proves that it does exist but only in a slight degree. The real aroma bacteria, which 

 develop by the side of the quercite colonies and correspond with these in all 

 other respects, do not possess the power of producing pyrogallol from quercite, hence, 

 though belonging to the same species, they represent other varieties. The quercite 

 bacterium might thus be named P. aromatica var. quercito-pyrogallica. That P. aro- 

 matica is so easily distinguished as a species, makes it in this case possible to in- 

 dicate a character by means of which forms found in nature and seemingly alike, 

 may be recognised as belonging to different varieties. The oxidation function here, 

 thus proves to be very variable, being present or lacking in closely allied forms 

 which are themselves constant and differ only in this quality. 



Another character by which the natural varieties of P. aromatica are mutually 

 distinct, consists in their very unequal power of liquefying gelatin, this power being 

 intense in some and quite absent in other varieties, with all intermediate degrees. 

 The same is to be observed in the quercite bacteria; hence the variability of this pro- 

 perty is in some degree a property of the whole group. 



All varieties, apparently without exception, produce in glucose bouillon about 

 3 cm. 3 N acid per 100 cm. 3 liquid. For growth, oxidation and acid formation, pep- 

 tones are wanted as source of nitrogen, ammonium salts and nitrates can hardly serve 

 as such and only in pure cultures, but by no means in free competition with other 

 microbes. 



Although aromatic milk contains a great many quercite bacteria, its flora chiefly 

 consists of other varieties of P. aromatica, but the following experiment, based on 

 the principle: slow rising of the concentration of a good nutrient medium apt to 

 produce a slight acidification, makes it possible almost exclusively to obtain the 

 quercite bacteria. 



Large glass beakers are filled with i L. of distilled water and therein are floated 

 a few small dialysators of parchment paper, manufactured by Schleicher and 

 S c h ii 1 1, of the shape of experiment tubes, each filled with about 15 cm. 3 of extract 

 of greenmalt. This extract is prepared by rubbing two parts of greenmalt with 

 three parts of water in a mortar and filtrating after some hours', digeration at room 

 temperature. The clear solution contains relatively little maltose and is of course 

 extremely fit for bacterial growth, where, likewise as in milk, lactic acid ferments 

 are able to develop, but only little lactic acid can be formed on account of the low 

 rate of sugar. Kept in a room where the temperature varied from 15 to 20 C. the 

 spontaneously corrupted infusions in the beakers produced at repeated experiments, 

 made in December 1910 and January 1911, so great an excess of quercite bacteria, 

 that other species could hardly be found in the black mass, obtained by streak 

 inoculations on brothagar quercite plates. 



If instead of submitting the malt extract to dialysis, different quantities of the 

 extract itself were directly added to the water, then, with for the rest like conditions, 

 much less quercite bacteria developed. 



The aroma formed in the malt extract is of the same nature as that found in 

 aromatic milk. 



Other bacteria but the above named, producing a pigment from quercite, have 

 not been found, neither by experiments with non-sporulating forms at higher tempera- 

 tures nor among the microbes that remain alive in pasteurised materials. 



