1 14 CHEMISTR y FOR A GRICUL TURAL S TUBE NTS 



be a compound of atoms of hydrogen and chlorine, a molecule 

 of ammonia must be a compound of atoms of hydrogen and 

 nitrogen, and the weight of each must be the sum of the 

 weights of its component atoms. If, then, the weight of a 

 molecule containing an atom of hydrogen and an atom of 

 chlorine be 36.5 times the weight of an atom of hydrogen, 

 and the weight of a molecule containing three atoms of 

 hydrogen and one atom of nitrogen be 17 times that of an 

 atom of hydrogen, the proportions in which hydrochloric acid 

 and ammonia will combine will also be 36.5 : 17. This was 

 found by experiment to be the case. 



The relative densities of four gases have been determined. 

 It is noticeable that the ratio of the relative densities of the 

 two compound gases, ammonia and carbon dioxide, is also the 

 ratio of the calculated molecular weights : — 



Relative Density. Molecular 



Air=i Hydrogen = I Weight. 



Ammonia .589 8.5 17 



Carbonic acid gas 1.524 22 44 



Hydrogen .069 i (2) 



Nitrogen .970 14 (28) 



This is found to be true of other volatile compounds, and 

 can only be explained by supposing that equal volumes of gases 

 contain an equal number of molecules (Avogadro's hypothesis). 

 Now the molecular weights of ammonia and carbon dioxide 

 are exactly double the densities compared with hydrogen as 

 unity ; and the relative densities of all gases being as the mole- 

 cular weights, it follows that the molecular weights of all other 

 gases must be double their relative densities. There is 

 abundant reason for believing that Avogadro's hypothesis holds 

 good for gaseous elements as well as gaseous compounds. 

 This being so, the molecular weights of hydrogen and nitrogen 

 must be 2 and 28 respectively, and therefore double the atomic 



