DIPHTHERIA. 307 



vacuo over sulphuric acid at a temperature of 25 C., a sub- 

 stance is left which is soluble in water, and possesses marked 

 toxic properties. It is insoluble in strong alcohol, and may 

 be precipitated by it from a watery solution in greyish white 

 flakes. It passes slowly through a dialyzing membrane. 

 The addition of lime water and a solution of phosphoric 

 acid to the filtered culture liquid causes an entangling 

 precipitation of the poison, phosphate of lime appearing to 

 hold it more tenaciously than any other substance. The 

 filtered fluid thus treated loses its toxicity, whilst the gela- 

 tinous precipitate inoculated into an animal kills with the 

 utmost certainty, though perhaps on account of the slightly 

 insoluble nature of the substance formed, somewhat more 

 slowly. To form some idea of the virulence of the poison 

 produced by these diphtheritic organisms, Roux and Yersin's 

 calculations that I cc. of the active liquid evaporated 

 in vacuo leaves I centigramme of dried residue, may be 

 accepted. Deducting from this the weight of the ash and 

 the portion soluble in alcohol which has no toxic action, 

 there remain four-tenths of a millegramme of organic 

 material, of which only a small proportion can be diph- 

 theritic poison ; even this quantity, however, is sufficient to 

 kill eight guinea-pigs, two rabbits, or one medium-sized dog. 

 If the latter does not succumb to the poison it remains ill 

 for some time. Like snake-bite poison, however (which 

 Waddell has shown to be weakened on being exposed to 

 peptic digestion, a change that is ascribed to the breaking 

 down of the albumose), it may be taken into the stomach 

 in much larger quantities without giving rise to any very 

 serious effect. This poison, when injected into the veins 

 or into the subcutaneous tissue, appears to act specially 

 on the walls of the blood vessels, giving rise to vascular 

 dilatations, minute haemorrhages, and the small cedematous 

 patches so characteristic of certain forms of the disease. 

 It can scarcely be too strongly insisted on that the activity of 

 a poison formed by a micro-organism is not at all the same 

 thing as virulence, which must be defined, according to 

 Roux and Yersin, as the power that a pathogenic organism 

 possesses of continuing to live and carry on its functions in 

 the tissues of the animal or human host. For example, one 

 might take a young culture of the diphtheria bacillus in which 

 the bacilli are vigorous, but in which the quantity of poison 



