increases with time also there is no concentration region in which r is independent of solution concentration for a high initial concentration of dissolution products i e initial concentration of 10000 mgl 1 and tap water the dissolution rate r is too low for practical use the rate being limited by the diffusion of water and dissolution products through the insoluble phosphate film which forms on the glass surface thus for phosphate glasses having 40 mole p 2 o 5 the dissolution rate rate can vary greatly depending on the concentration of dissolution products in the aqueous solution fig6 is a graph showing the relationship between weight loss in mg and leaching time in hours at 38 u00b0 c for a series of five buttons similar to those described above which were made glasses containing a constant amount of p 2 o 5 45 mole and a constant total of mo where mo cao mgo at 18 mole and na 2 o 37 mole the glass were varied in composition from the mo content being solely cao to cao being 10 mole and mgo 8 mole by replacing cao by mgo in 2 mole steps the buttons after etching were placed initially in 40 ml of tap water containing 1g litre of solution products and this was maintained substantially constant for 100 hours and the concentration was then increased to a substantially constant 2g l for the remainder of the experiment the solution was static fig6 shows that with a.