84 AGRICULTURAL CHEMISTRY. 



Pure air, as we inhale it, contains about 1-2500 of 

 this gas ; as we exhale it, it contains 1-25, a hundred 

 times as much as before; a very good reason, but 

 only one among many, why we should not unnecessa- 

 rily subject ourselves to the process of breathing the 

 same atmosphere', over and over again — a good reason 

 also, why the sexton should drive every particle of the 

 old air out of the church between •the morning and 

 afternoon service, and why the teacher should venti- 

 late thoroughly at noon and at the forenoon and after- 

 noon recess, if not oftener. 



49. Silicic Acid (SiO') is nothing else than sand, 

 quartz, flint-stone, commonly caled Sihca. A soil in 

 which it abounds is called Silicious. It is composed of 

 1 atom of Silica, 22, to 3 of Oxygen, 24, forty-six 

 pounds of it containing 22 lbs. Silicon, and 24 of Oxy- 

 gen. It exists in the soil in two conditions, soluble and 

 insoluble. When soluble, it is taken up by plants, and 

 forms the stiffening of stems, straw, husks, &c. One 

 office of manures, and especially of potash, soda and 

 other alkalies, is to render a portion of the sand in the 

 soil soluble, so that it may be available to plants. 

 Soluble silica is essential to the perfection of most 

 plants. Oats, grown on peat, for instance, will not ma- 

 ture straw sufficiently to support the grain. Nearly 

 all soils, with the exception of peat, contain from 60 to 

 90 per cent, of silica. 



50. Mtric Acid (NO') is composed of 14 lbs. 

 of Nitrogen to 40 of Oxygen. It is a very powerful 

 p,cid, known more commonly at the shops as aquafor- 



