PRECIOUS STONES DISTINGUISHED BY THEIR _PROPERTIES_. One precious stone is best distinguished from another just as substances of other types are distinguished, that is to say, by their _properties_. For example, salt and sugar are both _white_, both are _soluble in water_, and both are _odorless_. So far the italicized properties would not serve to distinguish the two substances. But sugar is _sweet_ while salt is _salty_ in taste. Here we have a distinguishing property. Now, just as salt and sugar have properties, so have all _precious stones_, and while, as was the case with salt and sugar, many precious stones have properties in common, yet each has also some properties which are distinctive, and which can be relied upon as differentiating the particular stone from other stones. In selecting properties for use in distinguishing precious stones, such properties as can be determined by quantity, and set down in numbers, are probably more trustworthy than those that can be observed by mere inspection. Those also which have to do with the behavior of light in passing through the stone are extremely valuable. IMPORTANCE OF NUMERICAL PROPERTIES. It is because gem dealers so often rely upon the more obvious sort of property, such as color, that they so frequently make mistakes. There may be several different types of stones of a given color, but each will be found to have its own numerical properties such as density, hardness, refractive power, dispersive power, etc., and it is only by an accurate determination of two or three of these that one can be sure what stone he has in hand. It must next be our task to find exactly what is meant by each of these numerical properties, and how one may determine each with ease and exactness. LESSON II REFRACTION EXPLANATION OF REFRACTION. Perhaps the surest single method of distinguishing precious stones is to find out the _refractive index_ of the material. To one not acquainted with the science of physics this calls for some explanation. The term _refraction_ is used to describe the bending which light undergoes when it passes (at any angle but a right angle) from one transparent medium to another. For example, when light passes from air into water, its path is bent at the surface of the water and it takes a new direction within the water. (See Fig. 1.) [Illustration: FIG. 1.] AB represents the path of light in the air and BC its path in the water.