



91.] Contributions to the Chemical Bacteriology of Sewage. 457 



The still living spores were then further isolated by plate cultivation, 

 ither with or without previous incubation of the broth tube. 



For the purpose of studying the absorptive power for free oxygen, 

 pure cultures were sown in sealed flasks with two necks, containing 

 25 c.c. of nutrient broth and 250 c.c. of air. These were incubated at 

 20 23 C. for seven days, after which time the flasks were opened 

 and the gases remaining abstracted for analysis. It was seen that 

 the various organisms exhibited great differences in their absorptive 

 power for free oxygen, some showing the feeblest absorption, whilst 

 others, abstracted nearly every trace of oxygen from an atmosphere 

 ten times as large as the culture liquid during seven days' incnba- 



-, 



w 



Ihe rate of absorption of dissolved oxygen was also determined for 

 a number of the organisms by sowing tap- water aerated under known 

 inditions, and containing a definite amount of dissolved oxygen, 

 ith 1 per cent, of a pure broth culture of the organism which had been 

 cubated for two days after sowing. It is shown, in the case of 

 ose organisms which absorb oxygen rapidly from the air, that the 

 ater is completely de-aerated in fourteen hours. 

 It is shown that certain organisms which are capable of growing in 

 atmosphere devoid of oxygen, i.e., anaerobic, are yet incapable of 

 iquefying gelatine without the presence of that element, although 

 when grown in air such liquefaction is extremely rapid. 



Cultivations were made in the form of flask referred to for 

 .naerobic organisms, in which the organisms were sown in molten 

 gelatine, through which pure hydrogen was passed for half an hour. 

 The flask was then sealed. After five days' incubation, no liquefaction 

 hatever took place, although, when exposed to air, the normal rapid 

 quefaction of the gelatine afterwards occurred. 

 It is also shown, both in the case of aerobic and anaerobic organ - 

 ns, that a very appreciable diminution of the liquefying power of 

 Tganisms takes place after repeated sub-cultivation in nutrient 

 latine. 



The method employed for photographing the micro-organisms is also 

 escribed. In all cases the bacteria were stained with methyl violet, 

 >ut, as this stain transmits chemically active rays, it was necessary, 

 order to obtain actinic contrast, to use a coloured screen and iso- 

 matic plates. The screen adopted (a weak solution of potassium 

 ichromate) was spectroscopically adjusted to the stain employed, so 

 at the objects appeared black on a bright yellow background. The 

 iparatus employed was of the simplest kind, and the source of illu- 



,tion was a common duplex paraffin lamp. 



The organisms isolated from the sewage under examination are 

 escribed and illustrated photographically, as regards microscopic 

 reparations and plate- and tube-cultures. 



