The forms in which iron exists in glass were investigated by colour analysis, using Moore and Prasad's method. The results obtained from the colour analysis of a number of silicate and high-lead glasses showed that it was necessary to redetermine the specific optical densities of ferrous iron and of the brown ferrie and the grey ferroso-ferric oxides given by Moore and Prasad. The specific optical densities of the ferrous iron were determined from a study of a number of silica-soda-lithia glasses, those of the brown ferric iron were calculated by studying the colours in high-lead and alkali-borate glasses and those of the ferroso-ferrio oxide were determined from a number of silica-boric oxide-soda glasses. The magnetic properties of the various glasses were also determined, and the results obtained were in complete agreement with the conclusions drawn from the colour analysis, namely, that iron exists in glass in the following five forms: (1) ferrous iron, contributing a blue colour, exists as Fe²⁺ ions; (2) ferric oxide which contributes a brown colour and is present as a colloidal dispersion; (3) colloidal ferroso-ferric oxide which contributes a grey colour and is paramagnetic when present in small amounts (mass susceptibility = 425 x 10_-6 at 20°), but shows ferromagnetic behaviour where it is present in high proportions or after the glass has been heated for 200 hours at 550°C; (4) colourless iron which exists as FeO₄ tetrahedra; (5) a highly oxidised form of iron, provisionally identified as existing in the form of an alkali ferrate which produces an absorption in the region of 480-520 mmu additional to that produced by the brown colouring iron; this form is present only in glasses containing high proportions of the alkalis. The results obtained from density measurements carried out on a number of silica-soda glasses indicated that the ferrous ions are probably enclosed in holes in the silica network.