GEOPHYSICAL LABORATORY. 169 



the potentiometer principle with a constant battery, but avoiding the stand- 

 ard cell, and measuring current with a cahbrated galvanometer. Similar in 

 result but different in principle is the new Harrison-Foote instrument, where 

 the circuit resistance can be very quickly adjusted to the correct value. All 

 these special instruments avoid the main difficulty of a direct reader, namely, 

 the error from uncertain or variable resistance. It is necessary to use the 

 regular potentiometer in order to avoid also the uncertainty (perhaps 1 per 

 mille) of the cahbration of the direct reader. With a slide-wire a simple and 

 portable potentiometer is made, good to about 10 microvolts, or 0.25° with 

 most thermo-couples. The sKde-wire also permits readings to 1 microvolt, 

 though not altogether satisfactorily. Two special designs of potentiometer, 

 the Diesselhorst- Wolff and the White, enable readings to be made to 0.1 

 microvolt or better, and the White potentiometer is very Httle affected by 

 corrosive gases. Both these are deflection potentiometers, enabling part of 

 the readings to be taken direct from the galvanometer with a gain in speed 

 and without sensible error. If the potentiometer is arranged as a double 

 potentiometer, speed can be still further gained in reading different instru- 

 ments simultaneously. The precision of these potentiometers exceeds that 

 needed in ordinary pyrometry, but is useful in fundamental standardization 

 work, in calorimetry, and in numerous other apphcations of the thermo- 

 element. 



(29) The rapid electrometric determination of iron in some optical glasses. J. B. Ferguson 



and J. C. Hostetter. J. Am. Ceram. Soc, 2, 608-621 (1919). (Papers on 

 Optical Glass, No. 16.) 



The results of the apphcation of the electrometric determination of iron 

 with stannous chloride and potassium dichromate are discussed in this paper. 

 The electrometric method enables one to make rapid and accurate analyses 

 for both ferric and ferrous iron, provided interfering substances are absent. 

 Under favorable conditions, such an analysis can be made in 10 minutes and 

 may be carried out in glassware. Four different procedures are described for 

 total iron and one for ferrous iron. A number of analytical results, including 

 many ferrous-iron determinations, are given. The ferrous-iron content of the 

 glasses proved to be dependent upon a number of factors and in some cases 

 reached values in excess of 35 per cent of the total iron present. 



(30) Hematite and rutile formed by the action of chlorine at high temperatures. H. E. 



Merwin and J. C. Hostetter. American Mineralogist, 4, 126-127 (1919). 



By the action of the vapors of a gas-fired furnace on the chlorides of iron 

 and titanium at 1000° C, well-faceted crystals of hematite with a maximum 

 diameter of 4 mm. were obtained. Excellent measurements were obtained 

 which are identical with those generally accepted for natural crystals. Mi- 

 croscopic faceted rutile crystals also were obtained. Suggestions are offered 

 for the growing of crystals of these substances. 



(31) Application of the thermionic ampUfier to conductivity measurements. R. E. Hall 



and L. H. Adams. J. Am. Chem. Soc, 41, 1515-1525 (1919). 



As a part of the general plan for the investigation of two-component systems 

 under pressure, a study is being made of the changes in solubihty of a salt 

 which occur when its aqueous solution, in contact with crystals of the soUd, 

 is subjected to pressure. In order to measure the changes of concentration 

 which occur in the solution, while leaving it in situ in the pressure bomb, we 

 note the changes in the conductance of the solution. For the attainment of 

 sufficient accuracy in these measurements, the telephone which indicates the 

 balance position of the wheatstone bridge must be extremely sensitive, since 



