In compound A, 1.00 gm of nitrogen unites with 0.57 gm of oxygen

In compound B, 2.00 gm of nitrogen combines with 2.24 gm of oxygen

In compound C, 3.00 gm of nitrogen combines with 5.11 gm of oxygen

These result’s obey the following law :

From the given data, in compound A 1.00 gm of nitrogen combines with 0.57 gm of oxygen.

In compound B 2.00 gm of nitrogen combines with 2.24 gm of oxygen. Dividing the both values 2,

    $\frac{2.0}{2.00}=1\text{g}$ $\frac{2.24}{2.00}=1.12\text{g}$    

That means in compound B 1.00 gm of nitrogen combines with 1.12 gm of oxygen.

In compound C 3.00 gm of nitrogen combines with 5.11 gm of oxygen.

Dividing the both values with 3 then we will get the following values.

     $\frac{3.00}{3.00}=1\text{g}$  $\frac{5.11}{3.00}=1.70\text{\hspace{0.17em}g}$ 

In all the three cases the mass of nitrogen is fixed (1g) which combines with the masses of oxygen (0.57g, 1.12g, 1.70g) having simple whole number ratio (1 : 2 : 3).