"Saltpetre-nitratine, as it is also known as, finds use in many things, such as fireworks."
The Box of Salitre is an item found in Prisoners of the Elder One. It is found in a pile of books in the Library, and is given to Lilly in exchange for Fairy Dust. Salitre, or sodium nitrate, can be used in small-scale explosives, which is likely why Lilly takes such keen interest in it.