The effects produced upon bacteria by environmental conditions constitute factors which until recently have been studied with but too little regard to scientific accuracy, even by the bacteriologist investigating the conditions under which pathogenic bacteria occur in nature, and whilst probably leading a saprophytic existence.

The following paper embodies the main facts obtained during a prolonged investigation as regards the pollution of Tidal Waters and of Shell Fish, undertaken on behalf of the Royal Commission on Sewage Disposal.

Futher experience with the method of measuring the precipitum obtained when experimenting with precipitating antisera has shown that finer degrees of reaction cannot be satisfactorily determined by means of the apparatus described by Nuttall. In the latter apparatus capillary tubes were used in which to measure the precipitum therein deposited. It is true that capillaries as far as possible of the same calibre were used, nevertheless we have found that different readings were obtained when experimenting with tubes of varying calibre, variations evidently due to physical causes which prevent an equal degree of “packing” of the deposit in tubes of unequal width. We have modified the method after finding that calibrated tubes of uniform bore gave more constant results, and have devised a simple apparatus which greatly facilitates the making of measurements.

It will perhaps be generally admitted that few of the practical problems which have from time to be faced by the Medical Officer of Health in his capacity of statistician present more difficulty than is often involved by the estimation of populations. This problem may be said naturally to present itself in three different forms, according as the estimate is required for a past, a present, or a future date. Practically, however, there are but two primary divisions of the problem, the one relating to dates since which a census has been taken (and its results published), and the other to dates subsequent to that of the last published census. This distinction is pointed out by Dr Cressy Wilbur in the “Thirty-second Annual Report (for 1898) on the Vital Statistics of the State of Michigan.” He there refers to the first class of cases as estimates of inter-censal, and the second as estimates of post-censal populations.

On visiting a farm near Grahamstown for the purpose of examining some cattle, our attention was drawn to a young dog which was sick. The owner stated that he had been unable to keep a dog on the place, that they all died.

The disease which is the subject of this paper is one of a group of diseases affecting different animals, and caused by parasites possessing very similar characters. The diseases produced by parasites of the genus Piroplasma appear to closely resemble each other, and for this reason it is well to follow the terminology adopted by French investigators in speaking of any one of the diseases in question as a Piroplasmosis. Not only are the parasites similar in these diseases, but they also appear, in all cases where the subject has been worked out, to be transmitted by species of Ixodoidea or Ticks.

1. Diphtheria bacilli have been found in a considerable proportion of persons who have come into contact with cases of diphtheria, or with other infected persons.

2. Such persons have been shown to be a grave danger to the public health, especially when frequenting schools or institutions, and to constitute the usual channel by which the disease is spread.

3. Very satisfactory results have followed on the isolation of convalescents from the disease and of “infected contacts,” where two, or more, consecutive negative examinations have been required before release.

4. Carefully conducted investigations amongst healthy persons, who have not at a recent date been in contact with diphtheria cases or infected contacts, have shown that virulent diphtheria bacilli are very seldom (2 examples amongst 1511 persons) present in the mouths of the normal population. This fact renders the discovery and isolation of infected persons a practicable possibility, and offers a fair prospect of discovering and isolating the majority of them during any outbreak.

5. Of the 113 examples of the diphtheria bacillus tested for virulence during this outbreak, 87 were fully virulent, and 25 were completely devoid of virulence. One virulent bacillus for reasons explained did not kill the inoculated animal till the 12th day (p. 282). No partially attenuated bacilli have been found.

6. In the majority of persons in whom diphtheria bacilli were found, who had recently been in contact with cases of the disease, the bacilli were virulent.

7. Non-virulent bacilli were discovered in 1—2 out of every hundred persons examined, whether contacts or non-contacts. The proportion of persons infected with this organism is therefore the same amongst contacts and persons who have not recently been in contact with the disease.

8. The absence of polar bodies is no indication of a want of virulence in diphtheria bacilli, and their presence is no indication of the possession of virulence.

9. Hofmann's pseudo-diphtheria bacillus is a very common inhabitant of the mouths of poorer class children. It is less common amongst adults, even of the same class. The proportion of persons infected with this organism bears no relation to the proportion infected with the virulent diphtheria bacillus. Notified persons and infected contacts harboured this organism in the same proportions as the healthy school children with whom they had been associated.

Examples of the Hofmann's bacillus isolated from the first cultures obtained from diphtheria cases were totally non-virulent to guinea-pigs. There is no evidence that it is in any way pathogenic to man.

The distribution of this bacillus points to the conclusion that it is carried from mouth to mouth in the same ways as the diptheria bacillus, and therefore its widespread prevalence in schools attended by the poorer children is significant, as showing how widely spread and uncontrollable an outbreak of diphtheria may become, unless measures are early taken to deal with infected contacts.

10. Organisms morphologically resembling diphtheria bacilli are not infrequently found in the throats of healthy persons, and require careful examination by culture before they can be identified.

11. The xerosis bacillus is a common inhabitant of the normal conjunctival sac, and organisms closely resembling it are present in the eyes of some animals.

12. Virulent diptheria bacilli have undoubtedly been found in ear discharges, but diphtheria-like organisms appear to be extremely common in the ear discharges of scarlet fever patients, and in the ears of normal persons. Consequently no conclusions as to the frequency of the diptheria bacillus in the ears of scarlet fever patients can be made without the thorough examination of any organisms which may be discovered both by cultural, and virulence, tests.

13. Diphtheria-like organisms occur in the throats of healthy birds.