246 CHEMICAL FERTILIZERS 



more soluble in water than in fats. Hence, washing easily 

 removes the rancidity. Ordinary sodium bicarbonate is 

 quite suitable. 



Sodium hydroxide is used in solution to rid trees of aphis 

 and many other pests. A \ % solution may be sprayed on to 

 trees, which have strong thick bark, to kill insects and fungi. It 

 is, however, mostly used in conjunction with soap (seep. 251). 



Boron Compounds. Excepting in preservatives of 

 cream, boron plays little part in agriculture. For the 

 preservation of cream a mixture of equal parts of crystallized 

 boric acid and borax is better than either alone. Cream so 

 preserved must be labelled accordingly. From J to J % boric 

 acid in the cream is sufficient. 



Sodium Silicate. Water-glass is made by fusing fine, 

 white sand and sodium carbonate in the proportions of 

 170 parts of sand to 100 parts of soda ash. Such a mixture 

 gives a ratio of 3$iO 2 : Na 2 O. Sometimes the ratio is 

 reduced to 2SiO 2 : Na 2 O, or about no parts of sand to 100 

 of soda ash. The fused silicate needs long boiling under 

 pressure to obtain a solution. Alternatively finety ground 

 flint or other siliceous material is boiled under pressure with 

 caustic soda. The latter process does not easily yield the 

 higher ratios of silica. The resulting solution is sold as a 

 thick syrup containing 37 % silica and 16 % of soda (Na 2 O) 

 and approximates to a hydrated form of NaH 3 SiO 4 . 



The thick syrup diluted to about one-fifteenth strength 

 is used for preserving eggs. It may also partly replace 

 soda or soap in sprays, q.v. pp. 245, 251. 



Sulphides and Polysulphides. These materials are 

 very useful insecticides. By the action of the carbonic acid 

 generated by any injurious insect or destructive bacterium, 

 hydrogen sulphide is liberated and free sulphur is precipitated 

 in a fine condition. The former is directly poisonous and 

 the latter chokes the breathing pores of insects. As these 

 sulphides often contain free alkali, the protective wax 

 coatings of many insects fails in its waterproofing properties 

 and the insect may be drowned. 



Lime Sulphur Wash. This is one of the simplest and 



