A form of ultramicroscope apparatus is described which enables the evaporation of mist droplets to be followed for long periods. A rough method of estimating the size distribution in aerosols is also given.The rates of evaporation of particles of various phenol solutions have been studied, the solutions chosen being of possible interest as aerial germicides. It is shown that the typical evaporation curve for a binary mixture consists of three parts, which in actual cases need not all be present. Sometimes the more detailed history of an evaporation can be deduced from the results.The approximate effective size distributions in mists of hexyl-resorcinol—propylene-glycol solutions have been measured and compared with the evaporation rates.It is shown that the efficiency of aerial germicides is partly a function of their volatility, and three rough classes are distinguished. But biological evidence is adduced which suggests that the solubility of the germicides in water may be of equal importance, and that the liquid in which the germicide is dissolved is not without effect.A centrifuge is described which enables the maximum particle size in a mist to be limited to a desired value.Methods of estimating quantitatively germicidal aerosols are discussed, and a method described which has been found to give reasonably accurate results in practice. Tentative experiments have been made to determine whether the materials studied are likely to cause corrosion of metals, etc., after pro-longed use.The collision theory of aerial disinfection is discussed, as is also the applicability of Langmuir's treatment of the evaporation of fine particles in mixtures.The development of the experimental technique involved in the biological investigations appertaining to air disinfection has been discussed, together with the numerous fallacies arising and the means by which these have been overcome.The construction of a special pair of experimental chambers is described, and details given, both as regards the bacteria and germicides, of the exact experimental conditions under which the most consistent results can be obtained.The requisite characteristics of germicidal mixtures which could be utilized for air disinfection are considered, and examples of suitable germicides and solvents are given. The method of recording and evaluating the results of a varied assortment of experiments is explained to facilitate inter se comparison.Examples are given of the lethal effectiveness in the air of several germicides in broth emulsions, both with and without the presence of serum, on stock cultures of selected types of bacteria. The sensitivity to the germicides of the bacteria making up the flora of the average normal saliva is compared with that of broth emulsions of bacteria.10% hexyl-resorcinol dissolved in propylene-glycol+0·05 % sulphonated lorol (“S2”) was the most effective all round germicidal mixture of those tested. Various other germicides were found equal in germicidal activity in the air, but for reasons stated have been deemed unsuited for use in the presence of man.The relationship between germicidal efficiency in the air and mist particle size and persistence has been studied by means of centrifuged mists. Particles of 0·5–1·0μ radius are shown to be the most useful in dealing with bacterial particles containing organic matter. Laboratory cultures, emulsified in broth, have been sterilized in mist dispersions by droplets of bactericides at least as small as 0·25μ radius.Tests on the penetrating abilities of bactericidal mists and bacteria through certain cloths have been performed.Experiments to demonstrate the surface sterilizing properties of aerosols have been performed, and examples of differences in effect on horizontal and vertical surfaces are noted.The degree of tolerance of man and animals (mice) for a number of phenolic substances and some organic solvents has been compared with the degree of tolerance of bacteria for the same substances.The tests on man have been confined to the detection by the special senses of the substances suspended in the air as a fine mist.The tests on animals were designed to show acute and chronic effects, the results being recorded in terms of clinical symptoms and pathological changes in the organs. A record was also kept of the findings as regards protozoal parasites of the host, and compared with the normal incidence among ordinary laboratory stock.Tests on bacteria were instituted for the purpose of gaining information on the host-parasite tolerance ratio for our germicides when in the air and when in the test-tube.

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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 canlle. 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.In seeking an explanation for this disconcerting state of affairs, the most obvious variation in our experimental conditions was a seasonal one, and in viewv of the fact that the room in which our experimental test chambers are located is thermostatically controlled, and the temperature variation is rarely more than 3°C., it seemed possible that the humidity of the air might be an important factor. Like other workers in the domain of aerial disinfection, we had kept in mind the probability of temperature and moisture being factors in influencing the efficacy of a germicidal aerosol, and, since our earliest experiments (Twort, Baker, Finn & Powell, 1940, p. 298), we had taken the precaution of recording temperature and relative humidity. Unfortunately, owing to stress of circumstances, during last year, when our team of workers was being disintegrated, no such records were kept, so that the data available for making an analysis of humidity in relation to lethal effectiveness of J. Hygiene 41 S

The laboraturiea of the Manehater Cmmittee on Cancer.WE propose here to describe some experiments, carried out several years ago, on the effect of applying carcinogenic agents at intervals or simultaneously on different areas of the skin of mice. The problems with which we were concerned were :-( 1) whether animals which were relatively sensitive on one area of the skin were also relatively sensitive on another, (2) whether the presence of a tumour per se on one area retarded or accelerated the development of a tumour on another area, (3) whether the application of carcinogenic agents on one area accelerated or retarded tumour formation on another area, (4) whether the type of tumour varied in different areas and (5) whether previous stimulation of an area had any effect (a) on the area itself, ( b ) on other areas.Sensitivity of the skin of the sacral region. Details are first given of a sample experiment wherein 100 mice were painted simultaneously five times per week for twenty-five weeks on the skin of the interscapular and sacral regions with a 1 per cent. solution of our synthetic tar in chloroform-tar B/19. This tar, with which we have performed nearly 100 experiments, was made by subjecting pinene to a temperature of 850" C. in an electric furnace in an atmosphere of nitrogen, fraction 19 being the tar concentrated by chemical and physical means. As a result of this experiment 75 of the animals developed tumours in the interscapular region and 57 in the sacral region ; a proportion of approximately 1.3 to 1. Of the interscapular tumours 50 became mahgnant before the end of the experiment and 25 remained benign, a proportion of 2 to 1, while of the sacral tumours 27 became malignant and 30 remained benign ; a proportion of only 0.9 to 1. Thus the interscapular region appeared to be more sensitive than the sacral region, especially as regards the development of malignant turnours. Of the 75 animals which bore tumours in the interscapular region 19 had no tumours in the sacral region, and of SO3

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