Dry distillation is the heating of solid materials to produce liquid or gaseous products. As the products distill off, they are condensed and collected. This method usually requires higher temperatures than liquid distillation. The method can be used to obtain liquid fuels from coal and wood. It can also be used to break down mineral salts such as sulfates, in this case producing sulfur dioxide and sulphur trioxide gas which can be dissolved in water to obtain sulfuric acid. This is how sulfuric acid was first produced.