The temperature has considerable influence in the process of impregnation, for in hot weather the gas will not combine so readily, nor will the water absorb an equal volume of gas. In summer time, therefore, soda-water should be made before the heat of the day, and ice should be added to the water. When the receiver is fully charged, and the operation of bottling begins, every bottle-full that is drawn off diminishes the pressure on the water that remains; and if no means were taken to add more gas, the soda-water would gradually become weaker and weaker as each bottle was drawn off. It is usual, in the best arranged apparatus, to have two tubes connected with the force-pump, one of which feeds it with water, the other with gas, by which contrivance water and gas, in their proper proportions, are continually forced into the receiver, which may thus be always kept nearly full. The process of bottling requires great manual dexterity. The neck of the bottle is pressed by a lever against a collar of leather fixed to a flange on the tap, so that, when the soda-water rushes in, no air nor gas can escape. The pressure inside the bottle therefore quickly becomes equal to that of the receiver, and the water ceases to flow through the tap, until some of the air is allowed to escape. When the bottle is nearly full, the operator quickly withdraws it with one hand, and having a cork ready in the other, he puts it in before the water can rush out. The cork is then forced in further by pressure, and fastened down by wires or strings. A bottling apparatus has been invented for facilitating the process; but a man accustomed to bottle by hand can do it more quickly, and with as little waste of gas and water as with a machine. Much depends, however, upon the state of the soda-water in the receiver; for if the gas be well digested, and the temperature low, it rushes into the bottle with much less force, though the water may contain a greater quantity of gas, than when it is newly made, and apparently more brisk. The bottles very frequently burst during the operation with great violence, and unless they are enclosed in a guard, the men are liable to be severely injured. Glass bottles have now generally supplanted those made of earthenware, which were formerly used; and though the glass bottles are much stronger than the earthenware ones, the bursting of them, when it does occur, is far more dangerous.