densimeter devised at this laboratory. In this apparatus a Torricellian
vacuum is used as a means of displacing the air surrounding the grains
of powder, and through very simple manipulation the true density of
black powder is determined with a high degree of accuracy. In Building
No. 17 there is an apparatus for separating or grading the sizes of
black powder (Fig. 1, Plate X).

By means of two factors, the moisture coefficient and the hygroscopic
coefficient, which have been worked out at this laboratory, a number of
important observations can be made on black powder, in determining the
relative efficiency of the graphite coating to resist moisture, and also
as a means of judging the thoroughness with which the components of the
powder are mixed. The moisture coefficient relates to the amount of
moisture which is taken up by the grains of the powder in a definite
time under standard conditions of saturation; and the hygroscopic
coefficient relates to the affinity of the constituents of the powder
for moisture under the same standard conditions.

Besides the examination of explosives used at the testing station, those
for the Reclamation Service, the Isthmian Canal Commission, and other
divisions of the Government, are also inspected and analyzed at the
explosives laboratory. At the present time, the Isthmian Canal
Commission is probably the largest user of explosives in the world, and
samples used in its work are inspected, tested, and analyzed at this
laboratory, and at the branch laboratories at Gibbstown and Pompton
Lakes, N.J., and at Xenia, Ohio.

Aside from the usual analysis of explosives for the Isthmian Canal
Commission, special tests are made to determine the liability of the
explosive to exude nitro-glycerine, and to deteriorate in unfavorable