194 THE BLOOD. 



of nitrite of amyl are added to a solution of haemoglobin. The 

 color of the latter almost instantly becomes brown. On adding 

 reducing agents to solutions so altered, reduced haemoglobin 

 (see 18) appears a fact which seems to square best with the 

 assumption that the action of the nitrites on haemoglobin is to 

 peroxidize it, and that on reduction, oxyhsemoglobin is first 

 formed, then reduced. The precise nature of the reaction is 

 still matter for investigation. 



18. Optical Properties of Haemoglobin. Crystals. 

 The crystals are doubly refractive, i. e., they look luminous 

 when examined with the aid of the polarization microscope (see 

 Part I., Chap. IV.), between crossed Nicols. They shine in 

 sunlight with a lustre compared by Preyer to that of silk. 

 When formed in liquids freely exposed to air or oxygen, they 

 are of the color of arterial blood, but have the wonderful pro- 

 perty of becoming dark without altering their form when placed 

 in vacuo at a low temperature. They then exhibit two colors, 

 looking green along the aretts, purplish-red elsewhere. On the 

 admission of air or oxygen, the color is restored. If a glass 

 plate to which crystals of haemoglobin adhere is placed in front 

 of the slit of the spectroscope, two characteristic absorption 

 bands (Hoppe-Seyler) are seen in the yellow between the Frau- 

 enhofer's lines D and E (see Fig. 195, 1). Solution. The bands 

 just mentioned are also seen when solution of haemoglobin or 

 of blood corpuscles is placed in the same position : they can 

 be distinguished even when the solution contains only one ten- 

 thousandth of its weight of coloring matter. The bands differ, 

 however, in their characters according to the degree of dilu- 

 tion. According to the experiments of Preyer, solutions vary- 

 ing in strength from one to five per 10,000, show both bands 

 faintlj 7 ; in solutions of six per 10,000, it can be distinguished 

 that the band next the line I) is the darker of the two, the other 

 being broader and fainter (see Fig. 195, 5) ; in solutions of 

 thirty per 10,000. the violet end of the spectrum is completely 

 absorbed, and the blue partially. As the concentration is in- 

 creased the two bands approach each other, until finally (when 

 the solution contains seventy per 10,000) they form a single 

 band, while the whole of the more refrangible rays are absorbed, 

 so that the spectrum does not extend beyond the limits of the 

 green (see Fig. 195, 6). 



In 1862 it was discovered by Stokes that hemoglobin exists 

 in the blood in two states of oxidation, which are distinguished 

 alike by color and by the spectroscope; that the oxygenized 

 haemoglobin, or (as it has since been called) oxy haemoglobin, is 

 deprived by reducing agents of its oxygen, and that when it 

 has been so reduced, it can be restored to its original state t>y 

 agitation with air. The nature of the change of color is ex- 

 j)ressed in two facts, which can be observed with the aid of the 



