ACTION OF LIGHT ON MICRO-ORGANISMS 373 



It is to be regretted that Buchner did not employ 

 one variety of organism only at the various depths, as 

 by not doing so he has introduced another factor, it 

 being quite possible that each variety of organism is 

 endowed with individual powers of resistance to insola- 

 tion. The experiments show, however, very clearly 

 that the sun's rays only exert a bactericidal action in 

 the upper layers of water, and we know that bacteria are 

 present in large numbers at depths very considerably 

 below that within the reach of insolation, so that sun- 

 shine must exert only a superficial effect in the bacterial 

 purification of water. 



Buchner includes in his paper some investigations 

 carried out towards the end of September 1892 by 

 Minck and Neumayer on the bacterial condition of the 

 river Isar during the day and night respectively. These 

 experiments were made to find out if the number of 

 microbes in the river water during the night, and there- 

 fore in the absence of the deleterious influence of light, 

 was greater than in the daytime. The spot selected 

 was 10 km. above Munich, and the gelatine plates were 

 prepared immediately after the collection of the samples. 

 The following table gives the results of their examina- 

 tions : 



Bacterial Condition of the Eiver Isar in the Day and Night 

 Eespectively (Minck and Neumayer) 



Time of Taking Sample Number of Organisms on 1 c.c. of Water 



6^ evening ...... 160 



8f 5 



11 

 12 



If morning ...... 380 



8 

 107 



460 

 520 

 510 

 250 



It is obvious that an investigation of this kind must 



