In former days the salt used for household and mercantile purposes was almost entirely prepared by the evaporation of sea-water. We no longer depend on this, however; and in England the sea-salt trade has gone down greatly before that of rock-salt, which is found to be the better for table use. It has not the same tendency to stick together in lumps, after being packed in sacks. Great districts of rock-salt are found in many places--such as those in the Carpathian Mountains, in the Swiss Alps, in Germany, and in Great Britain. One huge mine in Galicia has been worked for six hundred years; and this supply is said to reach through about five hundred miles. From British works alone the quantity carried away every year amounts to a cubic mile of salt. But land-supplies grow pale and insignificant before the quantities which float in the ocean. It has been reckoned that, if the waters of the whole ocean could be dried up, the amount of salt left lying on the ocean-bed would be something like four-and-a-half millions of cubic miles. Such an enormous mass hardly conveys a clear idea. Let us think of one single cubic mile of sea-water, separated from the ocean, and see how much it would contain. First, the whole of that cubic mile of water has to be dried up; and then the materials left behind have to be weighed. We should find about thirty-three millions of tons of various kinds of substances, the names of which need not be given. We should also find of common salt a supply which, when weighed, would reach the great figure of _one hundred and seventeen millions of tons_. All this, be it remembered, floating unseen in a single cubic mile of water. No wonder the sea tastes salt. In its make Water is always the same. Whether it be cold or hot, freezing or boiling, causes no difference. It consists of two gases, united; and the union is remarkable in kind. The two gases are not merely mixed together, as sand and sugar may be mixed. They are by the union changed into a fresh substance. For the time the gases exist no longer. In their stead, water has been formed. And when the gases enter into this very close relationship, they do it always in the same manner. There is just so much of the one, and just so much of the other. One portion of hydrogen has to join with eight times as much by weight of oxygen--neither more nor less of either. The same is true, whether we are speaking of a great mass of water, or of the tiniest speck. It is not actually correct to say, as is said above, that water “consists” of the two gases. So long as the water exists, the gases do not exist. And when, through the action of heat or electricity, the water is broken up and the gases reappear, then the water no longer exists. But at least we may say that it is the result of the uniting of those two gases, and that it can be made in no other way.