402 SUMMAKY OF CURRENT RESEARCHES RKLATING TO 



grains 1 /a (or less) in diameter which should be (3) barely opaque to all 

 colours. All the ordinary black pigments possess the third characteris- 

 tic approximately, some of them the second, but only one of them, bone 

 black in its various forms, has approximately the best refractive index. 



A white pigment should have a high refractive index, should be 

 ■even grained, with grains about • 5 to • 1 /^ in diameter. Finer grains 

 diffuse blue light more than red light under certain conditions, for 

 •example, when mixed in oil with black or dark red pigments. Thus 

 blue greys and purples result. Most white pigments contain enough 

 very fine grains to give bluish greys. Control of size of grain is 

 important. It is probable that the refractive index of certain zinc 

 oxides made by the American process can be controlled advantageously. 



Colouring efficiency must be considered as much from the stand- 

 point of the hue and tone desired as of the absolute " quantity of 

 colour " obtained. Thus, although extremely fine division may favour 

 tlie last factor mentioned, when the pigment is used in a mixLure with 

 a strongly diffusing pigment, yet under other conditions size and shape 

 of grain and refractive index are of great importance. For example, 

 Harrison red in grains of one shape has about 10 p.c. better diffusing 

 power than in grains of another shape. The shifting of hue, especially 

 of orange and yellow paints toward the green, due to admixed black, is 

 considered in detail. The optical properties which determine whether 

 a pigment is best suited for producing tints or shades are discussed. 



Special methods of studying the optical properties of pigments have 

 been used, and a considerable number of constants determined. These 

 properties can be applied in determinative work or to problems in 

 chromatics. 



This investigation has been conducted in the Geophysical Laboratory 

 of the Carnegie Institution of Washington. J. E. B 



(6) ^Miscellaneous. 



Chemical Microscopy. — E. M. ('hamot {Trans. American Microscop. 

 Soc, 1917, 3, 139-57, 2 pis.). This was an address delivered before 

 a joint meeting of the Chicago Section of the American Chem'ical Society 

 and the State Microscopical Society of Illinois, and was an appeal from 

 a chemist for a wider and more intelligent application of the microscope 

 in everyday chemical practice. The author criticizes the modern micro- 

 scope, and points out that it is not an ideal instrument for a chemical 

 laboratory. He considers that the mirror should be mounted on a 

 swinging bar capable of movements far to one side, or even above the 

 stage ; that the objective should be of small angle, long available 

 working distance and marked penetrating power ; and that the instru- 

 ment should be built of materials capable of resisting the corrosive 

 atmosphere of an industrial laboratory. With such an instrument, 

 however, many of the processes of qualitative chemical analysis can be 

 readily and speedily performed. The author discusses in some detail the 

 application of microscopy to such investigations as " water analysis, 

 cements, foods and beverages, metallurgical industries, pigments and 

 textiles. A. N. D. 



