“‘How is it made?’
“‘There are three ways of obtaining it:—First, in warm climates, where the sun is strong, the sea-water is collected into shallow pools, and there left until it is evaporated by the sun’s rays. The ground where these pools are made must neither be muddy nor porous, else the salt would get mixed with the mud and sand. Of course the people who manufacture it in this way take care to choose firm, hard ground for the bottoms of their pools. There are sluices attached to these pools by which any water that may not evaporate is drawn off. Salt is made in this manner in many southern countries—in Spain and Portugal, in France, and other countries that lie around the Mediterranean; also in India, China, Siam, and the island of Ceylon.
“‘The second way of making salt from sea-water is precisely the same as that I have described—except that, instead of these artificial pools, the evaporation takes place in broad tracts of country over which the sea has spread in time of high springtides. When the sea falls again to its proper level, it leaves behind it a quantity of water in these tracts, which is evaporated by the sun, leaving behind it fields of pure salt. Nothing remains to be done but to scrape this salt into heaps and cart it off; and at the next spring-tide a fresh influx of sea-water produces a new crop of salt, and so on. This kind is better than that which is made in the artificial pools—though neither of them is equal to the salt of the mines. They are both known in commerce under the name of “bay-salt,” to distinguish them from the “rock-salt” of the mines. Great natural beds of the bay-salt are found in the Cape de Verde islands; also in Turk’s island and Saint Martin’s in the West Indies, and on Kangaroo Island, near the coast of Australia.
“‘There is still a third plan of making salt out of the sea. That is, by boiling the water, as we are doing; but this makes the worst of all salt; besides, it is far more expensive to manufacture salt in this way than to buy it from other countries. Indeed, this last plan would never be adopted, were it not that some foolish governments force their people to pay a heavy duty for importing salt into their country, thus making it still cheaper for them, costly as it is, to manufacture the article at home.’
“‘What makes the sea salt, papa?’
“‘That is one of the phenomena about which naturalists have a difference of opinion. Some of them say there are vast beds of salt at the bottom which keep the water always impregnated. I think this notion is very childish; and they who hold it offer only childish arguments to support it. Others assert that the salt water of the ocean is a primitive fluid—that it was always as it now is—which you will perceive is giving no reason at all, more than saying, “it is salt, because it was salt always.” This is an equally irrational theory. Others, again, believe that the saltness of the ocean is caused by the flowing into it of salt rivers. These, I think, hold the true opinion; but unfortunately they have failed, as far as I know, to answer the objections which have been raised against it. Your papa has reflected a good deal upon this subject, and believes that he can explain away all the difficulties that oppose this last theory. Probably he may take an early opportunity of doing so; but it will require more time than he can spare at present.’
“‘Is the sea equally salt at all places?’ inquired the philosopher Frank, after a short pause, during which he had been busily reflecting on what had been said.
“‘No; it is more so at the Equator than in the colder regions around the Poles. It is less salt in gulfs and inland seas than in the open ocean. This I believe I can also explain, because it would support the theory regarding the rivers of which we have just spoken. The difference of saltness in different parts of the sea is, however, very trifling.’
“‘How much salt is there in the sea-water?’
“‘Three and a half per cent nearly. That is, if you boil down one hundred pounds of sea-water, it will yield you about three pounds and a half of salt.’