HEMOGLOBIN CRYSTALS OF RODENTS' BLOOD. 197 



the reverse experiment already related, in which crystals simu- 

 lating hexagons were obtained by mixing together the blood of 

 the rat and guinea-pig, and in which the same result was 

 obtained from a mixture of the solutions of the pure haemo- 

 globin of the same animals. 



6. Conclusions and Remarks. 



What the difference between the various forms of haemo- 

 globin may be, it cannot be a very deep or essential one. The 

 difference in crystalline form is associated with a difference of 

 solubility in water and other reagents ; but the spectroscopic 

 characters, the decomposition products, the compounds it forms, 

 of which haemin is a readily obtained example, are universally 

 the same. Not only so, but Hoppe-Seyler has shown 1 that in 

 various animals dried haemoglobin has the same or nearly the 

 same elementary composition. 



Have we then to deal w r ith a case of polymorphism? The 

 terms dimorphism and polymorphism cannot be applied to any 

 substance which crystallises in two or more forms, unless the 

 composition of that substance be exactly the same in all cases. 

 Instances of dimorphism in the mineral world are carbon and 

 sulphur among the elements, and sal ammoniac, potassium 

 iodide, cuprous oxide, &c, among compounds. The conditions 

 on which dimorphism depend are two: first, temperature, 

 secondly, the solvent from which the substance crystallises. 

 If, as in the case of many miueral salts, the compounds are 

 united with different proportions of water of crystallization, we 

 have to deal with different hydrates, and the case is not one of 

 true dimorphism; an instance of this is sulphate of soda. 



The case seems to me to narrow itself down to this in the 

 case of haemoglobin ; either we have here a case of poly- 

 morphism, or the crystalline forms are due to the combina- 

 tion with varying proportions of water of crystallisation. In 

 the absence of a rational formula for haemoglobin it would 

 be unsafe to affirm the former of these two alternatives. More- 

 over, the conditions that are known to produce dimorphism in 

 1 'Physiologische Chemie,' p. 377. 



