2098. The sash-frames of lights are very apt to become twisted, when the panes of glass are of a very large size; and when this is the ease, the glass is not only liable to be broken, but the frames will not fit close. On this account it is now generally recommended that no panes of glass should be used for horticultural purposes which are larger than 9 in. wide by 18 in. long, and that the thickness should not exceed 18 oz. to the foot. In cases where the sashes have to be shifted frequently, even this is found too large, and it is said that the panes of glass should never exceed 7 in. or 8 in. in width, and 10 in. in length.