1 1 2 CHE MIS TR V FOR A GRlCUL TURAL STUDENTS 



Now mix some dry sulphate of ammonium and chalk together, 

 and warm the mixture. Note that ammonia is evolved. On 

 testing the residue it will be found to contain gypsum. Hence 

 the reverse change has now occurred, and — 



(NH4)2S04 + CaCOg = (NH4),C03 + CaSO^. 



Explain why, under different conditions, the reaction should 

 be reversed. 



Both these reactions are of agricultural importance. When, from a 

 fermenting manure heap, ammonia is found to be escaping, it is customary 

 to sprinkle the heap with gypsum to " fix " the ammonia, in accordance 

 with the first reaction. The occurrence of the second reaction shows the 

 inadvisability of mixing together a salt of ammonia with any substance 

 containing chalk previous to its application as a manure. 



XXVIII. CONCLUSIONS 



In Part I. it was observed that, in the case of certain oxygen 

 compounds, the elements were united in unalterable propor- 

 tions. In Part II. this observation has been extended to so 

 large a number of other compounds of various elements that 

 it may be stated as a primary law of chemical combination 

 that elements unite in definite proportions by weight. These 

 proportional weights, compared with hydrogen as unity, are 

 termed the combining weights of the elements. 



It has, however, been noticed that elements frequently unite 

 in more than one proportion by weight, and it is consequently 

 difficult to assign fixed combining weights. There is, however, 

 always a simple relationship between the proportions, so that, 

 taking one of the proportions in which each element enters into 

 combination as the combining weight, it may be stated as a 

 second law of chemical combination, that elements unite in the 



