The substance to be operated upon should be placed in the tube, about half an inch from the end, and the flame applied at first very cautiously, increasing gradually to the required temperature.  The tube, in all these roasting operations, as they are termed, should be held in an inclined position.  The nearer perpendicular the tube is held, the stronger is the draught of air that passes through it.  If but little heat is required in the open tube operation, the spirit-lamp is the best method of applying the heat.  But if a greater temperature is required, then recourse must be had to the blowpipe.  Upon the angle of inclination of the tube depends the amount of air that passes through it, and therefore, the rapidity of the draught may be easily regulated at the will of the operator.  The inclination of the tube may, as a general rule, be about the angle represented in Fig. 14.

[Illustration Fig. 14.]

The length of the tube must be about six inches, so that the portion upon which the substance rested in a previous examination may be cut off.  The portion of the tube left will answer for several similar operations.

When the substance is under examination, we should devote our attention to the nature of the sublimates, and to that of the odors of the gases.  If sulphur be in the substance experimented upon, the characteristic odor of sulphurous acid gas will readily indicate the sulphur.  If metallic sulphides, for instance, are experimented upon, the sulphurous acid gas eliminated will readily reveal their presence.  As it is a property of this gas to bleach, a piece of Brazil-wood test paper should be held in the mouth of the tube, when its loss of color will indicate the presence of the sulphurous acid.  It often happens, too, that a slight deposition of sulphur will be observed upon the cool portion of the tube.  This is particularly the case with those sulphides, which yield sublimates of sulphur when heated in the bulb.

Selenium undergoes but slight oxidation, but it becomes readily volatilized, and may be observed on the cool portion of the tube.  At the same time the nose, if applied close to the end of the tube, will detect the characteristic odor of rotten horse-radish.  Arsenic also gives its peculiar alliaceous odor, which is so characteristic that it can be easily detected.  A few of the arsenides produce this odor.  The sublimates should be carefully observed, as they indicate often with great certainty the presence of certain substances; for instance, that of arsenic.  The sublimate, in this case, presents itself as the arsenious acid, or the metallic arsenic itself.  If it be the former, it may be discerned by aid of the magnifying glass as beautiful glittering octohedral crystals.  If the latter, the metallic lustre will reveal it.

But it will be observed that while some of the arsenides are sublimed at a comparatively low temperature, others require a very high one.