CHEMISTRY OF THE EARTH 87 



hydrofluoric, hydrobromic and hydriodic acids 

 respectively. 



Proceeding on the axiomatic basis that 

 things which are equal to the same are equal 

 to one another, if it is found that other 

 elements unite with chlorine, one to one, 

 such, for example, as the alkali metals, 

 lithium, sodium and potassium, which form 

 lithium chloride, sodium chloride, and potas- 

 sium chloride respectively, then it is possible 

 to say that the valency of these three metals 

 is the same as that of hydrogen. It is 

 necessary to follow this deductive step 

 closely, for much depends on it. The argument 

 is that two atoms, viz., one of hydrogen and 

 one of chlorine, unite to form a compound 

 containing one atom of each, their valencies 

 are accordingly equal, now hydrogen (the 

 measure of valency) cannot be united with 

 lithium, sodium and potassium, since it 

 possesses practically no affinity, or chemical 

 avidity, for them, but chlorine unites with 

 the atoms of these elements in the ratio 

 of equality to form molecules known as 

 chlorides, which possess one atom each of 

 chlorine and of one of these elements. The 

 inference is that lithium, sodium, and potas- 

 sium are equal in valency with hydrogen 

 and chlorine. Experimentation on this basis 



