In our world, it is widely accepted that black powder was created by the Chinese in an attempt to find a compound for immortality. What if this doesn't happen, and gunpowder isn't invented until at least a good understanding of atoms, molecules, and generally why certain compounds work as they do, in the late 19th century or beyond. Presumably, at this point, it would be reasonable that chemists would be able to devise black powder and other explosive compounds using atomic theory (I'm not a chemist so if this point is earlier please correct me).
Barring any accidents before this point in time/scientific advancement, what would weapons technology look like right before this point? Presumably, technologies such as the steam engine would already be created, and metallurgy has advanced much since then, so I can see some interesting development for warfare.