An inorganic Compound $\text{'}\mathrm{X}\text{'}$ on treatment with concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ produces brown fumes and gives dark brown ring with ${\mathrm{FeSO}}_4$ in presence of concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$. Also Compound $\text{'}\mathrm{X}\text{'}$ gives precipitate $\text{'}\mathrm{Y}\text{'}$, when its solution in dilute $\mathrm{HCl}$ is treated with ${\mathrm{H}}_2\mathrm{S}$ gas. The precipitate $\text{'}\mathrm{Y}\text{'}$ on treatment with concentrated ${\mathrm{HNO}}_3$ followed by excess of ${\mathrm{NH}}_4\mathrm{OH}$ further gives deep blue coloured solution, Compound $\text{'}\mathrm{X}\text{'}$ is: