68 OUTLINES OF BACTERIOLOGY 



influence the flavour of the beverage treated. Again, the effect of 

 electricity has been tried on the chromogenic power of bacteria. Bac. 

 pyocyaneus, which produced a blue pigment, was placed in the cavity 

 of a solenoid, and subjected to a current of 10,000 volts. An exposure 

 of twenty minutes almost completely destroyed the chromogenic 

 power of this bacillus. 



Magnetism. In all experiments in which bacteria have been placed 

 in a magnetic field only negative results have been obtained. It 

 must, however, be stated that the question has not been sufficiently 

 investigated. 



§ 3. LIGHT. 



The importance of light as a germicidal agent is an important point 

 to note in the cultivation of bacteria, as the vast majority not only 

 thrive better in the dark, but if exposed to the light, and especially 

 to strong sunlight, are very soon killed off. This was proved as 

 early as 1877, when it was found that the direct rays of the sun 

 were prejudicial to the growth of bacteria. Diffuse daylight is also 

 inimical, though less so than the direct rays. At this time it was 

 also proved that the blue and violet rays of the spectrum were the 

 most active destroyers, and next to them the red and orange-red 

 rays. The question of the germicidal power of the various rays of 

 the spectrum has recently been again investigated, and it is now 

 known that from red to green the effect is almost nil, from red to 

 the violet end of the blue the effect increases, reaching its maximum 

 at the latter point, and beyond it in the ultra-violet rays there is a 

 falling off in effect. The rays of the electric light are also germicidal, 

 but less so than those of direct sunlight. The effect of light seems 

 to be independent of temperature, for Tyndall found that even on 

 the Alps the growth of organisms which he had taken up enclosed 

 in a flask, was influenced by exposure to sunlight. 



As to the exact method by which the light destroys bacteria all 

 observers are not agreed, but it is held by some that light produces 

 decomposition products in the nutrient media, which act inimically on 

 the bacteria. By others it is held that light acts directly on the 

 organisms apart from its effect on the surrounding medium. It is, 

 nowadays, generally supposed that in nature both factors are instru- 

 mental in accomplishing the same end ; thus light acting on urine 

 produces hydrogen peroxide, which is an antiseptic, and therefore 

 kills off the bacteria that are feeding on the urine. In the majority 



