Water in All Substances.—Water was considered by the ancient philosophers as one of the four elements out of which all visible objects were constructed ; and, in reality, it enters to a greater or less extent into the composition of nearly all natural substances. Thus, for example, some vegetables, like cabbage or celery, contain as much as ninety-five per cent. of water ; and, on the other hand, close-grained marble may contain as much as four per cent. of water, or almost a quart to the cubic yard. On account of its remarkable solvent powers, which enable it to take up a smaller or larger quantity of nearly every substance with which it comes in contact, water is never found pure in a natural state; and, indeed, absolutely pure water for chemical purposes can only be obtained by repeated careful distillations.

Its hard to imagine that for your every day person, the concept that liquids were often comprised of water as opposed to be a completely different substance all together was relatively new.  People may have suspected, but many scientists had not confirmed (or reconfirmed) this fact and they were just beginning to put together the concept of atoms and how they might form together.  So for some people this apparently obvious paragraph today might have been as foreign if not more to them as a digital cameras function might have been 50 or 100 years ago.