Common glass ought to be fused at the _maximum_ point of heat; but glass that contains oxides capable of being reduced at that temperature (such as flint-glass) require to be worked in that part of the flame which possesses the highest oxidating power. If you operate without taking this precaution, you run the risk of decomposing the glass. Thus, for example, in the case of flint-glass, you may reduce the oxide of lead, which is one of its constituents, to the state of metallic lead. The consequence of such a reduction is the production of a black and opaque stain upon the work, which can only be removed by exposing the glass, during a very long time, to the extremity of the jet. You must invariably take the greatest care to keep the flame from passing into the interior of the tube; for when it gets there it deposits a greasy vapour, which is the ordinary cause of the dirt which accumulates in instruments that have been constructed without sufficient precaution as to this matter. In order that you may not blacken your work, you should take care to snuff the wick of the lamp whenever you perceive the flame to deposit soot. You can judge of the _consistence_ of the tubes under operation as much by the _feel_ as by the _look_ of the glass. The degree of heat necessary to be applied to particular tubes, depends entirely upon the objects for which they are destined. As soon as the glass begins to feel soft, at a _brownish-red heat_, for example, you are at the temperature most favourable to good _bending_. But is it intended to _blow a bulb_? The glass must, in this case, be completely melted, and subjected to a full _reddish-white heat_. We shall take care, when speaking hereafter of the different operations to be performed, to mention the temperature at which _each_ can be performed with most success. When an instrument upon which you have been occupied is finished, you should remove it from the flame _gradually_, taking care to _turn_ it continually, until the glass has acquired sufficient consistence to support its own weight without becoming deformed. Every instrument formed thus of glass requires to undergo a species of _annealing_, to enable it to be preserved and employed. To give the instrument this annealing, it is only necessary to remove it from the flame very gradually, allowing it to repose some time in each _cooler_ place to which you successively remove it. The thicker or the more equal the sides of the glass, the more carefully it requires to be annealed. No instrument should be permitted to touch cold or wet bodies while it is warm. THE ART OF GLASS-BLOWING. III.--_Fundamental Operations in Glass-Blowing_.