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 portion 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 gum-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 back your salt and 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 very 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 almost all the spirit, while the greater part of the water would remain. Water, again, is more evaporable than oil. Some solid substances are much disposed to evaporate: thus, smelling salts may by a little heat be entirely 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 seawater 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.
Pup. But what is a sirup?
Tut. That is when so much sugar is dissolved as sensibly to thicken the liquor, but not to separate from it. Well—now to your question about tea. On exposing it to considerable heat, those fine particles in which its flavour consists, being as volatile or evaporable as the water, would fly off along with it; and when the liquor came to dryness, there would be left only those particles in which its roughness and colour consist. This would make what is called an extract of a plant.
Pup. What becomes of the water that evaporates?