the specific gravity is necessary for certain tests, such as hydrometer analysis. It is also necessary for  computations  involving  volume  and  weight relationships. The specific gravity of a soil mass can be expressed in one of three different forms as  follows: SPECIFIC GRAVITY OF SOLIDS  (G,) is the ratio of the weight in air of a given volume of soil particles  to  the  weight  of  an  equal  volume  of distilled  water,  both  at  a  stated  temperature.  The specific  gravity  of  solids  is  only  applied  to  that fraction  of  a  soil  that  passes  a  No.  4  sieve. APPARENT   SPECIFIC   GRAVITY   (Ga) is the  ratio  of  the  weight  in  air  of  a  given  volume of the impermeable portion of soil particles to the weight in air of an equal volume of distilled water, both  at  a  stated  temperature.  The  impermeable portion of a porous material, such as most large soil  grains,  includes  the  solid  material  plus  im- permeable  pores  or  voids  within  the  particles. BULK SPECIFIC GRAVITY (G~)  is the ratio of  the  weight  in  air  of  a  given  volume  of permeable  material  (including  permeable  and  im- permeable  voids)  to  the  weight  of  an  equal  volume of distilled water at a stated temperature. Sample  Selection For specific gravity tests, the soil samples may be either disturbed or undisturbed. Care must be taken,  however,  to  ensure  that  representative samples are obtained. When the sample contains both large and small particles, the sample should be separated on a No. 4 sieve. Then the specific gravity   of   the   fine   fraction   is   determined separately from the coarse fraction. A composite specific gravity for the entire soil sample is then calculated  in  the  manner  to  be  described  later. For samples smaller than the No. 4 sieve, it is easier to begin the test with an ovendried sam- ple. However, some soils, particularly those with high  organic  content,  should  be  tested  at  their natural  water  content;  the  ovendried  weight  deter- mined at the end of the test. Specific Gravity of Solids As  discussed  earlier,  the  specific  gravity  of solids is applied to soil that passes a No. 4 sieve. However, when the specific gravity is to be used in  conjunction  with  hydrometer  analysis,  it  is determined  only  on  the  fraction  that  passes  a  No. 200 sieve. In either case, the specific gravity may be  determined  for  soil  at  natural  water  content or  ovendried. APPARATUS.— A 500-milliliter (ml) volumetric flask is required for this test. For the discussion  in  this  TRAMAN,  it  is  assumed  that the flask has been calibrated. This means that the weight of the flask and water has been calibrated over a range of temperatures that would likely be encountered in the laboratory. As a matter of in- terest,  calibration  procedures  are  located  in  ASTM D  854.  Some  other  apparatus  used  to  perform  test are  as  follows: Balance,  2,000-g  capacity Balance,  200-g  capacity Cans,  moisture  content Dishes,  evaporating Funnel Mortar  and  pestle Pump,   vacuum   (optional) Stirrer,  soil  dispersion  (optional) Thermometer,  general  laboratory PROCEDURE.—  AS  mentioned  previously, you can perform the specific gravity test on soils at  natural  water  content.  When  possible,  however, you should first oven-dry the sample, as this makes it  easier  to  perform  the  test.  The  procedure  for performing the specific gravity test is as follows: 1. Record all identifying information regard- ing the sample on a data sheet similar to figure 15-33.  Also,  record  identifying  information  for the flask and dish (or moisture can) that will be used for the test. 2.  Air-or  oven-dry  the  sample  and  breakup all lumps with a mortar and pestle. About 50 g of  clay  and  about  100  g  of  coarser  samples  are the usual quantities. 3.  Weigh  and  record  the  tare  weight  of  a moisture can. Then fill the can with the dry sam- ple;  oven-dry  and  determine  the  weight  to  the nearest 0.01 g. This weight minus the tare weight is the weight of the dry soil (W.) entered in block 6g of figure 15-33. This weight is critical to the accuracy of the test. YOU MUST TAKE GREAT CARE NOT TO LOSE ANY OF THE MATE- RIAL  DURING  THE  REMAINDER  OF  THE TEST. 4.  Transfer  the  material  to  the  volumetric flask,  using  a  funnel.  Use  a  battery  filler,  or syringe,   to   CAREFULLY   wash   ALL   material from  the  can  and  funnel  into  the  flask. 5. Fill the flask two-thirds full of clean water (for exact analysis use distilled or demineralized water).  Allow  the  material  to  soak  from  4  to  6 hr,  except  for  clean,  sandy  soil,  which  does  not require  soaking. 15-26