It is well known that hydrated sulphuric acid loses sulphuric anhydrid on heating, and that its specific gravity decreases in consequence. It is of some practical importance to know the exact circumstances under which this takes place, and the following experiments may prove useful in this respect. Sometimes, in distilling and subsequently evaporating sulphuric acid, I obtained a hydrate of sp. gr. 1·848, and at other times not exceeding 1·842.