926 Sir James Dewar [Jan. 21, 



located on the surface layer of the culture medium. The exposed 

 culture is taken out and allowed to warm up in the air. No bright- 

 ness supervenes even after the culture is kept for several hours. 

 A few colonies will occasionally develop on keeping for a day, but 

 this is because the growth of the bacteria has extended down- 

 wards into the medium, and are thus protected from the radiations, 

 since a thin layer of the culture medium is sufficient to absorb the 

 ultra-violet. If a metal plate, out of which a cross has been cut, or 

 perforated metal is placed on the surface of the frozen bacteria 

 before exposure to light, the appearance of the surface of the bacteria 

 when allowed to heat up to the ordinary temperature is shown in 

 Plate IV. fig. II. 



There are sources of deception possible in this work, unless proper 

 precautions are taken. If the dish containing the culture is floated 

 direct on the liquid air, and especially if it be heavy, causing the 

 level of the culture to be below that of the exterior liquid air, it fre- 

 quently happens that a thin layer of liquid oxygen makes its appearance 

 on the surface of the culture, either by condensation at the lower 

 level, or by capillary action, or even spraying. In this case we have 

 the complications produced by the absorption of the ultra-violet light 

 by the thin oxygen layer, and the possible action of the ozone pro- 

 duced. In many cases surprising results were obtained, in which the 

 organisms seemed to remain alive even after long exposures. In 

 addition to this there was the possibility of condensation on the cold 

 culture of both moisture and oxides of nitrogen, the latter always 

 present in the air round such a mercury lamp, although while the 

 culture is in the liquid air, the atmosphere over it is for the most 

 part kept clear of contamination by the evaporating oxygen and 

 nitrogen. Another arrangement was used in order to allow the 

 ultra-violet radiation to act on the bacteria in an atmosphere of 

 hydrogen or pure nitrogen, thus eliminating any of the secondary 

 reactions mentioned above. Plate II. fig. ii. shows such an an'ange- 

 ment. A simple thin sheet metal box M is fitted with a quartz 

 cover N, which slides tightly into it. The dish A, containing the 

 culture, is placed in M, into which also a narrow lead pipe P opens. 

 The whole an-angement can be supported by P in any position in 

 the vessel of liquid air. Through P hydrogen or nitrogen can be 

 passed, the excess escaping round the joint of M and N. Adopting 

 such precautions made no difference in the efficieucj of the ultra- 

 violet in killing oft* the bacteria at low temperatures. 



The luminosity of these bacteria may remain latent for consider- 

 able periods. Oxygen is necessary for their activity, but they 

 will remain alive, although dark, if air be excluded for several 

 weeks, although they then need a little time to develop their activity. 

 The curves of Plate lY. fig. i. are interesting, and show the rate of 

 growth of two identical colonies in air and a vacuous flask respec- 

 tively. The ordinates represent the diameter of the colonies in milli- 

 metres. The abscissae show the age in days. Curve No. (ii) 



