NATURE OF THE "ESSENTIAL SUBSTANCE^' 95 



The fact that our strain of the bovine tubercle 

 bacillus, when grown on Dorset's egg medium, appears 

 to contain no essential substance does not prove that 

 no essential substance is formed, but that no reserve is 

 formed that can be extracted; in other words, that only 

 just sufficient is formed for the immediate require- 

 ments of the bacillus; and the same is probably true 

 in the case of B. phlei grown on glycerine-liver broth. 

 Therefore in most of these experiments that we have 

 classed as negative a small quantity of the essential 

 substance may have been formed for the immediate 

 use of the bacillus. 



Our experiments also showed that certain strains of 

 the human tubercle bacillus seemed to form but little 

 reserve essential substance when first isolated from 

 the body on Dorset's Qgg medium ; and we thought 

 that the difficulty experienced in isolating this bacillus 

 from certain body fluids, such as pleural fluids, even 

 though their presence can be demonstrated by the 

 inoculation of a guinea-pig, may be due to a temporary 

 inability of the bacillus to form the essential substance. 

 To test this we tried to isolate the bacillus from these 

 fluids on media containing the essential substance, and 

 we tested tubercular pus from cows in the same way. 

 Our experiments were limited in number, but they 

 certainly indicated that a small quantity of the essential 

 substance (or of bacilli containing the essential sub- 

 stance) improves a medium used for isolating these 

 bacilli. 



On reviewing the experiments so far carried out, it 

 is obvious not only that the essential substance is 

 soluble in water and present in the alcoholic extract 

 of B. phleiy only in a very small quantity, but also 

 that the medium on which the bacillus is grown plays 

 an important part in determining the production or 



