In 1931 Cummins, Weatherall & Waters investigated the adsorption of tuberculin by anthracite dust and the subject was further investigated by Cummins & Williams in 1938. After the publication by Kettle (1934) of his observation that the intra-tracheal inoculation of kaolin dust mixed with dead tubercle bacilli gave rise to a much greater reaction than did the inoculation of kaolin alone, the result of the mixture being comparable to an early silicotic lesion, the author, thinking that this effect might have been due to adsorption of the products of the bacilli on to the dust, tried the same experiment with a number of other dusts, met with in mines, such as those of silica, sericite and anthracite. He obtained comparable results with all of them, though of different degrees according to the dust used (Cummins, 1940b).

1. Certain appearances of the colonies and some biochemical reactions of forty strains of B. dysenteriae Sonne have been investigated.

2. In thirty-five strains definite raised papillae occurred on parent colonies after 3–5 days' growth on MacConkey's medium or ordinary agar.

3. Papillae were produced both by strains that fermented lactose in fluid culture and those that did not; in the former case they were pink, in the latter pale.

4. Papilla-producing and other strains could not be differentiated by various biochemical reactions or by straight agglutination tests.

A Technique for drying serum proteins from the frozen state has been described by Greaves & Adair (1939). For war requirements the procedure has been to dry in 12 oz. medical flats into each of which is put 200 c.c. of serum. The bottles are then placed in a −20° C. cold room on shelves slightly inclined to the horizontal. The inclination is to allow the maximum quantity of serum to be dried consistent with the exposure of a reasonable surface area from which to dry. This method of freezing produces a wedge-shaped block of serum in the bottle (Fig. 1) and is very efficient. Large amounts of serum for the use of the Army have been dried in this manner by Greaves and his colleagues.

During the course of our experiments on the lethal effectiveness of organic smokes to bacteria in the air (Baker & Twort, 1940, p. 587), we experienced, as time went on, more and more difficulty in obtaining satisfactory results, whereas in the early days of these investigations our findings were relatively consistent. In the first place, our charcoal-incense ‘candles’, when smouldered, failed to give the results expected, and in-seeking an explanation for the failure it was thought possible that the temperature of smouldering was too high and that there was pyrolysis of the emitted smoke as it passed over the glowing point of the candle. Attempts were made to elucidate the matter by (1) smouldering the ‘candle’ inverted, and (2) heating the crushed ‘candle’ on a hot plate of regulated temperature. Both methods gave somewhat improved results, but these still remained very poor, and our next procedure was to examine the effect of the smokes obtained from incense supplied from different sources. The results were irregular, as they were with several other substances which were re-tested. Only Peru balsam continued to give good ‘kills’ for some time, but, finally, we were unable to reproduce our original results even with this product. While there seemed to be evidence that the smoke, generated by smouldering on a hot plate such substances as cardboard and the incense-charcoal mix, was somewhat more lethally effective than when generated by smouldering as a ‘candle’ or strip, the experimental results as a whole had, by this time, become consistently bad.

It has not yet been possible to breed hedgehogs in laboratory cages, but they have been bred and reared successfully in pens in the open, twenty-seven young being produced by five females in seven litters. The methods adopted in breeding and maintaining this stock are described.

A detailed study is made of methods available for estimating the significance of difference in degree of pollution of water by conform bacteria and of obtaining practically useful estimates of such differences.

For estimating the significance of a difference in a single pair of samples it was considered preferable to employ as the variate the difference in the total number of fertile tubes, or n.f.t., rather than the difference in the so-called most probable number of bacteria per 100 ml. A table (Table 3) is included by which the significance of any difference in n.f.t. may be determined at a glance. The variate is also useful in determining whether a series of differences may be regarded as homogeneous and such as might be expected to arise by chance in a high proportion of trials if corresponding members in a series of pairs of samples had been taken from sources identical as to their degree of pollution. Table 5 is included to facilitate the application of a comprehensive test of identity, in degree of pollution, of the members of each pair of samples.

The application of the binomial distribution in testing the significance of consistency in sign of a series of differences is explained, and Table 13 is included to facilitate the application of the binomial for a series of pairs twenty or less in number. The use of the normal distribution as an approximation to the binomial for pairs over twenty in number is explained.

The distribution of differences in the logarithms of the ‘most probable number of bacteria per 100 ml.’, or m.p.n., between samples in a certain series, A, and those of another series, B, in an experiment carried out by Dr L. F. L. Clegg, was found to be approximately normal in form. Thus the normal test of the mean logarithmic difference in m.p.n. is applicable and the t-test is applicable to a small series of differences. The advantages of employing the logarithmic difference as the variate are pointed out.

All tests applied to Dr Clegg's data agree in showing that the degree of pollution estimated from the results of a certain method, B, was higher than that indicated by the results of another method, A. According to method A the tubes were inoculated and- incubated at once at 44° C, while according to method B the inoculated tubes were incubated at 37° C, those proving fertile at that temperature being used for inoculating fresh tubes which were then incubated at 44° C. The mean ratio, B : A was found to be 1·416 and the 0·025 points of the distribution were found to be B : A = 9·6, A : B = 4·8 respectively.

The choice of a level of significance is discussed.

The results of the application of some statistical tests of significance to data from a control experiment are discussed. In this experiment the members of each pair of samples were taken from the same water sample and treated by identical methods.

The methods of replacing a difference by the corresponding value of the normal abscissa and of applying the normal test of significance of a mean to the mean of such normal abscissae is explained. This method is applied in testing the significance of a mean difference in n.f.t. and in testing the significance of a mean error from zero.

The theoretical bases of tests of the significance of differences in m.p.n. and in n.f.t. are compared.

The advantage of knowledge of the exact distribution of a single difference in tests of the significance of a series of differences is explained and examples are given which demonstrate this advantage very clearly.

The combination of results taken at different times and places for testing the significance of time or locality effects is explained and a, suggestion is made as to the use of replicated observations when the value of P for each observation is known.

When a number of persons is exposed to a particular pneumoconiosis-producing dust for the same period under apparently similar conditions, some of them develop the disease while others escape. It is possible that the less easily affected group comprises merely those who by chance or through some inborn or acquired local physical difference, e.g. nose-breathing, receive smaller doses of the noxious dust into the lungs. It is also possible that they are a selected group of peculiar constitutional type. The latter view has been accepted by some authorities and used in practice in selecting entrants to the mining industry, on the ground that by excluding certain anthropometric types the incidence of pneumoconiosis, or of pneumoconiosis complicated by tuberculosis, would thereby be reduced, though no adequate statistical data have so far been published to support this hypothesis.

1. The primary aim of the experiment was to find out if certain combinations of container and gas tube give a greater proportion of positive results than others (with the same strength of inoculation) and if possible to elucidate the causes of any differences between combinations.

The earlier results indicated that the best combination was a 7 × 1 ½ in. test-tube with a 4 × ¾ in. gas tube. Later experiments failed to maintain this preference but did show a definite disadvantage in using a very small gas tube. Whether this is due to the absolute sinallness of the gas tube or the smallness of its volume relative to the total amount of liquid used cannot be decided, for the latter was the same in all combinations. This was the only shape or sizefactor that was definitely influential on the evidence of these experiments. Yet if size of gas tube is intrinsically important the absence of reactions with acid but no gas is puzzling.

Attempts to break down the points involved in changes in shape and size of gas tube and container into a number of simple factors showed that several considerations might be involved, too many to cover in one experimental lay-out; but by including extreme combinations it was hoped that some indication might be obtained of the more important factors. With the new and successful combination of 7 × 1½ in. test-tube with 4 × ¾ in. gas tube, the gas tube holds 22% of the total volume of medium plus inoculum, the same proportion as the standard 6 × ⅝ in. test-tifbe and in. gas tube. This seems to be in support of standard practice if it is relative volumes that are important.

2. The analysis of plate counts shows that the opacity method, with the strain employed, gives complete control of the population and the three methods of assessing the plate count were in good agreement with one another.

The χ2 values were as a rule subnormal. This result excludes the possibility of any lack of control of the population in any one set of plates, but the reason for the subnormality is of some interest. It may plausibly be attributed to the factor of competition among developing colonies for the nutrient medium available so that the chance of development of a colony is smaller when a large number of others are present than when there are only a few. A toxic emanation from a colony which excluded the formation of another colony within a certain distance of it would have the same effect.

3. In the main experiment the percentage of positive tubes was about half (20% as against 45%) that to be expected from the plate counts on the usual Poisson hypothesis; or looking at the matter the other way the mean number of organisms per tube (0·22) deduced from the percentage of positives was about half the 0·5 expected from the plate counts. We have found no reason for this and in two subsequent experiments the agreement was satisfactory.

4. No difference was found between 2 × ⅜ and If in. gas tubes with a 6 × ⅝ in. test-tube or between rimmed and rimless gas tubes.

1. A slit sampler for air-borne bacteria is described, in which air is sucked through a narrow slit on to the surface of agar in an ordinary Petri dish.

2. This method has proved very rapid and simple in use, and has appeared more efficient than other methods against which it has been tested.

3. The collection efficiency for an aerosol consisting of Staph. albus, sprayed from distilled water as single cocci, is about 96%. The method thus collects the finest bacteria-carrying, particles almost as efficiently as coarser ones.

4. In contrast to this, the exposure of plain open Petri dishes appears to collect the larger particles in the air of crowded rooms at least 200 times as efficiently as it does single washed bacteria. This highly selective effect renders it advisable to use great caution in evaluating tests made in plain open dishes.

5. The methods of determining sampling efficiency, and their errors are discussed in detail.