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ing the necessary conditions under which the maximum amount of work is obtainable from these novel bacterial labourers.

Two different classes of bacteria are required to carry on the purification of sewage: those which flourish in the absence of air and are known as anaërobic bacteria, and those to which the presence of air is essential for the exercise of their functions, the latter being therefore called aërobic bacteria.

The work of the anaërobic labourers consists in breaking down the complex organic compounds present in sewage, whilst the completion of the process of purification is left to the aërobic varieties. In the ordinary course of nature both these processes are going on side by side, but it has been found advisable to separate these two different classes of bacteria as far as possible, and allot distinct premises to the anaërobic and aërobic varieties respectively engaged in the purification of sewage, for by so doing experience has shown that the work is not only more expeditiously, but also more efficiently, carried out.

Now the anaërobic bacteria are supplied along with the sewage, and the retention of their services offers practically no difficulty as long as an ample allowance of space and time is given them in which to carry on their labours. The aërobic bacteria,

however, besides demanding space and time, insist upon their workshops being well ventilated, and if the supply of fresh air is in any way curtailed they stop work entirely. Hence the ventilation of the aërobic workshops becomes a matter of primary importance if the valuable services of these labourers are to be retained. To ensure a sufficient supply of air being provided, it has been found advisable to have two or more aërobic workshops or bacteria contact beds, and the sewage is passed from one on to a second, and so on, until the purification is complete. Under proper management the sewage should leave the works as an inodorous, almost pellucid liquid, incapable of putrefaction, which may be turned into rivers or other waterways without fear of rousing the wrath of local riparian authorities.

But whilst the commercial side of bacteriology, so to speak, has made such great strides, the purely scientific applications which have been made of the facts it has furnished have by no means lagged behind. Chemists, from Pasteur downwards, have made use repeatedly of special bacteria to perform delicate operations in the laboratory which other methods have either failed to accomplish or have performed in a clumsy and less expeditious manner.

There can be no doubt that, as our knowledge

grows from day to day, we shall find more and more how much depends upon the work of individual bacteria, and how much importance attaches to the selection of just those varieties which are of value, and the banishment of those which are detrimental; and thus the many applications which bacteria already admit of render their easy access a matter of increasing consequence, enhancing the value of bacterial institutions such as already exist on the Continent.

But whilst the easy access of bacteria for experimental and scientific purposes is of great importance to the investigator, their indiscriminate distribution would equally be a source of uneasiness and danger to the community at large. Already sensational fiction has made considerable capital out of the pathogenic microbe, and with the winged aid of penny publications it does not take long for suggestions of such kinds to spread in society and assume practical shape, and whilst the administration of bacterial poisons offers comparatively but little difficulty, their identification would be a far greater problem for experts than that presented by particular chemical poisons. To cope with this danger to the public, specimens of disease-germs from these bacterial depôts may not be supplied to applicants unless the latter can prove to the satisfaction of the director that

they are connected with responsible public institutions.

In recent times, indeed, one of the most remarkable practical uses to which bacteria have been put is that of poisoning-agents on a large scale, or in other words vermin exterminators; if this new rôle for bacteria becomes extended, as no doubt it will, the law for the sale of noxious drugs and preparations will also doubtless be amended to cover the distribution of bacterialpoisons.

It was in the year 1889 that Professor Loeffler, while experimenting with mice in his laboratory at Greifswald, discovered a micro-organism which was extremely fatal to all kinds of mice. The happy idea occurred to the Professor that this lethal little microbe, which he christened Bacillus typhi murium, might be turned to excellent account in combating plagues of field mice in grain-fields, where the devastation committed by these voracious rodents had become in parts of Greece and Russia a serious source of loss to agriculturists. Experiments were accordingly made on a small scale to test the efficiency of this bacterial poisoner in destroying field mice, and so successful were the results that Loeffler confidently announced the possibility of keeping down these pests by distributing food material

infected with these bacteria over fields which were invaded by them. The Greek Government took up the question, and Loeffler's method was applied with brilliant results; the disease was disseminated with extraordinary rapidity and severity, and the mice were readily destroyed.

It is highly satisfactory to find that the character of this mouse-bacillus has stood the test of time, for after a period of more than ten years most encouraging reports concerning its efficiency still continue to be received. In one of the latest of these, drawn up by the Director of the Experimental Agricultural Institute in Vienna, we read that in no less than seventy per cent of the cases in which it was employed it was completely successful in its work of extermination, and it is interesting to note that in a considerable number of these instances it was the domestic mouse against which its energies were directed. The rat has, however, until recently escaped the hand of the bacterial executioner, but his knell has also now been sounded in the announcement that a rat-bacillus has been discovered.

Considering the undesirable notoriety which these rodents have of late obtained in connection with their undoubted culpability in the dissemination of plague, this discovery, if correct, should be warmly welcomed. That there is plenty of work

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