THE ELEMENTS OF PLANT FOOD 55 



Calcium Sulphate (CaSO 4 ). --This compound is 

 found in the mineral form, anhydrous, but a much more 

 familiar form is its combination, with H 2 O, called gyp- 

 sum. When gypsum is heated to about 250 F., it 

 loses a portion of the water bound up in its crystals, 

 called water of crystallization, and when ground into a 

 powder, is called plaster of Paris. Gypsum, either 

 burned or unburned, is called plaster or land plaster, 

 and has been used extensively as a fertilizer. As such 

 it has much the same effect as lime, making the potas- 

 sium compounds more soluble. It may be used to 

 great advantage as an absorber of ammonia on the 

 floors of stables and under the roosts in poultry houses. 



Note. Plaster of Paris is a white powder used in making the 

 putty coat, or outside coat, of plaster on walls, in making casts of vari- 

 ous objects, and as a cement for sticking glass and metal, sucn as the 

 brass rings to the top of lamps. When plaster of Paris is mixed to a 

 paste with water, it sets with increase of volume. This characteristic 

 makes it particularly valuable in making casts and taking copies of 

 medallions. 



MAGNESIUM (Mg) 



Description and Occurrence. Magnesium is a metal 

 of a silver-white appearance which is easily tarnished 

 in moist air. It is prepared for commerce either as 

 a ribbon or as a powder. The ribbon, when burned, 

 produces a very bright light, having much the same 

 effect as sunlight. In the powdered form it is used 

 to make the flash light for taking photographs in 

 darkened rooms. 



Magnesium is an essential element for the growth 

 of plants, but the compounds of magnesium found in 

 the soil are everywhere so plentiful that plants never 

 suffer for the want of it. 



