In regard to the use of the “glass tubes,” it may be remarked that they are used principally for the examination of minerals which yield a volatile substance on being heated therein, and to detect the presence of water and the like. It is important to make a distinction between the closed and the open tubes. When a mineral fragment is placed in a tube, closed at one end, whatever takes place will be in presence of very little air, or oxygen; on the other hand, when the tube is open at both ends, and is inclined during the experiment, a constant stream of oxygen passes through the tube, and the mineral is being dealt with in presence of that. The employment of this oxygen makes a great deal of difference in the results obtained, as a few elementary experiments will show. If we place a piece of sulphur in a tube, closed at one end, and heat it gently, we notice that a yellow coating takes place inside the tube; but if we now employ a tube open at both ends and heat it very slowly indeed, we notice that the sulphur goes off as an invisible gas, and if the experiment has been properly conducted, there should hardly be a trace of the sulphur left on the glass. A number of experiments of a similar nature might be quoted, but enough has been said for the present to show the utility of the tubes. The “chemical reagents” alluded to have already been sufficiently described to render any further discussion on them unnecessary for our immediate purpose. In regard to the “miscellaneous articles” mentioned, it may be remarked that the test papers are employed in the detection of certain acids and bases; whilst a strip of brazil-wood paper is for the detection of fluorine. The hammer and anvil are for breaking the substance to be tested into small fragments; the magnet for withdrawing particles of iron from the pulverised material; the three-cornered file for assisting in determining the relative hardness of minerals, &c., &c. In examining substances before the blowpipe, it is highly desirable that the various operations should be carried out in some definite order. The following has been found convenient:-- _a._ In a glass tube closed at one end. _b._ In an open tube. _c._ On charcoal. _d._ With borax and microcosmic salt. _e._ As to flame colouration. _f._ With other reagents. The size of the fragment to be dealt with in an examination, depends on circumstances, but for ordinary purposes a piece of the size of a small rabbit-shot will be found sufficient. It is convenient in this place to describe a few chemical reactions without the use of the blowpipe; that will render the effects on certain minerals, presently to be mentioned, clearer to the reader.