FERTILIZERS. 63 



The rectified guano is sold in immense quantities in Europe, where 

 its use has increased uniformly since its first introduction. 



Prof. Atwater added that the indirect action of fertilizers counts 

 for a great deal. Fertilizers often help plants to get food from the 

 soil which otherwise would be inaccessible to them. This helps to 

 explain the action of plaster. 



Fertilizers give rise to chemical changes which liberate other 

 plant food in the soil ; as to how the}' eflect this, we are much in 

 the dark. We should not however despair of ascertaining. Twenty 

 years ago no one could say whether potash was necessarj' for the 

 food of plants, but now we know it is. The increase in our knowl- 

 edge of the laws of plant growth, and of what has to do with suc- 

 cessful crop growing and stock raising, has, during a comparativel}' 

 few years past, been simply immense. Take away from our agri- 

 culture what has been added to it during the last fifty 3'ears by that 

 accurat(i and rational observation and experimenting which con- 

 stitutes true science, and it would be very little in advance of that 

 of the Romans. 



Dr. Peter Pineo thought that a rational and satisfactor}' explana- 

 tion of the effect of gypsum or sulphate of lime, which produces 

 remarkable results on some soils, while in others its action is 

 negative, might be found in the well-known principles of chemical 

 affinity, where two substances of difljerent chemical combination 

 came into contact — as in solution — forming new compounds, and 

 liberating gases. For instance, carbonate or phosphate of soda or 

 potash, coming in contact with tartaric acid, or sulphuric acid, in 

 solution, will have set free carbonic acid, or the phosphates, and 

 tartrate or sulphate of soda or potash will be formed. It is well 

 known that wood ashes coming in contact with hen manure will set 

 free the ammonia. 



We have a well marked case of double elective affinity, in the 

 familiar experiment of sulphate of zinc and acetate of lead, which, 

 when brought together in solution, produce two new compounds. 

 The sulphuric acid leaves the zinc and combines with the lead, for 

 which it has a stronger affinit}', forming sulphate of lead, while the 

 acetic acid combines with the zinc, forming acetate of zinc. These 

 processes which we see in the chemical laborator}' are clearly 

 explanator}' of those wonderful effects obtained in the vast labora- 

 tory of Nature. 



The gypsum, or sulphate of lime, applied to some soils, will 



