When negatively charged colloid-like $A{s}_2{S}_3$ sol is added to +vely charged $Fe(-OH{)}_3$ sol in suitable amounts:

When -vely charged colloid-like $A{s}_2{S}_3$ sol is added to +vely charged $Fe(OH{)}_3$ sol in suitable amounts both the sols are precipitated simultaneously it is due to the fact that negatively charged particles of the arsenious sulphide sol take up positive ions, resulting neutralisation of the charge on the colloidal particles and hence lowering the stability of the sol and both precipitate simultaneously.