THE EOYAL AHTILLEltY INSTITUTION. 
113 
but also of the varying porosity of powder, which allows each grain to burn 
more or less rapidly according as the density is less or more, and more or less 
unevenly according to the number and size of the pores. 
Porosity and density, though intimately allied, are not necessarily identical; 
and this helps to a great extent to explain some of the apparent anomalies. 
Powder of a density greater than 1*82 or 1*84 does burn uniformly; and 
Captain Castan* says that such powder when picked up unconsumed, after 
having been fired from guns, shows none of the pit marks which are to be 
found with powders of less density. These pit marks are doubtless due to 
the penetration of the flame into the porous channels in the grains, which 
allow the flame to enter further into the interior than would otherwise be the 
case, and thus accelerate the combustion. The effect of porosity, however, is 
mitigated by the amount of moisture the powder may contain, and which, from 
some cause or other, seems to have an effect on the rapidity of combustion 
much greater than would be anticipated. Possibly it may generate steam of 
a tension sufficient to counteract the pressure of the flame. It is not, how¬ 
ever, the actual amount of the moisture that affects the rate of combustion so 
much as the position it occupies. Moisture in the interior of a grain affects 
the combustion but slightly, but moisture in the exterior moderates consider¬ 
ably the rate of combustion at the period when the effects of greatest 
importance are produced. 
Waltham Abbey inch cubes of high density were thoroughly dried, and 
the mild character of the sample, 1*82, is to be regarded as entirely due to the 
high density. It is not to be compared with Messrs. Hall and Son's sample 
of the same density, which was made with a different charcoal—viz., dogwood, 
instead of alder. Some of the discrepancies obtained with the other samples, 
however, are not so easily explained, and the cases where the lower densities 
gave more moderate results than the higher are probably exceptional; though 
it possibly may be that the lower densities may have allowed the moisture to 
penetrate by absorption further into the pebbles, and that each grain was 
somewhat in a condition which may be represented in the following figure, 
where the shaded portions represent the moisture :— 
Fig. 13. Scale 
If such be the case, it is manifest that correct inferences cannot be drawn 
till the whole grain has reached its normal state of moisture, and that only 
thus can the relative effects of size of grain and density be fully determined. 
Size of grain and density are evidently the two main considerations, 
and that they to a great extent may be made to compensate for one another 
* “ Avec les poudres qui depassent 1-800 de den site, la forme semblable a celle du grain primitif 
est encore mieux conservee, et la profondeur des alveoles, diminuant tres-notablement, devient a 
peu pres nulle quand on opere sur des densites de 1-840 ou plus.”— Etude des poudres pour le 
nouveau materiel de Vartillerie de terre, par F, Castan , Capitaine d'Artillerie a la Poudrerie du 
Bouchet. 1873. 
