THE (ESTRUS-PRODUCIXG HORMONE 89 



were rarely obtained purer than -i mgm. per mouse unit. 

 Since the weight of the mouse unit can now be reduced to -ooi 

 mgm. and even lower, at least 99^0 of the solid matter of these 

 fat extracts must have been inert material. Probably the 

 figure is at least 99-9 °o- This fact quite invalidates such chemi- 

 cal analyses as those of Fraenkel and Fonda (218), Hartmann 

 (280), and Hartmann and Isler (281), regarding the nature of the 

 hormone itself. Even now it is doubtful whether the positive 

 properties of active extracts give any information about the 

 hormone, and even negative reactions, which might give 

 information, may be greatly influenced by the nature and 

 amount of the impurities. To obtain 5 gms. of material of the 

 present maximum purity for chemical work would involve the 

 preparation of fifty million mouse units, equivalent to 50-100 

 tons of ovaries, placentae, or urine of pregnancy. Furthermore, 

 it should be emphasized that until methods of quantitative 

 estimation of activity become much more accurate, it is im- 

 possible to say whether or not the hormone shows complete 

 resistance to such procedures as saponification. 



Solubility. There can be little doubt that the active substance 

 is readily soluble in fat solvents such as alcohol, ether and 

 acetone, benzene, chloroform, etc. In petroleum ether, however, 

 the solubility is comparatively low, as shown by Ralls, Jordan 

 and Doisy (524), and Glimm and Wadehn (246), who have used 

 this characteristic in the purification of the hormone. 



Its essential solubility in organic fat solvents has been 

 emphasized by many workers, including Herrmann, Fellner, and 

 Allen and Doisy, but it has been shown that under certain 

 conditions the hormone can be secured in an aqueous solution. 

 A number of workers (see p. 87) have obtained perfectly clear 

 aqueous preparations with a high activity. These are probably 

 true solutions rather than colloidal suspensions, because all these 

 workers have reported that the active material can be dialysed, 

 and Zondek and Brahn report the absence of a Tyndall pheno- 

 menon. Slotta (570) has pointed out, however, that the solu- 

 bility must be very slight, not more than -005 °o. In any case, 

 it is far less soluble in water than in fat solvents, because it can 

 be re-extracted from aqueous preparations by ether. 



Thermostabilitv. The active substance is resistant to 



