Why do the window glass of old old building (a) look milky and (b) become thick at the bottom?

Explanation

a. Due to heating during the day and cooling at night, i.e., annealing over a number of years, glass acquires some crystalline character.
b. Glass is not a true solid but a super cooling liquid of high viscosity (called pseudosolid). Therefore, it has the property to flow (i.e., fluidity).