176 CRYSTALLOGRAPHY OF THE HEMOGLOBINS 



shown on plate 17, fig. 99, with the orthodome (101) as the plane of twinning and the com- 

 position face (text figure 64). This form was rather common. From an examination 

 of such twins it would seem that this orthodome makes an angle of exactly 90 with the 

 base. From this, if /? were accurately determined, the value of 6 could be easily calcu- 

 lated. Taking /? at 48 the value of 6 becomes 2.6685. 



Pleochroism is very marked, a pale yellowish-red, b deep red, c very deep red. 

 The plane of the optic axes is the plane of symmetry; the orientation of the elasticity 

 axes is a A a = 17 in the obtuse angle; b=&; c A 6 = 25 in the obtuse angle. On the 

 flat the extinction is symmetrical; on edge view, looking along b, the extinction angle 

 is 17 from the edge 001-010 or from the trace of 001. On the flat the interference figure 

 is seen, somewhat unsymmetrically arranged, and the acute bisectrix, Bx a =c. The 

 optical character is hence positive. 



y hemoglobin of Didelphis virginiana. 



Hexagonal or pseudo-hexagonal. No axial ratio determinable. 



Forms observed: Prism (1010), base (0001). 



Angles: Prism angle 1010 A 0110=60 (120), prism to base 1010 A 0001=90. 



Habit tabular, thick or thin plates consisting of prism and base, with great develop- 

 ment of the base (text figure 65). The /?-oxyhemoglobin crystals appear after the a-oxy- 

 hemoglobin crystals, and the appearance of the former is accompanied by the resolution 

 and disappearance of the latter. They form first in the protein ring, but later may appear 

 anywhere in the slide. Apparently the solution of the a-oxyhemoglobin is due to the 

 action of bacteria, but frequently the /?-oxyhemoglobin crystals appear growing on the 

 a-oxyhemoglobin crystals as regular growth and with the a-crystal unaffected. The 

 orientation of the regular growth appears to be such that the edges of the plate of the 

 a-crystal are approximately normal to the edges of the /3-crystal in some cases, and par- 

 allel to them in others. Etching figures are seen, elongated normal to the edges of the 

 /3-oxyhemoglobin plates. Some of them appear to be composite crystals (see plate 17, 

 fig. 98), as in the mica twins that are nearly uniaxial, and yet on edge the layers of the 

 twin are so thin, or so intergrown, that polarized light does not seem to show any trace 

 of composite character. No regular twins of these crystals occur, but parallel growth 

 is common. 



The color of these /3-oxyhemoglobin crystals is brighter red, more scarlet than the 

 color of the a-oxyhemoglobin crystals, but this is evidently due to the fact that they 

 show little or no pleochroism; and the spectroscope does not show any difference from 

 the normal oxyhemoglobin spectrum. Pleochroism is not noticeable on the basal view, 

 and there is practically no pleochroism or absorption on the side or edge view, looking 

 normal to the vertical axis. There is no double refraction that can be detected on the 

 basal view; on edge view the double refraction is easily seen and the extinction is straight. 

 The vertical axis is the axis of least elasticity, e > w, and the crystal is positive. On the 

 base, in convergent, light, the uniaxial cross is readily seen; in some cases the crystal 

 is slightly biaxial, as in the nearly uniaxial micas, and the axis of least elasticity is normal 

 to a prism face. The biaxial crystals are also distinctly positive. The separation of the 

 brushes is only very slight, the angle 2E is very small. 



While these crystals are seen in all sizes, and do not appear to be composite, there 

 can be little doubt that they are really mimetic hexagonal only and are twins of the 

 a-oxyhcmoglobin on the base in one of the two forms of twinning that have been described 

 under the a-oxyhemoglobin. If the twin laminae were thin enough, the polarization test 

 would not show the composite character and this would be especially true if, as is usually 

 the case in these twins, the same layer did not run as a plane entirely across the basal 

 surface. In looking through from side to side the different orientation of the layers 

 would hence average, and neutralize each other. Of course, this averaging would happen 

 on the flat view to a still greater degree, and the elasticity axes a and b in different orien- 

 tation would completely extinguish each other, making a uniaxial effect. This may be 

 done artificially with only three plates of mica, twinned as these a-oxyhemoglobin crystals 



