GEOPHYSICAL LABORATORY. 149 



naiy black pigments possess the third characteristic approximately, some of 

 them the second, but only one of them — boneblack in its various fonns — has 

 the best refractive index. 



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

 grained with grains about 0.5 to 1.0^ in diameter. Finer grains diffuse blue 

 light more than red light under certain conditions, e. g., when mixed in oil with 

 black or dark-red pigments. Thus blue-grays and purples result. Most 

 white pigments contain enough very fine grains to give bluish grays. 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. 



Coloring efficiency must be considered as much from the standpoint of the 

 hue and tone desired as of the absolute "quantity of color" obtained. Thus, 

 although extremely fine division may favor the last factor mentioned when the 

 pigment is used in a mixture 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 per cent 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 deter- 

 mine whether a pigment will be best suited for producing tints or shades are 

 discussed . Special methods of stud jdng the optical properties of pigments have 

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

 properties can be applied in determinative work or to problems in chromatics. 



(23) Adirondack intrusives. N. L. Bowen. J. Geol., 25, 509-512 (1917). 



A reply to criticism by Professor Cushing of certain statements relative to 

 Adirondack structure occurring in the paper "The problem of the anortho- 

 sites" (reviewed under No. 15 above). 



(24) The determination of iron in glass sand. John B. Ferguson. J. Ind. Eng. Chem., 



9, 941-943 (1917). 



In the past, sand analyses generally have shown too low an iron content, 

 and since the Geophysical Laboratory has taken up the problem of optical 

 glass this has been the source of some rather vexatious delays. In fact, it has 

 been necessary for us to reanalyze practically all the sands under consideration, 

 and in this paper is proposed a new procedure, based on our recent experiences, 

 to replace the older procedures which have been found to be inadequate. 

 Before the correct iron content of a sand can be determined the sand must be 

 completely decomposed. Simple treatment with hydrofluoric acid and sul- 

 phm'ic acid is not sufficient. Fusion of the residue with potassium pyro- 

 sulphate must be resorted to and even subsequent fusion with sodium car- 

 bonate in rare cases. 



(25) The effect of strain on heterogeneous equilibrium. E. D. WilUamson. Phys. Rev., 



N. S., 10, 275-283 (1917). 



In previous discussions of this subject there has been a great deal of mis- 

 understanding owing to the confounding of stress with what has been termed 

 ''unequal pressure," i. e., a difference of the hydrostatic pressure on two phases 

 in equilibrium. Since the effect of "unequal pressure" is of a different order 

 of magnitude from the effect of a stress, this has resulted in some confusion. 

 In the present paper equations are deduced for both cases and the underlying 

 assumptions are discussed at some length. 



