September 6, 1894] 



NA TURE 



45: 



material, inasmuch as a marked decrease was also 

 observed in those samples kept in the dark. To meet this 

 objection, in the next series unsterile water was used, and 

 to a litre and a half as much as l c.c. of the broth-culture 

 of the particular organism was added, thus affording 

 ample provision, both in light and darkness, for the 

 support of the bacteria under observation. Instead of a 

 decrease taking place in the samples kept in the dark, 

 the numbers rose ; on the other hand, in the samples 

 placed in the sunshine, three hours' exposure in the case 

 of the typhoid, colon, and pyocyaneus bacilli brought 

 about their entire destruction, thus placing beyond doubt 

 the direct bactericidal action which had taken place 

 during insolation. 



The amount of water used being small, no indication 

 was given, in these experiments, of the depth to which 

 the bactericidal action of the sun's rays could extend. 

 Fol and Saracen (" Sur la penetration de la lumicre du 

 jour dans les eaux du lac de Geneve," Comptes Reiidus, 

 1884) have shown by the exposure of gelatine-bromide 

 plates that daylight penetrates to a depth of 170 metres 

 in the water of the Lake of Geneva, the degree of light 

 at this depth being about equal to that which we find 

 during a bright but moonless night, whilst at a depth of 1 20 

 metres the strength of light is still considerable. These 

 investigators also made the curious observation that in 

 the experiments they conducted, the light penetrated 

 far deeper into the water in September, during cloudy 

 weather, than in the month of August with a perfectly 

 clear sky. Thus not only does the power of light vary 

 at different depths and, doubtless, in different waters, but 

 it is also influenced by the time of year ; and what, 

 therefore, may be correct of a given water under certain 

 circumstances, may not necessarily apply to it on another 

 occasion, and hence a good deal of uncertainty attaches 

 to the exact degree of light capable of transmission in 

 any particular mass of water. 



Prof. Buchner has endeavoured to ascertain at what 

 depth in the water of the Starnberger Lake, near Munich, 

 light ceases to have any bactericidal action. For this 

 purpose he used his well-known process (described in 

 the Ccntrathlatt fiir Baktcriologie, vol. xii. August 1892; 

 of exposing partially protected agar-agar dish cultures. 

 This ingenious method consists in covering over parts 

 of a glass dish containing agar-agar, in which certain 

 varieties of bacteria have been evenly distributed, with 

 variously-shaped strips of black paper or lead, so that 

 the light is screened from these particular portions of 

 the surface. In this manner the bacteria immediately 

 beneath the covered part of the culture-medium are pro- 

 tected from the antiseptic action of light, whilst the rest 

 of the agar-agar and its contents is freely exposed ; the 

 result of which is that, in the shaded part of the dish the 

 colonies make their appearance, but in the remainder, 

 having been subjected to the action of light, no bacterial 

 growths, or only very feeble ones, are visible. This is 

 beautifully exhibited in a few days' time by the shape of 

 the black letters or other figures being sharply delineated 

 by the abundant growths which have taken place beneath 

 them from the blank remainder of the dish where nothing 

 IS visible, no colonies having developed. 



Recently infected agar-agar dishes, partially screened 

 with a leaden cross, were lowered to particular depths in 

 the Starnberger Lake. The day selected was very fine 

 and sunny, and the exposure was continued for 4^ hours, 

 the temperature of the water being 15'' R. The site was 

 the itai ting-place of the steamers, and the water was not 

 quite clear, this being doubtless due to the disturbance 

 caused by the plying to and fro of the vessels. 



It would have been more striking, perhaps, if Prof. 

 Buchner had used only one variety of organism through- 

 out, as then all chance of characteristic individual differ- 

 ences disturbing the progressive results would have been 

 obliterated. 



The following table shows the results obtained :- 



At a depth of r6 m. the bactericidal action of the sun's 

 rays, as shown by this method, is equal to that produced 

 outside the water ; but at 2'6 m., however, the action is 

 much less apparent, and in fact is only just perceptible. 

 Thus, as has been suggested elsewhere,' the antiseptic 

 potency of the sun's rays ceases a long time before the 

 light becomes affected by the depth of water it has to 

 traverse. 



These experiments are of particular interest and im- 

 portance, because they show very clearly that the agency 

 of light in purifying water cannot be regarded as of much 

 importance. So much stress has recently in Germany 

 been laid upon the self-purification of river-water, that 

 the advisability of permitting the sewage of cities of the 

 magnitude of Cologne to pass untreated into the Rhine, 

 has been publicly discussed on the assumption that in 

 its subsequent flow all objectionable matters will dis- 

 appear, one of the agencies cited as materially assisting 

 in this magic destruction being sunlight. It is, however, 

 sufficiently apparent that the action of light can only 

 affect a very small fraction of the whole mass of water, 

 for we know that bacteria exist in large numbers at depths 

 very considerably below those which insolation can em- 

 brace, whilst there are only a few months in the year, at 

 any rate in our northern climes, when the sun's action is 

 sufficiently strong or prolonged to produce any appre- 

 ciable effect even in the upper layers of the water. 



Prof. Buchner concludes his paper with some investi- 

 gations carried out by his assistants on the River Isar, 

 10 km. above Munich. These experiments were made 

 to ascertain if any increase from the numberof organisms 

 present during the daytime takes place in the night, as 

 in the absence of light might reasonably be anticipated. 

 Dr. Minck and Ur. Neumayer, therefore, undertook on a 

 September night to abstract samples from the river at 

 i m. below the surface at intervals of from 1-2 hours 

 from 6 o'clock in the evening until 6 o'clock the next 

 morning. The temperature of the water during this time 

 only varied between 9-10 R., and the samples were 

 examined immediately after collection. The results are 

 recorded in the following table : — 



Njjmber of microbes in about 

 20 drops of waler. 



i6o 



5^ 



%"- 



«o7 



380 



460 

 520 



S'o 



250 



It would be interesting to have further confirmation of 

 the results here given, other factors having doubtless 

 assisted besides the absence or presence of light ; but the 

 arduous nature of the experiments will doubtless greatly 

 militate against such a series being sufficiently often 

 made to permit of any definite conclusions being arrived 



' " U.-ictcrial Life and Li^hl," Longman's Magazine, September. i3j> 

 - No explanation is offered tor these abnormally low 6g>trC3. 



Time of taking sample. 



NO. 1297, VOL. 50] 



