38 BULLETIN 314, U. S. DEPARTMENT OF AGRICULTURE, 
porcelain evaporating dish and evaporated en a steam bath. The 
most scrupulous care must be taken at all times that no fiames are 
in its immediate vicinity. Evaporati on is carried on st a gentle 
heat, with continual stirring, until foaming practically ceases. It is 
advisable te have a large watch glass at hand to smother the flames 
quickly should the material ignite. As the foaming subsides, the 
heat of the steam bath may be gradually raised, and evaporation is 
continued until the bubbles beaten or stirred to the surface of the 
bitumen fail to give a blue flame or odor of sulphur dioxide when 
ignited by a small gas jet. The dish of bitumen should then be set 
in a hot-air oven maimtained at 105° ©. for about an hour, after which 
it is allowed to cool. Its general character is noted and any tests 
for bitumens that are necessary are then made upon it. 
GRADING THE MINERAL AGGREGATE. 
EQUIPMENT. 
1 set of 8-inch stone sieves with circular Speen oi 14, 4, 1, 2, §, and } imehes, 
respectively. — 
1 set of 8-inch brass sand sieves of 10, 20, 30, 40, 50, 80, 100, and 200 mesh, respectively, 
with pan and cover. 
1 rough balance, capacity 1 ae ae sensitive to 0.1 gram. 
i isinch stiff flat brush. 
Several sheets of manila paper. 
METHOD. 
While a mechanical sifter is employed in the Office of Public 
Roads and Rural Engineering, the fellowing hand method is given 
for the benefit of those who have not a machine of this character 
available. When a machine is used its method of operation should 
be checked against the hand methed described below to obtain 
practically equivalent results. 
For aggregates contaiming particles too large to pass a 10-mesh 
screen, the stone sieves are used, and are stacked in their regular order 
over a sheet of heavy paper, with the largest size required on top. 
The weighed amount of stone is placed on the largest sieve and is care- 
fully protected from drafts which might carry away any of the fine 
material. The upper sieve is then removed from the stack and shaken 
over a large sheet of paper until no more particles come through. 
The material thus retained, including any fragments caught in the 
meshes of the sieve, is weighed and that which passes is added to the 
contents of the succeeding sieve. This operation is repeated with 
each succeeding sieve. 
When grading sands or fine aggregates, it is customary to take a | 
i00-gram sample in order that the weights may give direct percentages 
to tenths of 1 per cent, The sieves are stacked in regular order with 
the 200-mesh sieve resting on the pan. xe sample is brushed on the 
