THE CHEMISTRY OF THE ANIMAL BODY. 519 



roots of trees, and ultimately much of this combines with mineral matter, or 

 contributes to form shells in marine life. 



Silicon, Si = 28. 



Silicon is found in the ash of plants, and in traces in the cells and tissues of 

 animals, being a constant constituent of hen's eggs. It appears in traces in the 

 human urine, and in considerable quantity in herbivorous urine. It is especially 

 present in hair and feathers. It does not seem to be of great importance to the 

 life of the plant, for if corn-stalks, whose ash usually contains 20 per cent, of 

 silica (Si0 2 ), be grown in a soil free from it, the plant flourishes though only 

 0.7 per cent, of silica is found in the ash, this having been derived from the 

 vessel holding the soil. 



Silicon Dioxide, or Silica, Si0 2 . — This is the oxide of the element, and is found in 

 quartz and sand, but not in the organism. 



Silicic Acids. — The ortho-silicic acid (H 4 SiOJ is formed by the action of an acid on a 

 metallic silicate, 



Ca.SiO* + 2H 2 C0 3 = 2CaC0 3 + H 4 Si0 4 . 



This reaction takes place in the soil, and the silicic acid so obtained is soluble in water and 

 is a colloid — that is to say, is of gelatinous consistence, will not crystallize, and does not 

 osmose through vegetable and animal membranes. However, it is in this form or in the 

 form of soluble alkaline silicate that it is probably received by the root of the plant. 1 



Metasilicic acid has the formula H 2 Si0 3 , while the polysilicic acids (H 2 Si0 5 ,H 6 Si 2 7 , etc. ) 

 are numerous, and constitute the acid radicals of most mineral silicates. If silicic acid be 

 evaporated and dried, it leaves a gritty residue of silica. 



The Metallic Elements. 

 The metals in the body are the alkalies potassium and sodium, the alkaline 

 earths calcium and magnesium, and the heavy metal iron. 



Potassium, K = 39. 



Potassium salts are found especially in all animal cells (see p. 499), and in the 

 milk which is manufactured from the disintegration of such cells. They are 

 found in the blood-corpuscle sometimes to the almost complete exclusion of ■ 

 sodium salts. Only to a small extent do they occur in the fluids of the body 

 and in the blood plasma (K 2 O = 0.02 per cent, in plasma). They are excreted 

 in the urine. Potassium salts are retained on the surface of the ground for the 

 use of vegetation, and occur in the plant not only as inorganic but also as 

 organic salts (tartrate, citrate, etc.). 



Potassium Chloride, KC1. — Potassium chloride is a constant constituent 

 of all animal cells and tissues, and may be absorbed with the food <>r be pro- 

 duced in the body after eating potassium carbonate or phosphate, since these salts 

 may react with the sodium chloride. If fed, it is ordinarily balanced by its ex- 

 cretion, but if 0.1 gram be introduced into the jugular vein of a medium-sized 

 dog, immediately paralysis of the heart ensues. It is a powerful poison fornerves 

 and nervous centres. It melts when heated to a low red heat, and volatilizes 

 at a higher heat. 



1 Bunge: Physiologixche Chemie, 3d ed., 1894, S. 25. 



