2089. The glazing of plant-houses has always been a subject of great interest and of considerable difficulty to gardeners. When tender exotic plants were first grown in this country, the houses in which they were sheltered having only glass fronts, the sashes composing them only differed from those of ordinary windows in being somewhat larger; but when glass roofs were introduced, numerous difficulties presented themselves, which rendered the ordinary mode of glazing windows impracticable for the sashes of plant-houses. The sashes forming the roofs of these houses being at an angle of not more than 45ï¾¦, and generally less, the glass in them was liable to be broken by any heavy substance that might chance to fall upon the roof; and in violent hailstorms, particularly, it was a common occurrence for almost every pane of glass in the roofs of the plant-houses exposed to them to be broken. The most obvious way of remedying this evil was, to make the glass used for the roofs of plant-houses exceedingly thick; but for many years this could not be done without incurring an enormous expense, as the duty on glass was paid by its weight, while it was sold to the consumer by measure, and consequently it became, as Dr. Lindley observes, 'the interest of the manufacturer to blow his glass to the thinnest possible state, and thus to increase, to the greatest degree, the brittleness of the most fragile commodity we know of.' (Gard. Chron. for 1845, p. 115.) The consequence of these difficulties was to induce gardeners to use their glass in very small panes; and this plan, though it entailed on cultivators many very serious inconveniences, was generally followed till the year 1845, when the duty on glass was taken off, and a great impulse was given to the glass trade by the extraordinary quantities that were required for horticultural purposes. Among the various inconveniences which had been occasioned by the glazing with small panes, that of obstructing the light was always considered one of the most serious. In glazing with small panes, it was necessary to make one lap over the other, to exclude the cold; and these laps were inconvenient in a great variety of ways. Water was continually lodging between them, and generating masses of minute confervï¾µ and other similar plants, which obstructed the light; and in frosty weather, the moisture between the laps was liable to freeze, and to break the glass. The great difficulty, however, was the obstruction of light; for the necessity of light to plants is too obvious to every cultivator to admit of any dispute. In consequence of this, when the duty was taken off glass, the first idea seems to have been to have the panes of glass used for horticultural purposes as large and as thick as possible; and, accordingly, we find that houses were glazed with immense panes, 2 or 3 feet long, 12 inches wide, and weighing from 18 to 26 ounces the square foot. Of course the light in houses glazed in this manner was almost as brilliant as in the open air; and it was confidently expected that this great improvement in glazing would form quite a new era in the cultivation of exotic plants. Time, however, which destroys so many brilliant hypotheses, has proved the fallacy of this; and it is found that the new mode of glazing is attended with difficulties which are quite as serious as those which proved so annoying to the cultivator of the old school. The principal of these is, the scorching of the plants; and minor ones are, the twisting and consequent ill-working of the frames of the sashes from the great weight and length of the glass.