another a piece of deep blue or green glass and a similar piece of ruby glass--the combination will be found to be very nearly opaque even when each glass by itself is practically transparent. The question which now naturally presents itself to us is, what is the essential difference between, for instance, a piece of red glass and a piece of “white” glass that confers upon the former the power of absorbing blue light? A perfectly complete and satisfactory answer to this question is not, in the writer’s opinion, available in the present state of our knowledge, but to a certain extent the difference between the two kinds of glass can be explained. The difference is _produced_, in the first instance by introducing into the colourless glass some additional chemical element or elements, the substances in question being generally known as “colouring oxides,” although they are by no means always introduced in the form of oxides, and are frequently present in the glass in entirely different forms. To a certain extent the colour of the glass may be ascribed to a definite “colouring” property of the chemical elements concerned; thus most of the chemical compounds of such elements as nickel, cobalt, iron, manganese and copper are more or less deeply coloured substances, and it would seem as if the atoms or “ions” of these elements had the specific power of absorbing certain varieties of light-waves while not materially affecting others. But this specific “colouring” property is not so easily explained when we recollect that the colours of iron compounds, for example, may be green or red according to the state of combination in which that element is present, and that iron has also the power of imparting either a green or a yellow colour to glass according to circumstances. The detailed discussion of these questions, however, lies outside our present scope, and we must confine ourselves to the broad statement that colouring substance in glass may be roughly divided into two kinds or groups; the first and probably the largest group are those bodies which occur in glass in true solution, the element itself being present in the combined state as a silicate or other such compound (borate, phosphate, etc.) which is soluble in the glass. In this class, the colouring effect upon the glass is specifically that of the element introduced, and is brought about in the same way as the colouring of water when a coloured salt--such as copper sulphate--is dissolved in it. The second class of colouring substances, however, behave in a different manner; they are probably present in the glass in a state of extremely fine division, and held not in true solution, but really in a sort of mechanical suspension that approximates to the condition of what is known as a “colloidal solution.” The point which is known beyond doubt, thanks to the researches of Siedentopf and Szigmondi on ultra-microscopical particles, is that in certain coloured glasses, of which ruby glass is