158 CARNEGIE INSTITUTION OF WASHINGTON. 



drate, Fe203.H20, has definite and reproducible properties as a crystal- 

 line mineral. It occurs, however, in two crystalline modifications, 

 goethite and lepidocrocite. 



It is characteristic of hydrated ferric oxide that it occurs quite 

 commonly in finely fibrous crystalline forms, as well as in an "amor- 

 phous" form. In the "amorphous" form it is in a condition so fine- 

 grained that its crystalline character — or sometimes even the fact that 

 it possesses any crystalHne character — can not be determined. In 

 both the fibrous and amorphous forms the oxide holds variable amounts 

 of adsorbed and capillary water. Limonite is the amorphous mono- 

 hydrate having associated with it this adsorbed and capillary water; 

 while turgite seems to be a solid solution of the crystalline mono- 

 hydrate, goethite, with the anhydrous, oxide hematite, the solid solu- 

 tion holding in addition adsorbed and capillary water. An amorphous 

 or crj^Dto-crystalline hydrate having a given percentage of water may 

 thus be either a limonite or a turgite, depending on the internal con- 

 stitution of its fibers or grains. 



Considerable progress has been made on the ternary system FeaOg— 

 SO3-H2O, which involves hematite as well as the hydrated oxides, but 

 the work will not be in press in time for inclusion in this report. 



PUBLICATIONS. 



Brief reviews of the papers pubhshed by members of the Laboratory 

 staff during the current year follow : 



(1) The identification of "stones" in glass. N. L. Bowen. J. Am. Ceram. Soc, 1, 



594-605 (1918). (Papers on Optical Glass, No. 4.) 



The petrographic microscope is a convenient and efficient instrument for 

 the determination of the nature and origin of "stones" or crystalline particles 

 occurring in glass. Stones are divided into four classes: (1) pot stones, (2) 

 batch stones, (3) crown drops, (4) devitrification stones. These classes have 

 distinctive features of structure and texture that are revealed by the micro- 

 scope. Moreover, the ciystalline phases contained in stones can be identified 

 by a determination of their optical properties. The results of a study of 

 stones by these methods are given in this paper. 



(2) A contribution to the methods of glass analysis, with special reference to boric acid and 



the two oxides of arsenic. E. T. Allen and E. G. Zies. J. Am. Ceram. Soc, 

 1, 739-786 (1918). (Papers on Optical Glass, No. 5.) 



Arsenic. — An accurate method for the separation and determination of 

 both trivalent and pentavalent arsenic in glasses is described. (1) The 

 separation depends on the volatilization of the trivalent arsenic as AsFs when 

 the glass is heated with hydrofluoric and sulphuric acids, while the penta- 

 valent arsenic remains in the residue. (2) The latter is determined by pre- 

 cipitation as sulphide which is then oxidized to arsenic acid, reduced by 

 hydriodic acid, and titrated with a standard iodine solution. (3) The triva- 

 lent arsenic is determined by difference between the pentavalent and total 

 arsenic. The determination was controlled by direct determination with the 

 aid of a platinum still. (4) The total arsenic is determined by fusing the 

 glass with sodium carbonate and niter, removing the sihca and excess nitric 



