is mounted via sealing gaskets 15 such that its coated surface 14 is exposed to an aqueous medium whereby the soluble layer 14 dissolves slowly at a controlled and uniform rate this continuously presents a fresh surface to the aqueous medium and thus effectively prevents the fouling and consequent loss of transparency of the window it will be apparent that it is not essential for the water soluble glass to be confined to surface layer 14 thus the entire body of the window may be formed from water soluble glass body 11 maybe formed by any of the conventional techniques of glass working e g it may be cast or cut from a drawn rod the layer 14 may be applied by the fusion of a piece of water soluble glass on a substrate of ordinary glass or by a vapor deposition process a number of glass compositions are suitable for use informing surface layer 14 however we prefer to employ phosphate glasses including but not limited to phosphorus pentoxide one or more alkale metals and one or more alkaline earth metals the dissolution rate of the glass is controlled via its composition an increase in the alkaline earth metal content will for example reduce the dissolution rate while an increase in the alkaline metal content will increase the dissolution rate a significant reduction of the dissolution rate can also be achieved by the addition of alumina to the glass the techniques of glass dissolution rate control are more fully.