After a preliminary survey with the electron microscope of various preparations of colloidal gold, a study was made of the process of nucleation and growth in gold colloids. It was shown that nucleating agents may be identified with reducing agents which form a mixed polymer with chlorauric ion before the reduction to the nucleus takes place. It was also shown that the law of growth is exponential. The average size, the deviation from the average size and the character of the particle size distribution curve are determined by the amount of gold, the nucleation process and the law of growth.Colloidal gold may be considered as a typical hydrophobic colloid with particle size falling below the resolution limit of the optical microscope. With the development of the electron microscope which has s resolution permitting the examination of the individual colloidal particles 1 it was natural to make an extensive study of the shape, mean size and size distribution of the various preparations of colloidal gold and determine the factors that govern these properties. Previous work on the subject was limited to one or two preparations.4 l1 Hauser and Lynn, Ex9eriments in Colloid Chemistry (McGraw Hill, 1g40), p. 18.

In the course of cytological studies of a few selected species of bacteria by the techniques of ultra-thin sectioning and electron microscopy, some new and si8nificant information has been obtained concerning cellular division in Bacillus cereus.

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VOLUME 2 1, NO. 4, APRIL 1949 475 one after the illumination had been corrected. In the latter procedure, the point representing the quality on the trichroic ratio graph was brought to the same location on the graph within a reasonable tolerance. There was one exception to this. In one case, the variation consisted of an intensity difference with the illuminance from a ribbon filament at one twentieth that of the first pair. This was brought to the quality represented by the point further along the line representing variation of the reference point with intensity level. It was found that the same reference point was not valid for the tungsten arc which contains lines of the mercury spectrum superposed on the tungsten spectrum, at least with the photocell filters listed for Type B films. The method was satisfactory for variations of quality produced by changing from tungsten filament to a 4.5-ampere and a 10-ampere arc and from transparent to vertical illumination. (A sheet of this test on Kodachrome film was shown at St. Louis as a color slide.} Probably the system can be used with a tungsten arc but with its own reference points on the graph; this has been tested by only a fe\v trials. This relative!}' new' system has proved valuable in routine work;it saves much time and justifies the confidence with w'hich exposed film is sent off for processing.

The early literature on the cytology of M1ycobacterium tutbercuilosis was reviewe(I by Knaysi (1929). Among the reports not reviewed by him are those of Feinberg (1900), Nakanishi (1901), 'Minder (1916), Kirchensteins (1922), and Petit (1926). Feinberg observed a red nucleus and a bluie cytoplasm in cells stainedl by a modified Romanowski's soluition. iNakanishi demonstrated by postvital staining wi-ith methylene blute a round body locatedl near the middle of the cell; sometimes there was a constricted body or there were two round bodies. 'Minder distinguished between MIuch's granules and polar bodies, although the latter were demonstrable by Mluch's method of staining. He stated that the grantules were neither spores nor decomposition products. Kirchensteins, using a complicated method of staining with basic fuchsin, concluded that a nuclear apparattus existed consisting of several granules united by fine filaments. Petit did not attribute to the granules a nuclear nature. Knaysi (1929) studiedI a strain, originally of the human type, which had lost its pathogenicity; his observations were summarized as follows:

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