This was the condition of things at the time the second Rivers Pollution Commission carried out its investigations, which in many respects, and rightly too, are still considered standard investigations. It cannot be surprising, therefore, that, being without proper means of biological examination, and having to rely chiefly on chemical methods only, the Commissioners came to the conclusion that the changes brought about in sewage purification were due to mechanical and chemical agencies! It is frequently a matter of the utmost difficulty to ascribe, after the lapse of half-a-century, a new theory to one special author, as several investigators may have been trending the same way quite independently of each other, but may not have been equally successful in the matter of their publications becoming generally known. Theories, as a rule, do not drop out of the clouds like meteorites, they force themselves gradually upon men’s minds and are elaborated by them until ripe. Bearing this in mind, and subject to further research, it would appear as if Alexander Müller had been the first to apply Pasteur’s general theories as to decomposition, fermentation and putrefaction to the problem of the self-purification of sewage. He made his experiments in 1869 and published them in 1873. Since that date a very large number of investigators have been at work on similar lines, and whilst it would lead too far to deal with them minutely, it ought to be stated that the results of their labour confirmed the view of living organisms playing a very important part in the decomposition of sewage. Among the many names prominent in this respect are those of Schloesing, Müntz, Hatton, Warrington, Sorby, Winogradsky, Percy Frankland, Dupré, Emich and Dibdin. That set of researches, however, which has done more than any other to consolidate the theory of bio-chemical changes taking place in the self-purification of sewage are the investigations of the Massachusetts State Board of Health, which were commenced in November 1887, and are still being continued. Since 1895 a large number of additional experiments have been made, which will be dealt with more in detail later on, but speaking generally they have not materially increased our knowledge of the processes taking place in sewage purification. Summarising the remarks on the theoretical aspect of this question, it may be said that, as to the agencies at work, we know now they are of a mechanical, chemical and biological nature; but as to the processes and products brought about by these agencies we know very little beyond the initial and terminal stages, as will be pointed out in some of the subsequent observations.