of pathogenic bacteria in water, whilst a separate chapter has been devoted to the comparatively novel subject of the bactericidal action of light. Finally, we have appended a concise description of the principal characters of all the micro-organisms, numbering upwards of 200, which, as far as we have been able to ascertain, have hitherto been found in water; and it is hoped that this descriptive catalogue of water bacteria may prove of some value in the at present bewildering task of identifying the different microbial forms met with in natural waters. DUNDEE April, 1894. P. F. F. G. C. F. IX. THE ACTION OF LIGHT ON MICRO-ORGANISMS IN WATER 395 521 10. ANAEROBIC CULTURE 1. BACILLUS RAMOSUS. 2. BACILLUS VIOLACEUS 3. BACILLUS ARBORESCENS 4. Roux's METHOD OF CULTIVATING ORGANISMS ON SLICES OF POTATO 5. HÆMATIMETER (after Jörgensen) 6. LEVELLING APPARATUS FOR MAKING PLATE-CULTIVATIONS 11. SUSPENDED DROP-CULTURES 2 30 4 22 29 32 34 35 40 41 59 12. APPARATUS FOR COLLECTING SAMPLES OF WATER AT DEFINITK DEPTHS (Miquel) 62 13. WOLFFHÜGEL'S COUNTING APPARATUS 78 14. PHOTOGRAPHIC REPRODUCTION OF A GELATINE-PLATE CULTURE 15. PASTEUR-CHAMBERLAND FILTER 81 178 16. MULTIPLICATION OF BACTERIA IN SPRING WATERS (Miquel). 17. MULTIPLICATION OF BACTERIA IN IMPURE WATERS (Miquel) 18. TYPHOID BACILLI FROM A PURE CULTURE 22. TYPHOID BACILLI IN AGAR-AGAR, EXPOSED TO DIRECT SUNSHINE FOR ONE HOUR INTRODUCTION UNTIL about twenty-five years ago it was generally supposed that some organic substances in the act of undergoing decomposition are capable of causing the alteration and decay of other organic substances with which they are placed in contact, and it was by the assumption of such communicated decomposition from one substance to another that Liebig sought to explain the various well-known phenomena of fermentation. Thus Liebig conceived of the ordinary alcoholic fermentation of sugar as being brought about, not by the living and growing yeast-cells, but, on the contrary, by the dead yeast undergoing decomposition. As long as this theory was the accepted doctrine of the day, it was not surprising to find chemists attaching great importance to the organic matter in water which analyses revealed, and which was known to have been derived from decomposing vegetable and animal substances with which the water had been in contact. It was not unnaturally supposed that such decomposing organic matters, if present in drinking water, would tend to set. up putrefactive and other injurious changes in the digestive organs with which they were brought in con But the theory, or rather dogma, of fermentation. enunciated by Liebig was completely broken down by the classical researches of Pasteur, by whom it was shown that the processes of fermentation and putrefaction were due, not to decomposing organic matter, but to living organisms, and that living organisms were also certainly the cause of some, and probably of all, zymotic diseases. The presence of organic matter in water was thus deprived of much of its direct import, the chief interest still attaching to it being that it might serve as an indication of the possible presence of living organisms endowed with virulent properties, whilst the interest attaching to the presence of micro-organisms in water was further greatly enhanced by the proof which was furnished by medical men that some zymotic diseases are undoubtedly communicated by drinking water. In the case of two diseases, at any rate, the evidence may be regarded as conclusive on the main point, and the communicability of Asiatic cholera and typhoid fever by water forms one of the cardinal principles of modern sanitary science, which year by year is becoming more widely recognised and more generally accepted. The germ theory of zymotic disease, which has gained with each successive decade of the past half-century a firmer hold on enlightened public opinion, was naturally soon impressed into the service of those who sought to explain the empirical fact that these particular diseases are frequently communicated by water. Having thus become so early interwoven with the consideration of potable waters, it is easy to understand how the acceptance of this germ theory of disease |