This is a proces called sublimation = directly from solid to vapour.
Some compounds evaporate without melting first. (Even ice does, but only at sub-zero temperatures.)
Campher is one of the compounds that doesn't melt at room temperature, but evaporates.
Another one is solid carbon dioxide, that's why it's called "dry ice".
Under different conditions of temperature and pressure, both compounds have a liquid "in between" phase.