The chemistry of glass is extremely complicated.The word " glass " is a generic term referring to vitreous material (material which softens gradually over a temperature range rather than melting sharply).Most commercial glasses are vitreous silicates with some type of additives which confers special properties.Boron decreases the coefficient of expansion in PYREX Glass.Potassium gives a brown light-resistant glass.The rare earths selectively absorb light of certain wavelengths. Glass may be considered as SODIUM SILICATE ( Na 4 SiO 4 ). Aqueous solutions may slowly become alkaline upon standing for prolonged times in soft glass containers.It is shown by the reaction given below: 4Na + + SiO 4 -4 + 3H 2 0 ⟶ H 3 SiO 4 - + 4Na +  + 3OH - The compendia usually specify the type of glass container if the drug is base-sensitive.The water for injection must be stored in glass of TYPE Ⅰ or Ⅱ as the above reaction is greatly speeded up during heat sterilization.Both the U.S.P