148 CARNEGIE INSTITUTION OF WASHINGTON. 



lized or amorphous, in individual units or in aggregates, coarse or fine-grained, 

 and of organic or inorganic nature. Refractive indices can be ascertained on 

 grains a few microns thick, and other optical properties on grains 0.01 mm. or 

 greater in diameter. 



(19) The crystallization of menthol. F. E. Wright. J. Am. Chem. Soc, 39, 1515-1524 



(1917). 



The crystallization of menthol is interesting not only to the crystallographer 

 but also to the physical chemist. Menthol appears in four different forms, 

 a, /3, 7, 5; three of these bear apparently monotropic relations to the stable 

 a-foi"m. Because of pronounced undercooling the melting temperatures of 

 all four forms can be realized and their mutual relations can be studied directly 

 under the microscope. On ciystallization all forms of menthol show a pro- 

 nounced tendency to the development of radial spherulites; these are roughly 

 spherical in shape in the case of crystallization from the melt, but noticeably 

 ellipsoidal on inversion of one crystal form into a second. The four forms of 

 menthol are readily distinguished under the petrographic microscope, a-men- 

 thol shows dextrorotatory polarization, while the melt is levorotatory. In 

 the formation of the different monotropic forms the initial temperature of 

 crystallization appears to be the determinative factor. 



(20) The thermodynamic reversibility of the equilibrium relations between a strained solid 



and its liquid. F. E. Wright and J. C. Hostetter. J. Wash. Acad. Sci., 7, 

 405-417 (1917). 



Experimental proof is offered in this paper of the reversibility of the relations 

 between a strained solid and its liquid. The mechanism of this action has 

 been found to be exactly that postulated in 1862 by James Thomson from a 

 purely theoretical basis, namely: that on crystallization each particle (atom 

 or group of atoms) enters into the crystal state in the condition of the crystal 

 at the point to which it becomes affixed; and that if the crystal be under a 

 state of strain the freshly deposited particle enters into the same state of strain. 

 This fact is essential if equilibrium relations are to exist between a strained 

 crystal and its liquid, because only under these conditions can the relations be 

 strictly reversible; and thermodynamic reversibility is necessary if the ther- 

 modynamic equations are to find vahd application. 



(21) Dispersion and other optical properties of carborundum. H. E. Merwin. J. Wash. 



Acad. Sci., 7, 445-447 (1917). 



The dispersion of carborundum in the visible spectrum was determined for 

 both CO and e by means of prisms and a basal plate. Crystals which are black 

 are fairly transparent in microscopic grains. Color has little effect on re- 

 fractive index for red light. Following are some of the observed values: 

 coLi = 2.633, €Li = 2.673; a;Na = 2.654, eNa = 2.697; cop = 2.700, €f = 2.749; 

 coG' = 2.741, €G' = 2.794. 



(22) Optical properties and theory of color of pigments and paints. H. E. Merwin. Proc. 



Am. Soc. Testing Materials, 17, 494-526 (1917). 



The hue, purity, and brightness of light diffused by a pigment or paint 

 depend upon the refractive index, color absorption, size, shape, and texture 

 of the pigment grains, and upon the refractive index, color, and continuity 

 of the vehicle; and also upon the distribution of the grains in the vehicle. 



A black pigment to be optically most effective should have (1) a refractive 

 index equal to that of the surrounding medium, and (2) grains 1/x (or less) 

 in diameter which should be (3) just barely opaque to all colors. All the ordi- 



