“‘Oh! I should like so much to see them!’ cried Harry, in a transport.
“‘But, papa,’ inquired Frank, who always sought after information on such subjects, ‘I never saw any of this rock-salt. How is it that it comes to us always crushed, or in great bricks, as if it had been baked? Do they break it fine before it is sent to market from the mines?’
“‘In some of these mines nothing more is required than to crush the rock; in others, however, the rock is not pure salt, but mixed with other substances, as oxide of iron and clay. In these cases it is first dissolved in water, to separate it from such impurities, and then evaporated back again into salt, precisely as we are doing now.’
“‘What colour is the salt-rock, papa?’
“‘When pure it is white; but it assumes various colours, according to what substances may be found mixed with it. It is often yellow, and flesh-coloured, and blue.’
“‘How pretty it must be!’ exclaimed Harry; ‘like precious stones, I declare.’
“‘Yes, it is a precious stone,’ rejoined his brother; ‘more precious, I take it, than all the diamonds in the world. Is it not, papa?’
“‘You are quite right,’ I replied. ‘Salt-rock is more valuable to the human race than diamonds; though they, too, have an absolute value, besides their value as a mere ornament. There are some important uses in arts and manufactures to which they can be applied.’
“‘But, papa,’ again inquired Frank, determined to know everything he could about the article of salt, ‘I have heard that salt is made of sea-water. Is it so?’
“‘Vast quantities of it.’