Among the features which distinguish modern houses from those existing in the early ages of English history, few have been more conducive to comfort than the adoption of _Glass-Windows_. Before the employment of that invaluable substance--glass--for this purpose, windows consisted either of uncovered holes in the wall of a house, whereby in order to admit light, the cold would also gain admittance; or else they were holes covered with oiled skin, oiled paper, thin horn, or some other partially transparent material, which would admit a dim light, and yet exclude wind and rain. It is only by placing ourselves in a room thus lighted, that we can form a correct idea of the increase of comfort resulting from the use of glass instead of such imperfectly transparent substances. The slow and imperfect modes of making glass soon after its introduction necessarily gave it a high value, and it could only be employed by the wealthy; but its price has gradually so much lessened, and its claim to a place among the necessaries of life so generally felt and acknowledged, that there are now but few persons in England, except those moving in the very humblest ranks of society, who have not a room with a glazed window. The Manufacture of Window Glass. The glass with which windows are generally glazed, is called _Crown glass_. It is formed of different materials in different manufactories. In some instances the materials consist of fine white sand, carbonate of lime, carbonate of soda, and clippings or waste pieces of old glass; while in other cases they consist of white sand, pearl-ash, saltpetre, borax, and arsenic, in certain proportions. On this point we shall not dwell, for almost every manufacturer has a favourite receipt of his own. Whatever substances are employed, they are intimately mixed before being melted. The melting takes place in large crucibles or melting pots, made of a particular kind of clay capable of enduring intense heat. Several such crucibles are placed in a furnace, a little door being situate in the furnace opposite to each crucible. Through this door the materials are introduced and are suffered to melt; and as soon as these become melted, other portions of the materials are added, until the crucible contains a given amount of melted material. A curious effect is then observable. Although most or all of the materials are nearly opaque in their separate states, it is found that when they are all melted together, they form a transparent liquid, which is _glass_.