216 HAEMOGLOBIN. 



appearance in the fluid. 1 A similar production and formation of 

 crystals is frequently observed when crystals of oxy-haemoglobin 

 are sealed up with Canada balsam under a cover-slip and kept for 

 some time. 2 The form of the crystals obtained from the blood of 

 different animals has not yet been fully investigated. They ex- 

 hibit to a marked degree the phenomena of pleochroism, being ap- 

 parently trichromatic. 3 



Pleochroism is that property possessed by many crystals of appear- 

 ing to differ more or less in colour, in accordance with the direction 

 from which they are viewed by transmitted light. The phenomena 

 are usually investigated by means of a single Nicol prism. For 

 further details consult some special work on mineralogy or the article on 

 this subject in the " Encyclopaedia Britannica," Vol. xvi. p. 375. 



As ordinarily seen the crystals of haemoglobin have a dark red 

 appearance, unlike the bright scarlet of oxy-haemoglobin, with a 

 strong purple or bluish tint They are extremely soluble in water, 

 much moie_aQ than the crystals of oxy-haemoglobin. The optical 

 properties of solutions of haemoglobin have already been suffi- 

 ciently described ( 346, and see below Fig. 36, No. 5). One of 

 the most remarkable properties of haemoglobin is its power of 

 uniting directly with any one of several gases, such as oxygen, 

 carbon monoxide, nitric oxide and, as recent research has shown, 

 possibly carbon dioxide ; the compounds which are thus formed 

 have in the case of the first three gases a definite and constant 

 composition, crystallising more or less readily in characteristic 

 forms and snowing in aqueous solutions absorption spectra which 

 are constant and characteristic for each. (See below.) 



The chemical composition of haemoglobin does nqt as vet admit 

 of being represented by any definite formula, and indeed its per- 

 centage composition has not been determined by direct analysis. 

 It must be inferred from a knowledge of the probable composi- 

 tion of the more stable and easily crystallisable oxy-haemoglobin 

 and of the quantitative relationships whicR hold good between 

 haemoglobin and oxygen during its conversion into oxy-haemo- 

 globin. As will be seen later on, analysis of purified crystals of 

 oxy-haemoglobin shows that these probably differ in composition 

 as prepared from the blood of different animals, and the same 

 statement therefore probably holds good for haemoglobin. When 

 decomposed in the absence of oxygen (air), as for instance by~The 

 action_of organic acids,'niore dilute mineral acids, orjasat of all by 

 caustic alkalis, it ;yields ajjroteid, of which but little is known 

 (see p. 32), and a coloured substance called by Hoppe-Seyler 

 haemochromogen. The latter on exposure to air absorbs oxygen 



1 Hiifner, Zt. f. physiol. Chem. Bd. iv. (1880), S. 382. Cf. Nencki u. Sieber, 

 Ber. d. d. cfiem. Gesell. Bd. xix. (1886), Sn. 129,410. 



2 A. Ewald, Zt.f. Biol. Bd. xxn. (1886), S. 459. 



3 A. Ewald, loc. cit. 



