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.

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

During the last year further investigations of germicidal mists, especially those produced by heat-volatilization of phenols, have been carried out. We are primarily interested in continuous disinfection of the air in the presence of man, and all our original experimental results appeared to show that a small quantity of the higher phenols in the mijt form was more potent to bacteria than when in the vapour form. Thus, persistence as a mist long enough to ensure a duration of lethal effectiveness covering the time taken for at least one air turn-over was considered to be a desirable characteristic of a germicide to be used for our purpose.In

IN our original paper on germicidal aerosols (Twort, Baker, Finn & Powell, 1940) it was shown that the most satisfactory results were obtained with a solution of 10% hexyl resorcinol in propylene glycol together with a little alkali and wetting agent, the mixture being mechanically atomized into mist particles of appropriate size. Although it was confirmed by other workers that a high degree of lethal effectiveness on air-borne bacteria was achieved by the above procedure, objections were raised to its use in practice at the moment on the grounds that:(1) The germicide base was difficult to obtain.(2) The solvent was only manufactured in America, and not in bulk there.(3) The mechanical apparatus advocated involved the use of metals now required for the making of war weapons.We endeavoured to overcome these objections, and as a result of further research were led to suggest the use of balsamic smokes and the like as a means of depleting the air of pathogenic bacteria. It was thought at one time 'that all forms of apparatus for generating the smokes could be dispensed with by smouldering candles, etc. of the crude substances we were using,.but it was soon found that as regards balsams such smokes did not work. It appears that in the case of balsams and balsam-charcoal mixtures employed in candle form combustion of the active bodies occurs at the glowing tip of the candle, but when the mixes are heated on a hot-plate distillation of the active principles takes place, the charcoal in the mix not glowing till a later stage. On the other hand, such substances as cardboard and wood smouldered on a hot plate or as a strip provide active smokes, the activity 'presumably being due to compounds formed during partial combustion.The knowledge that humidity of the air was playing a far greater part in influencing our results than we had originally thought likely (Baker & Twort, 1941) induced us to revert to the heating of pure phenolic substances, a procedure which, owing to the poorness of the 'kills' previously obtained (Twort et al. 1940, p. 319), due to conditions of low humidity, we had temporarily discarded as of little value. It was soon found that a hot plate was apparently as useful as a mechanical atomizer for making a mist when humidity conditions were satisfactory, and the former procedure for the time being became our choice for experimental purposes. The simplicity of the technique renders

A commercial preparation of sodium hypochlorite dispersed in the form of a mist has been tested for germicidal action on bacteria suspended in the air.The relative merits of acid, neutral and alkaline solutions have been discussed.A maximum of efficiency dependent on dilution has been shown to exist, and experiments with water and glycerol as diluents have been performed.The value of hypochlorous acid gas and chlorine as aerial bactericides has been tested, and experimental evidence advanced to show that while these are to some extent effective as gases, the acid, at any rate, is more lethal in solution as a mist.Further evidence of the importance in aerial disinfection of size and evaporation rate of mist particles has been obtained.Objections have been raised to the proprietary use of the preparation on account of the liberation of chlorine, and its high salt content, but the necessity for the presence of a certain amount of salt has been shown.Decrease in germicidal activity of the mists when required to sterilize atomized salivas has been indicated.The active persistence of sodium hypochlorite mists has been shown to be short.A few notes on the use of ultra-violet light and nitration methods for combating aerial infections have been added.It was found that the greater part of a magnesium oxide smoke inhaled by mice was retained in the upper respiratory passages.