SAMPLING AND TESTING HIGHWAY MATERIALS. 31 
and filtered (to remove a little aluminum and iron and perhaps 
calcium). When cool, 10 cubic centimeters of a saturated solu- 
tion of sodium-ammonium-h.\ drogen phosphate shall be added 
with constant stirring. When the crystalline ammonium-mag- 
nesium orthophosphate has formed, ammonia shall be added in 
moderate excess The solution shall be set aside for several 
hours in a cool place, filtered and washed with water containing 
2.5 per cent of NH 3 . The precipitate shall he dissolved in a 
small quantity of hydrochloric acid, the solution diluted to about 
100 cubic centimeters. 1 cubic centimeter of a saturated solution 
of sodium-ammonium-hydrogen phosphate added, and ammonia 
drop by drop, with constant stirring, until the precipitate is again 
formed as described and the ammonia is in moderate excess. 
The precipitate shall then be allowed to stand about two hours, 
filtered and washed as before. The paper and contents shall be 
placed in a weighed platinum crucible, the paper slowly charred, 
and the resulting carbon carefully burned off. The precipitate 
shall then be ignited to constant weight over a Meker burner, 
or a blast not strong enough to soften or melt the pyrophosphate. 
The weight of magnesium pyrophosphate obtained multiplied by 
72.5 gives the percentage of magnesia. The precipitate so ob- 
tained always contains some calcium and usually small quanti- 
ties of iron, aluminum, and manganese as phosphates. 
27. A permissible variation of 0.4 will be allowed, and all re- 
sults in excess of the specified limit but within this permissible 
variation shall be reported as 5.00 per cent. 
VIII. DETERMINATION OF SPECIFIC GRAVITY. 
28. The determination of specific gravity shall be made with Specific 
a standardized Le Chatelier apparatus. This apparatus is stand- gravity. 
ardized by the United States Bureau of Standards. Kerosene 
free from water, or benzene not lighter than 62° Baurne. shall 
be used in making this determination. 
29. The flask shall be filled with either of these liquids to a 
point on the stem between zero and 1 cubic centimeter, and 04 
grams of cement, of the same temperature as the liquid, shall be 
slowly introduced, taking care that the cement does not adhere 
to the inside of the flask above the liquid and to free the cement 
from air by rolling the flask in an inclined position. After 
all the cement is introduced, the level of the liquid will rise to 
some division of the graduated neck : the difference between read- 
ings is the volume displaced by 64 grams of the cement. The 
specific gravity shall then be obtained from the formula 
Specific gravity ^ ei ? ht ( ' f cement ( ^ ams) 
Displaced volume (cubic centimeters) 
30. The flask, during the operation, shall be kept immersed in 
water, in order to avoid variations in the temperature of the 
liquid in the flask, and any variation that occurs shall not exceed 
0.5° C. The results of repeated rests should agree within 0.01. 
31. The determination of specific gravity shall be made on the 
cement as received: if it falls below 3.10, a second determination 
shall be made after igniting the sample as described in section 20. 
IX. DETERMINATION OF FINENESS. 
32. Wire cloth for standard sieves for cement shall be woven Fineness. 
(not twilled) from brass, bronze, or other suitable wire, and 
mounted without distortion on frames nol less than 1\ inches 
below the top of the frame. The sieve frame- shall be cir- 
cular, approximately 8 inches in diameter, and may be provided 
with a pan and cover. 
33. A standard No. 200 sieve is one having nominally a 0.0< 
inch opening and 200 wires per inch, standardized by the United 
States Bureau of Standards, and conforming to the following re- 
quirements ; 
