The easiest method of obtaining crystals of any substance which is soluble in water is to make a saturated solution in this liquid, to put a small drop upon a slide, then to tilt the microscope slightly so that there is a thin film of solution at the upper side of the drop. The microscope must not be tilted so much that the liquid runs from the slide on to the stage. Where the film of liquid is thin crystals will be found first. A word of explanation is necessary concerning the term saturated solution, especially as we may have occasion to use it many times. When we add a solid to a liquid in which it is soluble, we shall find that the liquid will take up a certain amount of the solid and no more; when, on the addition of more solid it fails to dissolve, we have reached the saturation point. A saturated solution then is a liquid in which the maximum amount of solid is dissolved. We have already described the most simple way in which crystals may be formed, and we may easily make objects for our microscope in this manner of all the substances we can find which are soluble in water. With some we shall find that crystals do not form easily, in which event we may modify our tactics and warm one drop of saturated solution on the slide till nearly all the moisture is driven off, then there should be no difficulty in watching the crystals in process of formation. Common salt, sugar, alum, borax, washing soda, iron sulphate, called also green vitriol and copper sulphate, or blue vitriol, are common and easily obtained substances, all soluble in water. As we carry our experiments a little further we shall find that crystals formed from cold solutions as suggested in our first method differ from those formed from hot solutions. Again, if we use some other substance than water as our solvent the crystals which separate out will differ once more. Many very interesting experiments may be tried on these lines.