82 6 Handbook of Nature-Study 



The common alum we buy in crystal form, however, it is very much 

 broken. Its crystals are eight-sided and pretty. Alum is widely used in 

 dyes, in medicines, and in many other ways. It is very astringent, as every 

 child knows who has tried to eat it, and has found the lips and tongue much 

 puckered thereby 



Although we are more familiar with crystals formed from substances 

 dissolved in water, yet there are some minerals, like iron which crystallize 

 only when they are melted by heat; and there are other crystals, like the 

 snow, which are formed from vapor. Thus, substances must be molten 

 hot, or dissolved in a liquid, or in form of gas, in order to grow into crystals. 



LESSON CCXI 

 Crystal Growth 



Leading thought — Different substances when dissolved in water will 

 re-form as crystals; each substance forms crystals of its own peculiar color 

 and shape. 



Method — Take three test tubes, long vials or clear bottles. Fill one 

 with a solution made by dissolving one part of blue vitriol in three parts of 

 water; fill another by dissolving one part of bichromate of potash with 

 twenty-five parts of water ; fill another 'with one part of alum in three 

 parts of water. Suspend from the mouth of each test tube or vial, a piece of 

 white twine, the upper end tied to a tooth pick, which is placed across the 

 mouth of the vial ; the other end should reach the bottom of the vial. If 

 necessary, tie a pebble to the lower end so that it will hang straight. Place 

 the bottles on the window sill of the schoolroom, where the children may 

 observe what is happening. Allow them to stand for a time, until the 

 string in each case is encrusted with crystals ; then pull out the string and 

 the crystals. Dry them with a blotter, and let the children observe them 

 closely. Care should be taken to prevent the children from trying to eat 

 these beautiful crystals, by telling them that the red and blue crystals are 

 poisonous. 



Observations — i. In which bottle did the crystals form first? Which 

 string is the heaviest with the crystals? 



2. What was the color of the water in which the blue vitriol was 

 dissolved? Is it as briUiant in color now as it was when it was first made? 

 Do you think that the growth of the crystals took away from the blue 

 material of the water? Look at the blue vitriol crystals with a lens, and 

 describe their shape. Are the shapes of the large crystals of the vitriol the 

 same as those of the small ones? 



3. What is the shape of the crystals of the potassium bichromate? 

 What is the color? Are these crystals as large as those of the blue vitriol or 

 of the aliun? 



4. What shapes do you find among the crystals of alum? 



5. Do you think that vitriol and potassium bichromate and alvtm 

 will, under favorable circumstances, always form each its own shape of 

 crystal wherever it occurs in the world ? Do you think crystals could be 

 formed without the aid of water? 



6. How many kinds of crystals do you know? What is rock candy? 

 Do you think you could make a string of rock candy if you dissolved sugar 

 in water and placed a string in it? 



