When Borax ($Na_2B_4O_7 \cdot 10H_2O$) is heated, it first loses water of crystallisation and swells up. On further heating, it melts to form a clear, transparent, glassy bead composed of Sodium Metaborate ($NaBO_2$) and Boric Anhydride ($B_2O_3$). Reaction: $Na_2B_4O_7 \xrightarrow{\Delta} 2NaBO_2 + B_2O_3$ When this glassy bead is heated with a coloured metal oxide (like Cobalt oxide), the metal oxide reacts with the boric anhydride to form a characteristic coloured metaborate. Example: $CoO + B_2O_3 \rightarrow Co(BO_2)_2$ (Cobalt metaborate - Blue). (Note: This specific qualitative analysis test is a standard part of the chemistry curriculum for Boron compounds, though the reaction details are not explicitly found in the provided text snippets).