Fluorescein dispersed as powder into the air of occupied rooms was extracted in measurable quantities from dust after as many as twenty previous sweepings.

The value of fluorescein in showing the rate of removal of dust from occupied environments is discussed in relation to the transfer of infection.

(i) The theory of amoebic surveys and of related types of pathological surveys is discussed.

(ii) An inequality is derived,

where X is the total number of carriers and X1, X2 are the new carriers found at the first and second examinations.

(iii) Demonstrability is defined and several theoretical distributions of it are considered.

(iv) Demonstrability is shown not to be a constant in the populations considered.

(v) The danger of extrapolation is noted.

(vi) Life-table methods are introduced to treat some well-known published series.

(vii) A criterion for the adequacy of any estimate of the total carrier rate is given. The mean demonstrability of the outstanding carriers after successive examinations must form a decreasing series.

(viii) This criterion usually shows that the method of estimating the carrier rate for Entamoeba histolytica as three times the proportion of positives at a single examination, or as the total number of positives picked up in six examinations, both give too low a carrier rate.

(ix) It is suggested that the investigation of the ‘carrier’ rate may in certain cases be merely a measure of ‘demonstrability’.

(x) Possible future lines of investigation of the problem are noted.

I Know that you have finished your study in tropical medicine and hygiene at this school. And so, surely, I am not wide of the mark if I suppose there is very little in the way of facts and views you have not, at least, heard of. I think it rather useless to try to add one more small fact, or one more insignificant view, to the hoard of knowledge stored in your brains, or in your notebooks.

The success of detecting Mycobacterium tuberculosis in milk by microscopic examination of samples from each quarter of the udder and the use of this method in rapidly eliminating from a herd those cows which were excreting tubercie bacilli has been reported (Maitland, 1937). The advantages from the clinical point of view with illustrative clinical data have been recorded by Locke (1937), who collaborated in the investigation.

Hoyle & Fairbrother (1937) showed that tissues infected with influenza virus contained two distinct particles, the infective virus elementary body, and a smaller particle, the soluble antigen, which could be demonstrated by complement-fixation tests.

In the present lecture I shall have to go back to the very beginning of modern knowledge on malaria. I shall incur grave risks of telling you things you know already; as a matter of fact, it is more than a risk, it is a certainty. But I cannot help myself. All I can do is to pass over the initial stages as summarily as possible.

The value of ‘Statistical control’ in haematology has been demonstrated. After a short historical survey of the chief contributions to the theory of the technique of counting and of the application of the Poisson distribution in biology, the statistical theory of the distribution of cells in the haemocytometer is briefly described and examples are given of large sample methods of testing goodness of fit. Some of the author's counts are considered.

In the red cell counts a ‘crowding’ effect is noted by which the variance between the counts on the individual squares on the haemocytometer is reduced. This effect is trivial in white cell counting owing to the small proportion of the area occupied, but is important in reducing the variance between the numbers of red cells on individual small haemocytometer squares. A quadratic function adequately describes the regression of the variance on the mean for fixed size of haemocytometer square, for the usual range of red cell density. This function, calculated on the basis of the author's own counts also describes Berkson's findings on crowding adequately and is not, considering the approximations involved, inconsistent with the theoretical discussion on crowding. Reasons are given for believing that the crowding effect is only trivial if blocks of 16 squares are compared in red cell counting.

The methods of statistical control, suggested by R. A. Fisher's work, are introduced. It is shown that they give satisfactory results when applied to some of the author's own counts. The methods are used to discuss the consistency of the counts of certain medical graduates and technicians. The methods are suitable for a review of the literature with the object of examining the internal consistency of the counts made by various authors.

The viable bacteria in 10 mg. portions of dust were estimated by shaking with 5 ml. of Ringer's solution in 1 oz. bottles containing glass beads. The extract was allowed to stand for a few minutes, and 0·1 ml. amounts were then plated out at suitable dilutions. Successive portions from the same sample of dust showed very great variation in the bacterial counts, and this variation has been examined in series of twenty such portions. The distribution of the counts for three groups of organisms appears to approximate to a log-normal form, and the graphical estimate of the log-median is taken as a suitable parameter for the study of the influence of environmental factors on the survival of dust flora.

The behaviour of wild rats towards an unfamiliar food presents problems of both practical and theoretical interest (Chitty, 1950; Shorten, 1950; Thompson, 1948). When first eaten the food is often taken in very small amounts; if poison is present its effects soon make the rats stop feeding, and if they recover the rats are often found to be bait-shy; that is, they refuse the bait entirely (Rzoska, 1950). As far as rat control is concerned prebaiting with harmless food can overcome this difficulty by inducing rats to feed rapidly on the bait when poison is eventually added (see Barnett, 1948, for a review); and if there are even then a few bait-shy survivors they may be disposed of by a second prebaiting and poisoning with different materials. In large-scale practice, however, this elaborate programme presents considerable difficulties for organizations such as local health authorities, and they are often unable to adhere to it.

A quantal response is one in which a certain event either happens or does not happen. If, in an animal experiment, we record merely whether or not the animal dies, we are measuring a quantal response. The type of data with which we are concerned here is familiar to all workers in biological assay, and occurs constantly in bacteriological and immunological experiments. A number of animals is divided randomly into several groups, and all the animals in each group are treated with the same dose of a certain substance. The doses differ from group to group, and are frequently arranged so that successive doses differ by a common dilution factor. At each dose the numbers of animals which respond positively and negatively are recorded. The potency of the substance may be measured by that dose which would in the long run produce a positive response in exactly 50 % of the animals, and the main statistical problem is how to estimate this dose (the LD 50) from the available data. It is assumed that any inaccuracies in measuring the doses are negligible in comparison with the sampling errors due to the inevitable differences between experimental animals.

Since the introduction of toxoid-antitoxin floccules by Glenny & Pope (1927) little has been done to improve their value as an antigen. Watson, Taggart & Shaw (1941) and Barr (1949) confirmed the work of these investigators in showing that heating at 80° C. increased their antigenic value.

It has been shown in this article that about 80% of the toxoid contained in floccules may be recovered if they are dissolved in n/20-NaOH, the alkali being allowed to act for 1 hr. before being neutralized with acid. A purity of between 0·002 and 0·0025 mg. N/Lf is obtained regularly. Such a dissolved floccule solution is a relatively poor antigen when tested in guinea-pigs, but may be rendered highly antigenic when precipitated with alum or adsorbed on AIPO4.

The method has the advantage of simplicity. It would appear that any medium capable of yielding a toxin convertible into flucculable toxoid may be used, thus doing away with cumbersome media-making methods. However, for the best results in terms of both recovery and purity, a high-value, quickly flocculating, toxoid should be used. Further, the choice of antitoxin appears to be important where recovery is concerned. With most of the antitoxins used, recoveries of between 70 and 90 % were got, although with that used, for most of the work recorded here, the recoveries were much lower.

It is not yet established which is the antigen of choice—alum-precipitated (P.D.F.) or AIPO4-adsorbed (A.D.F.) dissolved floccules. A.D.F. has the advantage of ease of preparation and of constant mineral carrier content, but further work is necessary to establish the optimum amount of carrier before a definite answer can be given. A paper on immunity responses in man will be published shortly in the Lancet.

Dust from scarlet-fever wards was exposed to a controlled range of atmospheric humidities by enclosure in metal boxes containing anhydrous calcium chloride and saturated solutions of potassium carbonate, sodium nitrite, potassium bromide and sodium sulphate.

The death-rate of total organisms, Staphylococcus aureus and Streptococcus pyogenes in the dust was assessed by periodic sampling of series of twenty 10mg. portions. A positive correlation between atmospheric humidity and death-rate was observed for the three groups of organisms counted in three specimens of dust.

When mussels are allowed to function in sea water the main changes occurring in the water are depletion of dissolved oxygen and lowering of pH value. Provided the faeces and pseudo-faeces are not disturbed the increase in the content of organic matter is not appreciable. If the supernatant water is removed and aerated with diffused air the water is re-oxygenated, the pH value is restored to its original level, and the water so treated may be re-used for immersing a fresh batch of mussels. The process of re-use may apparently be continued indefinitely.

Under these conditions a high degree of cleansing is achieved by the mussels, the count of coli-aerogenes bacteria being reduced to a small fraction of the original; counts of bacteria in the water, on the other hand, are subject to large fluctuations. It was considered advisable for this reason to chlorinate the water between each cycle of cleansing in order to immerse the mussels on each occasion in water which itself was reasonably certain to be free from coliform bacteria, potentially including pathogens.

Experience showed that the greatest difficulty involved in the use of chlorine was in removing residual chlorine so that it would not inhibit functioning of the mussels when the water was re-used. After trials of various alternatives it was concluded that the most satisfactory method was to determine by means of small-scale tests the smallest quantity of chlorine (below the break-point) required to give a residual concentration of 0·05–0·10 p.p.m. in the re-used water and then to add the corresponding quantity with precision to the bulk of water by means of a dosing apparatus while the water was being pumped from the mussel tank to the aeration tank. After a period of contact of 1 hr. the water was aerated for a further hour. A series of trials in a semll-scale plant showed that this treatment ensured that residual chlorine in the water being added to the mussels did not exceed 0·05 p.p.m.; as a result the mussels functioned satisfactorily and the degree of cleansing attained was comparable with that attained in the existing mussel-cleansing tanks in which sterilized fresh sea water is used for each cycle of cleansing. Although the concentration of residual chlorine was small the mussels kept the water so clear that this concentration was effectively bactericidal and the bacterial quality of the water was usually comparable with that of good drinking water. Low temperatures retard the metabolic activity of mussels and below 4° C. this is so marked that the degree of cleansing achieved is unsatisfactory. For this reason it is recommended that the temperature of the re-used water should be maintained at 6° C. (43° F.) or above.

Oysters were found to be satisfactorily cleansed by a process of re-use similar to that adopted for mussels, provided the temperature of the water was maintained at 54° F. (12·2° C.) or rather higher.

Small-scale trials showed that artificial sea water, prepared by dissolving in fresh water suitable quantities of the major constituents of natural sea water, could be successfully re-used for cleansing mussels.

The effects of daylight, low-intensity ultra-violet radiation, fluorescent lighting, and tungsten-filament lighting on the survival of dust flora have been studied, at room humidities of about 60% and under dry conditions. The first three radiations all cause a significantly enhanced death-rate at room humidities for all the groups of organisms studied. The effect appears to be limited in extent and to be complete in about 10 days. The action of the radiations is much slower under dry conditions.

1. In the spring of 1948 the health visitors of 422 authorities in Britain obtained information from 4683 mothers on accidental injuries sustained by their young children during the first 2 years of life. This study formed part of a questionnaire inquiry into the health and development of these young children, who had all been born in one week, 3–9 March 1946.

2. The mothers seem to have remembered accurately the more serious accidents which received professional treatment during the 2 years under review. It is possible that some of the minor injuries which had not been treated were not recalled or entered on the questionnaires.

3. The rate for all serious injuries which received professional care was 79·6 per 1000 children in the sample. Treated cuts were almost as frequent as treated burns or scalds, each being sustained by about 37 per 1000 children. There were only 7·5 fractures per 1000 children. The sum of the rates for each type of injury (81·3 per 1000) slightly exceeds the combined rate for all children injured (79·6 per 1000). This is due to the fact that in calculating the former rates eight children who were each involved in two serious accidents of different types were counted twice, i.e. once under each type of injury sustained.

4. In 1946–8 treatment of young children for burns or scalds and for serious cuts was most frequently given by general practitioners. Children under 2 years of age were admitted to accident wards of hospitals chiefly for the treatment of serious burns or scalds. Fractures were usually treated at out-patients’ departments.

5. Although almost all accidents occurred at home, variations in housing conditions, as measured by crowding and the domestic facilities available, had little effect on the incidence of accidents among children under 2 years of age. Accidents happened most frequently when the children were beginning to walk and becoming inquisitive, and most injuries occurred either because they were not steady on their feet or because they had not developed ‘heat-sense’.

6. Since domestic help was rare, most children had to play in the kitchen or living room while their mothers were doing the housework, cooking and laundry. In these circumstances, they were often exposed to the risk of injury.

The appearance of vitamin E deficiency in rats and mice after the introduction of 2% of cod-liver oil to a diet containing 85% of whole-grain ground cereals is described. Evidence is given to show that the deficiency can be prevented by the reduction of the cod-liver oil content to 1%.

1. Repeated inoculation of thirty-six guinea-pig does with random blood of guinea-pig bucks failed to produce any demonstrable anti-red cell isoantibodies.

2. A note is included on the blood picture of a small series of newborn guinea-pigs aged between 12 and 24 hr.

1. Five generations of guinea-pigs bred successfully on diet 18 supplemented with crystalline ascorbic acid but without fresh green food.

2. No difference in reproductive performance was found between these animals and those of the normal breeding colony receiving unlimited supplies of fresh abbage.

3. Symptoms of vitamin E deficiency appeared among some young of the fourth and fifth generations, showing that diet 18 alone contained barely enough of this factor for continuous reproduction througy many generations.

4. Diet 18, supplemented by hay and fresh green food to supply vitamin C and additional vitamin E, provides all the essential factors required by the guinea-pig.

The bactericidal effect of ultra-violet radiation on dried films of β-haemolytic streptococci has been shown to be dependent both on the relative humidity and on the nature of the suspending medium from which the films are prepared. In particular, while the bactericidal action is most often more repaid under dry conditions, films prepared from suspensions in dust extract culture show a similar behaviour to that of naturally occurring dust flora and are most resistant to the effects of radiation under dry conditons.

An account is given of the behaviour of two porcine field strains of the virus of foot-and-mouth disease, with evidence of their adaptation to swine, based on experiments on infection by contact.