250 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
The first of these includes black gunpowder, smokeless powder, and 
black blasting powders; the second includes dynamite, nitroglycerin, 
guncotton, most of the ‘‘ permissible explosives,’’ and some blasting 
powders; the third class comprises chiefly fulminates and chlorates. 
For all classes the effect of explosion is dependent upon the quan- 
tity of gas and heat developed per unit of weight and volume of the 
explosive, the rapidity of the reaction, and the character of the con- 
finement, if any, given the explosive charge. 
Low explosives—The rapidity of reaction varies greatly with 
different explosive substances and with the manner in which the 
explosion is started. For certain explosives, such as smokeless 
powder, the explosion does not differ in principle from the burning of 
a piece of wood or other combustible. The combustion is very rapid 
but is a surface action proceeding from layer to layer until the grain 
is consumed. Such materials are known as low or progressive 
explosives, although the total power developed through the com- 
bustion of a unit weight may be very great and would be destructive 
unless properly controlled. 
High explosives—The combustion of another class of materials 
called high explosives is extremely rapid wher. suitably inaugurated, 
such action being known as a detonation. 
In these explosives, such as nitroglycerin, guncotton, the picrates, 
etc., the progress of the explosive reaction is not by burning from 
layer to layer, as described above, but the breaking up of the initial 
molecules gives rise to an explosive wave which is transmitted with 
ereat velocity in all directions throughout the mass and causes its 
almost instantaneous conversion into gas. The velocity of propaga- 
tion of the detonating wave has been determined for some materials 
to be more than 20,000 feet per second, or, approximately, 4 miles 
per second. A charge 1 foot long would thus be converted into gas 
in the very short interval of five one hundred thousandths of a 
second. The progressive emission of gas from a low explosive such 
as burning gunpowder produces a pushing effect upon a projectile 
throughout its movement without necessarily overstraining the gun, 
whereas the sudden conversion of an equal weight of material into 
gas, as would happen with a high explosive such as dynamite or nitro- 
glycerin would develop such high pressure and shattering effect as to 
rupture the gun. 
Fulminates—The action of fulminates is much more brusque and 
powerful than even that of the class of explosives just described. As 
they can be readily detonated by Shock or by the application of heat, 
they are used in primers and fuses to start the action of both the low 
and the high explosives. The most common fulminate is made by 
dissolving the metal mercury in strong nitric acid and pouring the 
solution into alcohol. After an apparently violent reaction there is 
