crystals take. In man and in most 

 of the mammalia hemoglobin is deposited in the form of rhombic 

 prisms; in the guinea pig it crystallizes in tetrahedra (d, Fig. 168), 

 and in the squirrel in hexagonal plates. This difference in crystal- 

 line form implies some difference in molecular structure, and taken 

 together with other known variations in property shown by hemo- 

 globin from different animals lead us to believe that the huge mole- 

 cule has a labile structure, and that it may differ somewhat in its 

 molecular composition or atomic arrangement without losing its 

 physiological property of an oxygen-carrier. In this connection 

 it is interesting to state that the hemoglobin of horses' blood, which 

 crystallizes ordinarily in large rhombic prisms, may be made to give 

 hexagonal crystals by allowing it to undergo putrefaction, and that 

 the form of the crystals may then be changed from hexagons to 

 rhombs by varying the temperature of the solutions.* The crystals 

 are readily soluble in water, and by repeated crystallization the 

 hemoglobin may be obtained perfectly pure. As- in the case of 

 other soluble proteid-like bodies, solutions of hemoglobin are 

 precipitated by alcohol, by mineral acids, by salts of the heavy 

 metals, by boiling, etc. Notwithstanding the fact that hemoglobin 

 crystallizes so readily, it is not easily dialyzable, behaving in this 

 respect like proteids and other colloidal bodies. The compounds 

 which hemoglobin forms with carbon monoxid (CO) and nitric oxid 

 (NO) are also crystallizable, the crystals being isomorphous with 

 those of oxyhemoglobin. 



Absorption Spectra. Solutions of hemoglobin and its deriv- 

 ative compounds, when examined with a spectroscope, give 

 distinctive absorption bands. 



Light, when made to pass through a glass prism, is broken up into its 

 constituent rays, giving the play of rainbow colors known as the spectrum. 

 A spectroscope is an apparatus for producing and observing a spectrum. A 

 simple form, which illustrates sufficiently well the construction of the appara- 

 tus, is shown in Fig. 169, P being the glass prism giving the spectrum. Light 

 falls upon this prism through the tube (A) to the left, known as the "colli- 

 mator tube." A slit at the end of this tube (S) admits a narrow slice of light 

 lamplight or sunlight which then, by means of a convex lens at the other 

 end of the tube, is made to fall upon the prism (P) with its rays parallel. In 

 passing through the prism the rays are dispersed by unequal refraction, giving 

 a spectrum. The spectrum thus produced is examined by the observer with 

 the aid of the telescope (B). When the telescope is properly focused for the 

 rays entering it from the prism (P), a clear picture of the spectrum is seen. 

 The length of the spectrum will depend upon the nature and the number of 

 the prisms through which the light is made to pass. For ordinary purposes a 

 short spectrum is preferable for hemoglobin bands, and a spectroscope with one 

 prism is generally used. If the source of light is a lamp flame of some kind, 

 the spectrum is continuous, the colors gradually merging one into another 

 from red to violet. If sunlight is used, the spectrum will be crossed by a 

 number of narrow dark lines known as the "Fraunhofer lines." The position 

 of these lines in the solar spectrum is fixed, and the more distinct ones are 



* Uhlik, "Archiv f. d. gesammte Physiologie," 104, 64, 1904. 



