When a vapour , at atmospheric pressure was gradually heated from $25^{\circ}$ its colour was found to deepen at first and then to fade as the temperature was raised above $160^{\circ}$ . At $6000^{\circ}$ the vapour was almost colourless , but its colour deepens when the pressure was raised at this temperature , the vapour was

Question

$\begin{array}{1 1} \text{The bromine} \\ \text{A mixture of nitrogen dioxide and denitrogen tetraoxide } \\ \text{Pure nitrogen dioxide } \\ \text{pure dinitrogen tetroxide } \end{array} $