14 PEACTICAL FARM CHEMISTRY. 



Exposed to frost, it will turn into ice, and assume 

 the solid form. We may put a piece of ice into a 

 vessel on a liot stove. It will melt and form water; 

 and this we can boil away, until the vessel is empty. 

 Still we have not annihilated the water by driving 

 it out of the vessel. In a new form, that of steam, 

 it is floating in the air, occupying a space 1,700 

 times as large as it did in the form of water. In 

 time, this steam or gaseous water again condenses 

 and becomes liquid, and under still lower tempera- 

 ture, turns to ice. 



Here we have observed a number of changes, but 

 all these were merely changes of form or condition, 

 not of chemical composition. Water changes its 

 forms on slight provocation, but whether gas, liquid 

 or solid, it is chemically the same thing — water. 



There are other changes, however, more violent, 

 more thorough, and often more permanent than 

 those just mentioned. These are chemical changes, 

 and I can give no better illustration than by com- 

 paring them to the changes which the youngster 

 makes with his building blocks. He puts up a 

 structure of some sort; then tears it down again, 

 and uses the same material, re-arranged, in the con- 

 struction of a building perhaps altogether different. 



Water is not a simple substance, or element, as 

 supposed by the old school of philosophers who 

 named water, air, earth and fire as the four elemen- 

 tary bodies. By passing an electric current through 

 it, and by various other means, we have it in our 

 power to decompose a quantity of water; 

 chaMM^ that is, sever the intimate connection be- 

 tween its two constituents — hydrogen and 

 oxygen. These are two gaseous bodies which 

 occupy still more room than steam, so that a few 



