72 



GRAVITATIONAL METHODS 



[Chap. 7 



and rocks are inclosed in a thin glass tube. Porous specimens are coated 

 thinly with wax, shellac, or paraffin. Clays should be weighed in machine 

 oil of high viscosity. To reduce interference due to surface irregularities, 

 fairly large samples should be used, since the surface is proportional to 

 the second power of the dimensions and the volume is proportional to the 

 third power. 



A number of balances have been designed for 

 the determination of the density of rocks by the 

 buoyancy method. Typical examples are the 

 Schwarz and Jolly balance. The former (see Fig. 

 7-la) is designed in the fashion of a letter scale 

 and may be used for light and heavy samples, two 

 ranges being provided by two lever arms and two 

 graduations. In the Jolly balance (Fig. 7-16) 

 the two pans are suspended from a coil spring 

 whose extension is read on a scale. If the speci- 

 men is placed in the upper pan and the index 

 lowered h scale divisions, and is then placed in the 

 lower pan corresponding to a reading of h' 

 divisions, the density 8 = h/{h — h'). 



4. The flotation method is used for the 

 determination of densities of very small 

 mineral specimens. A specimen is first 

 floated in a liquid of greater density which 

 is then diluted until the specimen neither 

 comes to the surface nor goes to the bot- 

 tom. At that moment the density of the 

 liquid as determined by a hydrometer is 

 equal to the density of the specimen. 

 Examples for heavy solutions are : bromo- 

 form (CHBrs), 5 = 2.9; mercury-potas- 

 sium-iodide (Thoulet's solution, 2Hgl2-- 

 2KI-3H20), 5 = 3.2; barium-mercury-iodidc (Hgl2-2BaI), 5 = 3.59; and 

 thalliumformiate, 5 = 4.76 at 90° C. 



Fig. 7-la. Schwarz bal- 

 ance for determining spe- 

 cific gravity [(after Keil- 

 hack). 



Fig. 7-16. Jolly balance. 



B. Factors Affecting Formation Densities 



In an evaluation of the applicability of gravitational methods to a given 

 geologic problem, and in the calculation of the effects of geologic bodies 

 of definite properties, it is necessary to make reasonable assumptions in 

 regard thereto or to extrapolate from tabulated values when samples for 

 the desired depth or locality are not available. In order to arrive at 

 correct values, an evaluation of the influence of changes in chemical compo- 



