64 TALKS ABOUT THE SOIL. 



and plants must have it in abundance, or they cannot 

 exist. 



The chemist tells these few facts, and we now dis- 

 cover the meaning and value of many of the observa- 

 tions we have already made. If plants must have 

 oxygen, we see the value of the rain and the atmos- 

 phere ; for these are composed in large part of oxygen. 

 If oxygen is greedy to combine with other elements 

 to form new compounds, and these compounds, as we 

 shall learn presently, are useful to all plants, we begin 

 to see the value of a light sandy soil that permits air 

 and water to pass through it and reach these elements. 

 We also begin to see why we must open the top of 

 the window or top of our greenhouse where plants are 

 growing, that the fresh air may reach them, and supply 

 them with oxygen. 



2. Hydrogen. This is another gas without taste 

 or smell. It will combine with oxygen, and burns 

 furiously with a pale blue flame and much heat. When 

 pure it is used in balloons, and in tanks it is often sold 

 for use in lime-lights. It forms a part of street-gas, 

 as the blue flame of a gas-stove plainly shows. It 

 unites with other elements to form some of the most 

 important materials in all soils and in all plants. 

 It unites with oxygen to form water, and in this 

 shape spreads through the soil, dissolving other ele- 

 ments and compounds of elements, and making 

 them fit for plant-food. It also combines with other 

 compounds, to make still other compounds useful to 

 plants. 



