A new form of photo-electric photometer or colorimeter is described which permits of accurate and objective readings in a wide range of colorimetric work with small quantities of solutions. The instrument is of the null-point type and hence requires calibration to suit the particular estimation.

The writer is indebted to the Earl of Moray Fund of the University of Edinburgh for financial assistance towards the expenses of the earlier part of the work.

The original model was made by A. H. Baird of Edinburgh, the model described above by Wm. Watson and Sons, London, to whom, and to Mr W. E. Watson-Baker, I am indebted for suggestions on a number of points.

A study of the growth phases of pleuropneumoma has been carried out by two new methods: (1) a new fixing and staining technique, (2) a new dark-ground method. The results obtained by the two methods are in good agreement, and the life cycle of the organism of bovine pleuropneumoma and related microbes as revealed by means of both methods has been described.

The mean time-survivor curves of bacteria immersed in two phenolic disinfectants show the same variation of type as the individual curves, that is, (a) an exponential rate, (b) a lag phase followed by an exponential rate. The significance of all these curves and of others is discussed in the next part of the paper.

1. Four β-haemolytic strains of Streptococcus, suspended in normal saline, showed progressive increase in mortality, as the pHwas lowered from 7·5 to 5·0; below pH 5·0 the mortality was greatly accentuated.

2. Ether and alcohol extracts of human skin and its appendages were powerfully, saline extracts more weakly and inconstantly, lethal to the haemolytic Streptococcus and to certain other organisms.

3. The long chain fatty acids and soaps are the chief, if not the only, bactericidal constituents of skin and its appendages.

4. Experiments are described illustra ting the streptococcocidal effect of various acids occurring in animal fats.

5. Cystein and blood inhibit the streptoeoccocidal activity of skin fats and certain fatty acids. Ultra-violet light may increase this activity, or abolish, the inhibition exercised by cystein.

6. The mechanism of the sterilizing power of fatty acids and soaps, the influence exerted by cystein, blood, and ultra-violet light on the sterilizing power, of skin fats and fatty acids, and the relationship of skin fatty acids to lysozyme are discussed.

The meaning of the footnote on p. 162 is somewhat ambiguous and the statement comprised in it is not true for all values of n, the number of differences in N.F.T. of which the significance of the mean is tested.

The statement that, by the use of a certain approximate distribution, significance of an observation is underestimated means that the estimated value of P in the neighbourhood of the significance level is greater than the true value.

The plate count is no longer considered the only test for the bacterial count of milk. There has recently been a tendency for the methylene-blue, or even the recently developed resazurin test, to displace the colony count as a measure of hygienic quality. Many workers, however, after experience with the dye reduction tests, have returned to the plate count, especially for advisory work, and consider that the new simple tests give inadequate information. Moreover, the plate count is still considered the best test for pasteurized milk. The questions of cost and availability of medium constituents, therefore, assume considerable importance in wartime.

Two media (Min. Health, 1937; Min. Agric. 1934) have received official recognition in England for routine plate counts of milk. Both media (yeastrel agar and milk agar) contain 0·5% peptone, so that a considerable amount can be used if a large number of samples is tested. A shortage of peptone has already been experienced and the price has increased by over 100%. Experiments were therefore undertaken to find an alternative satisfactory medium containing less peptone.

Bismuth sulphite agar is by far the most efficient of all single media devised for the isolation of typhoid bacilli from stools. When tested in comparison with other media it was found not only to afford the highest percentage of positive results but also the maximum inhibition of saprophytic growth. Its remarkable selectivity is demonstrated by the fact that negative plates are often completely sterile, and positive ones often consist exclusively of typhoid colonies (Loureiro & Matos, unpublished observations).