The Blast of Air.—­By using the blowpipe for chemical purposes, the effect intended to be produced is an uninterrupted steady stream of air for many minutes together, if necessary, without an instant’s cessation.  Therefore, the blowing can only be effected with the muscles of the cheeks, and not by the exertion of the lungs.  It is only by this means that a steady constant stream of air can be kept up, while the lungs will not be injured by the deprival of air.  The details of the proper manner of using the blowpipe are really more difficult to describe than to acquire by practice; therefore the pupil is requested to apply himself at once to its practice, by which he will soon learn to produce a steady current of air, and to distinguish the different flames from each other.  We would simply say that the tongue must be applied to the roof of the mouth, so as to interrupt the communication between the passage of the nostrils and the mouth.  The operator now fills his mouth with air, which is to be passed through the pipe by compressing the muscles of the cheeks, while he breathes through the nostrils, and uses the palate as a valve.  When the mouth becomes nearly empty, it is replenished by the lungs in an instant, while the tongue is momentarily withdrawn from the roof of the mouth.  The stream of air can be continued for a long time, without the least fatigue or injury to the lungs.  The easiest way for the student to accustom himself to the use of the blowpipe, is first to learn to fill the mouth with air, and while the lips are kept firmly closed to breathe freely through the nostrils.  Having effected this much, he may introduce the mouthpiece of the blowpipe between his lips.  By inflating the cheeks, and breathing through the nostrils, he will soon learn to use the instrument without the least fatigue.  The air is forced through the tube against the flame by the action of the muscles of the cheeks, while he continues to breathe without interruption through the nostrils.  Having become acquainted with this process, it only requires some practice to produce a steady jet of flame.  A defect in the nature of the combustible used, as bad oil, such as fish oil, or oil thickened by long standing or by dirt, dirty cotton wick, or an untrimmed one, or a dirty wickholder, or a want of steadiness of the hand that holds the blowpipe, will prevent a steady jet of flame.  But frequently the fault lies in the orifice of the jet, or too small a hole, or its partial stoppage by dirt, which will prevent a steady jet of air, and lead to difficulty.  With a good blowpipe the air projects the entire flame, forming a horizontal, blue cone of flame, which converges to a point at about an inch from the wick, with a larger, longer, and more luminous flame enveloping it, and terminating to a point beyond that of the blue flame.