162 CARNEGIE INSTITUTION OF WASHINGTON. 



concentration of the equilibrium solution (see No. 4, "The measurement of 

 the freezing-point depression of dilute solutions"), some trouble was expe- 

 rienced at first in obtaining thoroughly concordant readings; this was found to 

 be due to an alteration of the achromatic reference fringe produced by differ- 

 ences in optical dispersion. In order to guard against error from this source 

 it proved necessary to investigate the relationships in order to derive formulae 

 from which the exact amount of shift of the achromatic fringe could be pre- 

 dicted. The appropriate formulae for this type of instrument have apparently 

 not been worked out heretofore; consequently it has seemed worth Avhile to 

 call attention to these relationships and to put the formulae on record so as to 

 save trouble to future users of this most useful type of instrument. By means 

 of these formulae, one is enabled to interpret the readings of the instrument in 

 terms of refractive-index differences and to predict in advance the exact 

 amount of shift of the achromatic reference fringe due to differences in optical 

 dispersion. {Cf. No. 8, "The use of the interferometer for the analysis of 

 solutions.") 



(8) The use of the interferometer for the analysis of solutions. L. H. Adams. J. Am. 



Chem. Soc, 37, 1181-1194 (1915). 



Chemists have long used the refractometer as an aid in analytical work, 

 but have not made use of the interferometer to the extent that its precision 

 and general convenience would warrant. The use of the ordinary forms of 

 refractometer is limited by the circumstance that the change of refractive 

 index with temperature is usually such as to require regulation of temperature 

 to 0.01° in order to secure an accuracy of one unit in the sixth place in the 

 measurement of refractive index. By means of the interferometer, on the 

 other hand, it is a simple matter, requiring no special regulation of temperature, 

 to secure an accuracy of one unit in the seventh place; this is possible because 

 in the latter case we are comparing the refringence of one liquid (or gas) with 

 that of another of very nearly the same composition and hence possessing 

 almost the same temperature coefficient of refringence. In other words, with 

 the refractometer one can determine the composition of a solution to 2 parts 

 in 10,000 of solvent, but with the interferometer — provided that certain 

 simple precautions be observed — to 2 parts in 1,000,000. The interferometer 

 is adapted to the determination in any transparent mixture of a single varying 

 component; this component may be solute or solvent, electrolyte or non- 

 electrolyte, indeed, any substance which will not attack the instrument. 



This paper presents a brief description of a convenient form of interferom- 

 eter for chemical purposes, discusses its mode of operation, and points out 

 the precautions to be observed in its use. The only important source of error 

 arises from differences in optical dispersion; this can, however, be readily 

 obviated by the use of the methods recommended. 



(9) A vacuum furnace for the measurement of small dissociation pressures. R. B. Sosman 



and J. C. Hostetter. J. Wash. Acad. Sci., 5, 277-285 (1915). 



This paper describes a vacuum furnace and accessory apparatus adapted 

 to the measurement of a wide range of dissociation pressures of iron oxides 

 and silicates. The furnace consists essentially of two parts: (1) the furnace- 

 tube, which serves both as the furnace-wall inclosing the "inside vacuum" 

 and as the heating element; (2) the water-cooled jacket which surrounds the 

 furnace tube and incloses the "outside vacuum." The furnace-tube is 15 

 mm. inside diameter, and made of an alloy of 80 parts platinum and 20 

 rhodium. Three gages provide a possible range of pressure measurement 

 from 0.000001 mm. mercury up to 2.5 atmospheres. The uniformity of tem- 



