1910] on Light Reactions at Low Temperatures 925 



concentrated beam from the arc condensed in the liqnid by means 

 of a quartz lens D. After an hour's exposure ozone could easily be 

 recognized. 



The exposure at liquid hydrogen temperatures is somewhat too 

 dangerous to carry out on the lecture table, but solid oxygen in 

 liquid hydrogen contained in a quartz vacuum tube has also been 

 subjected to the ultra-violet radiation. When the liquid hydrogen 

 evaporated away, ozone came off from the liquefied oxygen during 

 its evaporation, so that the production of ozone was still possible 

 even at 20° A. from the impact of ultra-violet rays on solid oxygen. 



Experiments on the action of ultra-violet light on phosphorescent bac- 

 teria at low temperatures. — The Photohacterum phosphorescens is also 

 known as Photolacterium phosphor escens gelidus. It is the most widely 

 distributed and ])est known of all photogenic bacteria. It occurs on 

 the bodies of nearly all dead fish and is most easily obtained from 

 dead herring or mackerel. The light emitted by this Bacillus is of a 

 brilliant green colour which provides an easy means of identification. 

 The culture-medium must contain certain inorganic salts characteristic 

 of sea-water. A good artificial culture-medium has the following 

 composition : — 



1 litre of beef broth peptone gelatine. 



Sodic chloride 26 "5 grammes. 



Potassic chloride 0*75 ,, 



Magnesia chloride 3*25 „ 



The microscopic appearance of these bacteria at different ages, 

 and also when grown in different culture media, are shown in 

 Plate III. Several cultures of these organisms, kindly prepared by 

 Mr. Henry Crookes, are on the table. At the temperature of the 

 room they are very bright. This large shallow tin vessel contains 

 a good bright culture, and on to it some liquid air can be poured, 

 which rapidly destroys the luminosity of the bacteria. The organisms 

 are not killed, for on waving the tin in the air, thus allowing it 

 to warm up, the brightness again appears. While the organisms 

 are at Hquid air temperature they can be exposed to the action of 

 ultra-violet light. For this purpose a smaller culture in a tin dish is 

 floated on liquid air by the arrangement shown in Plate II. fig. i. 

 A is a tin or aluminium dish about 10 cm. wide and 2 cm. high, 

 in which is placed the smaller dish containing the culture. This 

 arrangement is supported by a cork float on the liquid air contained 

 in a glass silvered hemispherical vacuum vessel B. On B, a shield C 

 rests, consisting simply of two poHshed sheets of tinned iron, fixed at 

 a small distance apart, which protects the liquid air in B from the 

 heat radiation. C is pierced by a central hole rather wider than A, 

 in which a quartz dish D, aboat 1 cm. high, is placed, filled with 

 water to act as a heat filter. This arrangement can be placed either 

 under the arc or the mercury lamp, whichever is being employed. 



Five minutes' exposure at a distance of a few inches from the 

 mercury lamp is usually sufficient to kill the bacteria when they are 



