But when the cannon is fired,.e., when these very subtle and highly agitated particles have surrounded the coarse and solid parts of the saltpeter and have communicated their strongest and most violent motion to them, then the whole must explode, because the pores of the cannon (which left free passage in every direction to the subtle particles of which we speak when they were alone) are not large enough to give passage to the coarser particles of saltpeter, and to certain other particles of which the powder is composed, once they have received the agitation of the subtle particles that surround them.