14. For the purpose of ascertaining the names and characters of minerals, attention must be paid to their _form_, _surface_, _lustre_, _fracture_, or the appearance of their internal surface when broken; _structure_, _transparence_, _streak_, or the mark left when scratched by any hard body; _stain_, or trace left when rubbed upon paper; _cohesion_, whether solid, friable, or fluid: _hardness_, or the resistance which they oppose when scratched; _tenacity_, or the resistance which they oppose to the stroke of a hammer; _flexibility_, or their property of bending without breaking; _feel_, or the sensation communicated by their surfaces when handled; _smell_, _taste_, _adherence to the tongue_, _sound_, _specific gravity_, or weight in comparison with that of water; _colour_ and _electricity_. 15. To ascertain the _chemical properties_ of minerals, one of the most important instruments is the blow─pipe. This is a tube which terminates in a cavity as fine as a small wire, and through which the air is forced, and made to play upon the flame of a candle. The flame is thus concentrated, and directed against small particles of the mineral to be examined, which is placed upon a bit of charcoal in a spoon of platina or silver. The air is forced into the blow─pipe by the mouth of the person using it, or by bellows attached to it for that purpose. Under this operation we have an opportunity of trying the action of other bodies upon minerals at a very high temperature; and the properties which these experiments bring into view enable us, in many cases, to ascertain, not only the nature, but even the component parts, of minerals. SIMPLE MINERAL SUBSTANCES. 16. As a necessary introduction to the study of minerals, it is requisite to describe, in a brief manner, such simple substances as form their constituent parts. Few of these, it is true, are to be found in a separate, uncombined state; yet that they do exist, and that they are to be obtained from the minerals with which they are united, we have the proof of every day’s experience. UNCONFINABLE FLUIDS. 17. There are some kinds of unconfinable fluids, the existence even of which is manifested only by their contact with other bodies, or becoming separated from them. They are of a nature too subtile to be collected or confined in our vessels for the purpose of examination, and the investigation of their properties has consequently been attended with peculiar difficulty. Those at present known are _caloric_, _light_, _electricity_, and _magnetism_; but of these the first only is immediately connected with the subjects of our present investigation.