A word of warning is necessary concerning this and the following experiments; they may not always succeed, for success largely depends upon having the solutions at the right strength, and experience alone can teach us what is correct. Interesting and easily formed crystals may be obtained from barium sulphate, and the experiment may also be made to show the phenomenon which we have already mentioned, that the form of the crystals depends on the nature of their solution. A little barium sulphate should be dissolved in strong sulphuric acid. Here, by the way, another warning: every care should be taken in the use of all acids; they should never be allowed to come into contact with face, hands or clothes, nor should they touch any part of the microscope. Some acids give off fumes, and these should not be allowed to reach the eyes or nose, and the microscope must be protected from them. To continue our experiment: While hot a drop of the solution of barium sulphate in sulphuric acid should be transferred by means of a glass rod to a slide and allowed to cool. Examination with the microscope will show that the barium sulphate has formed small rectangular scales. With the remainder of the sulphuric acid we now make a saturated solution of barium sulphate, and find, on repeating the method described above, that the chemical has formed curious x-shaped crystals. Similar experiments may be tried with calcium sulphate with the certainty of interesting results. Many crystals of calcium, in the form of calcium oxalate, may be found in plants, and they are well worth looking for. They may best be seen in sections of the plants, but, if we have not mastered the art of cutting sections, we may find them by teasing the plant cells apart with our mounted needles. In the stems of rhubarb there is the substance in bundles of long needle-shaped crystals, to which the name of raphides has been given. In the seed of the garden poppy, just below the skin, there is a layer known as the crystal layer, where crystals of calcium oxalate occur as tiny balls called crystal sand. In the leaf stalks of begonias very beautiful and occasionally very large crystals of this substance may be found, whilst in shapes they are strikingly varied. In orris root there are enormous crystals of calcium oxalate; in fact it is common in many plants.