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then it would be a diffusion. But while the chalk was thus mixed with the liquor, it would lose its transparency, and not recover it again, till by standing, the chalk had all subsided, and left the liquor as it was before.
Pupil. How is the cream mixed with the tea ?
Tut. Why, that is only diffused, for it takes away the transparency of the tea. But the particles of cream being finer and lighter than those of chalk, it remains longer united with the liquor. However, in time the cream would separate too, and rise to the top, leaving the tea clear. Now, suppose you had a mixture of sugar, salt, chalk, and tea-leaves, and were to throw it into water, either hot or cold;—what would be the effect ?
Pup. The sugar and salt would be dissolved and disappear. The tea-leaves would yield their colour and taste. The
chalk -I do not know what would become of that.
Tut. Why, if the mixture were stirred, the chalk would be diffused through it, and make it turbid or muddy ; but on. standing, it would leave it unchanged.
Pup. Then there would remain at bottom the chalk and tea-leaves.
Tut. Yes. The clear liquor would contain in solution salt, sugar, and those particles of the tea, in which its colour and taste consisted; the remainder of the tea and the chalk would lie undissolved.
Pup. Then I suppose tea-leaves, after the tea is made, are lighter than at first.
Tut. Undoubtedly. If taken out and dried they would be found to have lost part of their weight, and the water would have gained it. Sometimes, however, it is an extremely small por'tion of a substance which is soluble, but it is that in which its most remarkable qualities reside.
Thus a small
piece of spice will communicate a strong flavour to a large quantity of liquid, with very little loss of weight.
Pup. Will all liquors dissolve the same things ?
Tut. By no means. Many dissolve in water, that will not in spirit of wine; and the contrary. And upon this difference many curious matters in the arts are founded. Thus, spirit varnish is made of a solution of various gums or resins in spirits that will not dissolve in water. Therefore, when it has been laid over any surface with a brush, and is become dry, the rain or moisture of the air will not affect it. This is the case with the beautiful varnish laid upon coaches. On the other hand, the varnish left by gum water could not be washed off by spirits.
Pup. I remember when I made water, upon setting the cup in a warm place, it all dried away, and left the
gum just as it was before. Would the same happen if I had sugar or salt dissolved in water?
Tut. Yes, upon exposing the solution to warmth, it would dry away, and you would get
sugar in a solid state as before.
Pup. But if I were to do so with a cup of tea, what should I get?
Tut. Not tea-leaves, certainly! But your question requires a little previous explanation. It is the property of heat to make most things fly off in vapour, which is called evaporation, or exhalation. But this it does in
different degrees, to different substances. Some are very easily made to evaporate; others very difficultly; and others not at all by the most violent fire we can raise. Fluids in general are easily evaporable ; but not equally so. Spirit of wine flies off in vapour much sooner than water; so that if you had a mixture of the two,
by applying a gentle heat you might drive off all the spirit, and leave the water pure. Water, again, is more evaporable than oil. Some solid substances are much disposed to evaporate, Thus, smelling salts by a little heat may entirely be driven away in the air. But in general, solids are more fixed than fluids; and therefore when a solid is dissolved in a fluid, it may commonly be recovered again by evaporation. By this operation common salt is got from sea-water and salt springs, both artificially, and in hot countries by the natural heat of the sun. When the water is no more than is just sufficient to dissolve the salt, it is called a saturated solution, and on evaporating the water further, the salt begins to separate, forming little regular masses called crystals. Sugar may be made in like manner to form crystals, and then it is sugar-candy.