18 CHEMISTRY OF FARM PRACTICE 



for ammonium carbonate (NH^COa, the NEL* and the 

 COs are each radicals. Radicals may be considered as 

 having free bonds, the same as elements. In HsPC^ the 

 POi has three bonds. This is indicated by the fact 

 that it requires three monovalent hydrogen atoms to sat- 

 urate it. 



In acids the portion after subtracting the acid hydrogen 

 is a radical, and its bonds are indicated by the number of 

 acid hydrogens subtracted, in salts the portion transferred 

 from the acid from which it was derived is a radical with 

 the same number of bonds as in the acid. In bases, the 

 hydroxyl, OH, is a radical, each hydroxyl having one free 

 bond. 



By following these directions, the formulas of the com- 

 mon compounds may be written, provided the bonds of 

 the elements and the formulas of the acids are in mind. 

 On the other hand, the bonds of elements and radicals 

 may be inferred if the formulas of compounds are known. 



The formula of the salt, calcium silicate, may be de- 

 rived as follows: This silicate salt is related to silicic acid. 

 Silicic acid has the formula H4SiO4. Its acid radical is 

 SiO4, which has four bonds, as shown by the four hydrogen 

 atoms. Calcium is a bivalent radical. This could be 

 inferred if a formula of any common substance containing 

 calcium is known, e.g., CaCl2 or Ca(OH)2. Chlorine, we 

 know, has one bond, and applying the criss-cross rule, we 

 have Ca"Cl2 r , therefore, calcium is bivalent. Now, writing 

 for calcium silicate, CaSiO4, and inserting the marks for 

 the bonds, Ca"(SiO4)"", and applying the criss-cross rule, 

 Ca4(Si04)2, and reducing to its lowest terms, we have the 

 correct formula Ca2SiO4. 



We may derive the formula for aluminium sulphate in 

 a similar way. A sulphate is related to sulphuric acid, 

 H2SO4. The acid radical SO4 has two bonds, because of 

 its two hydrogen atoms. Aluminium has three bonds, as 

 indicated by the formula for its oxide, Al2'"Oa". So, 



