the best example, the colouring substance, be it gold or cuprous oxide, is present in the form of minute but by no means atomic or molecular particles suspended in the glass. The presence of these particles has been made optically evident, although it can hardly be said that they have been rendered visible, and it is at all events probable that these suspended particles act each as a whole in absorbing the light-waves characteristic of the colour which they produce in glass. This being the case, it is easy to understand how readily the colour of such glasses is altered or spoilt by manipulations which involve heating and cooling at different rates--too rapid a rate of cooling producing a different grouping of the minute particles, altering their size or shape, or even obliterating them entirely by allowing the element in question to go into or to remain in solution in the glass. While it would be entirely foreign to the purpose of this volume to give in this place a series of recipes for the production of various kinds of coloured glass, it will be desirable to state in general terms the colours or range of colours which can be produced in various kinds of glass by the introduction of those chemical elements which are ordinarily used in this way. In general terms it may be said that the lighter elements do not as a rule tend to the production of coloured glasses, while the heavier elements, so far as they can be retained in the glass in either solution or suspension, tend to produce an intense colouring effect. The element lead appears to form a striking exception to this rule, but this is due to the fact that while the silicates of most of the other heavy elements are more or less unstable, the silicate of lead is very stable, and can only be decomposed by the action of reducing agents. When lead silicates are decomposed in this way, however, the resulting glass immediately receives an exceedingly deep colour, being turned a deep opaque black, although in very thin layers the colour is decidedly brown. On the other hand, glasses very rich in lead are always decidedly yellow in colour, and it has been shown that this coloration is due to the natural colour of lead silicates and not to the presence of impurities. What has just been said of lead applies, with only very slight modification, also to the rare metal thallium and its compounds, which have been introduced into glass for special purposes. Leaving these two exceptional bodies on one side, we now pass to a consideration of the elements in the order of their chemical grouping. The rare elements will not be considered except in certain cases where their presence in traces is liable to affect results attained in practice. The _Alkali Metals_, sodium, potassium, lithium, etc., and their compounds, have no specific colouring effect, although the presence of soda or of potash in a glass affects the colours produced by such substances as manganese, nickel, selenium, etc.