50 



THE POPULAR EDUCATOR. 



ordinary laws of refraction, but in some cases the ray is divided 

 Into two. This phenomenon is termed double refraction, and is 

 readily exhibited if a piece of clear calc spar be placed over a 

 line; the line will appear double, as represented in Fig. 1. 

 Upon turning the crystal in one position, the lines will overlap 

 each other, and the maximum separation will be found when 

 the crystal is turned 90 from the position in which they 

 overlapped. 



If the greatest solid angles at opposite corners of the rhomb 

 of spar were cut off, and the new faces polished, wo should find 

 that a ray passing through the crystal in that direction was not 

 divided. This line is called the optic axis, or the axis of double 

 refraction. 



This property belongs to all crystals having unequal axes of 

 symmetry (this and other terms now necessarily used will be 

 understood when the next lesson is reached), and as there 

 is one system in which the axes are all equal, crystals of 

 that system do not possess the property of double refraction. 

 There are crystals, however, which have two optic axes, that is, 

 which have two directions in which the ray is not divided, and 

 these belong to the three systems which have their lateral axes 

 unequal ; so that, concisely 



1. The Tes*ular (I.) system has no double refraction. 



2. The Dimetric (II.) and Hexagonal (VI.) systems have one 

 optic axis. 



3. The Trimctric (III.), the Monoclinic (IV.), and Triclinic 

 (V.) have two optic axes. 



The relative hardness of minerals is most useful in deter- 

 mining them. Kirwan was the first to arrange the table or 

 scale of hardness now universally adopted, which is known by 

 the name of Moh's scale ; for that mineralogist gave the idea 

 most publicity. 



1. = the hardness of talc. 



2. rocksalt. 



3. calo spar. 



4. fluor spar. 



5. apatite. 



6. = the hardness of felspar. 



7. quartz. 



8. ,, topaz. 



9. sapphire. 

 10. ,, , diamond. 



That mineral which will scratch another is the harder of the 

 two, so that by trying a mineral with the minerals named on 

 the list, its relative hardness may at once be determined, and at 

 least it may be pronounced what it is not. A good way to try 

 the hardness of two minerals is to draw a file across them, and 

 the way in which each is affected by the file will at once 

 indicate their relative hardnesses. 



Fluorescence is a property possessed by a few minerals of 

 retaining the light of the sun for a short time after they have 

 been in a dark place. Fluor spar possesses this quality. 



The specific gravity is another means of discriminating a 

 mineral specimen. The specific gravity is the relative iveight 

 which a mineral 'bears to an equal volume of distilled water at 

 60 Fahr. 



It is easily obtained by attaching the mineral to one scale of 

 a balance by a hair, and then weighing it as it is immersed in a 

 glass of water beneath the scale. Subtract this weight from 

 the ordinary weight of the mineral to find the weight of the 

 : water displaced, that is, of a volume of water equal to that of 

 the mineral, and the ordinary weight of the mineral divided by 

 this will be its specific gravity. 



There is a second method, which is applicable to porous 

 minerals and those which can only be obtained in powder. 



A light glass bottle, capable of containing 1,000 grs. of water, 

 is filled up to the mark on its neck with distilled water at 

 60 Fahr.; a few drops are poured out, and sufficient of the 

 mineral is now added to make the water again reach the mark. 

 The bottle is now weighed. The difference between this weight 

 and 1,000 grs. divided by the weight of the water poured out, 

 gives the required specific gravity. 



Mineral tallow or hatchetine is the lightest of the known 

 minerals, its specific gravity being 0'C078, whilst the ore of 

 indium, whose specific gravity is 19'5, is the heaviest. 



CRYSTALLISATION. 



"When from any cause a mineral has been deprived of its 

 cohesion and its particles caused to separate, if the particles 

 are permitted to associate themselves again to form the solid, 

 in such a way that they can follow their own inclinations, the 

 solid will give indications of being constructed according to 

 certain laws. That is, the force of cohesion does not act 



equally in every direction, but in the great majority of instances 

 sets itself to construct regular geometrical solids, called 

 crystals. 



The student can readily assure himself of the fact, by taking 

 any ordinary salt, common salt, or saltpetre, or alum, and adding 

 it to boiling water until the water will dissolve no more ; suspend 

 in the water a bunch of threads, and allow the solution to stand 

 all night ; in the morning the string will be found covered witn. 

 crystals. The common salt will be in cubes, the alum in four- 

 sided pyramids placed base to base. The larger the quantity 

 of solution and the more slowly it cools, the larger will be 

 the crystals ; muddy solutions also increase their size. The 

 presence of a substance which does not crystallise with the salt, 

 may modify the shape of the crystals ; thus, if in the solution 

 of common salt urea be present, the crystals will no longer 

 be cubes, but, like those of alum, octohedra. 



Many are the peculiarities of crystallisation. We might 

 almost say that crystals in their formation exhibited signs of 

 instinct. If a damaged crystal be suspended in a saturated 

 solution of the salt which composes it, the salt out of the solu- 

 tion will begin to repair the damage, so that in a little time the 

 general contour of the crystal will be restored. If in a solu- 

 tion there be small and great crystals, and the solution by 

 an alteration of temperature be made alternately saturated and 

 non-saturated, it will be found that the small crystals entirely 

 become dissolved, while the large crystals grow. Crystals may 

 also be got from a vapour condensing sulphur, arsenic, iodine, 

 offer examples of this or from a liquid cooling. If, for in- 

 stance, 8 or lOlbs. of sulphur or bismuth be melted and allowed 

 to cool, if when a crust has been formed it is removed, and the 

 yet liquid substance bo poured out, the cavity will be found 

 lined with crystals ; and often when a metal has been molten, 

 and in its cooled state exhibits no signs of crystallisation, yet 

 the existence of the phenomenon may be shown, if a weak solvent 

 be applied to remove those particles which mask the formation. 

 If a sheet of tin, while hot, be washed over with a weak solution 

 of hydrochloric acid, the crystals which make the tin moire'e 

 mdtallique, and which previously existed, will appear. A bar 

 of nickel, placed in dilute nitric acid, becomes covered with 

 tetrahedra, because the acid dissolves the intervening uncrys- 

 tallised metal. But, perhaps, the tendency of particles to 

 arrange themselves in some order of polarity is most strikingly 

 illustrated in solids which are constantly submitted to processes 

 which move their particles. For example, the axle, or tire of 

 the wheel of a railway carriage, by constant vibration, gives 

 the particles of which it is composed the opportunity of taking 

 positions according to the polarity of their kind. Of this 

 opportunity they take advantage, and the consequence is that 

 many axles, when broken after years of service, exhibit through- 

 out their mass crystals of iron. 



A very slight acquaintance with crystals will assure the 

 observer that those of the same mineral have a close relation- 

 ship, whenever, that is, the same forms are studied. This 

 will be illustrated by a glance at the snow crystals represented 

 in Fig. 2. 



Although a great diversity is apparent, yet all the angles are 

 equal, being those of an equilateral triangle, 60; and it is the 

 angles which are the constants in Mineralogy they never 

 vary ; but the faces of the same form are always equally 

 inclined. To measure the angles an instrument called a gonio* 

 meter is used. 



LESSONS IN SPANISH. XV. 



IEEEGULAE VEEBS OP THE SECOND CONJUGATION 



(continued) . 



19. The irregular verb valer, to be worth, is thus con- 

 jugated : 



INF. Past Participle. Valido. Gerund. Valiendo. 

 ISTD. Present. Valgo (no other Persons irregular). .First Future. 

 , valdras, valdra ; valdremos, valdreis, valdran. 



IMP. Valga, , valga; valgamos, , valgan. 



SUB. Present. Valga, valgas, valga ; valgamos, valgais, valgan. Im- 

 perfect. Valdria, valdrias, valdria ; valdriamos, valdriais, valdrfan. 



20. Tha irregular verb ver, to see, is thus conjugated : 

 INF. Post Participle. Visto. Gerund. Viendo. 



IND. Present. Veo (no other Persons irregular). Imperfect. Veia or via, 

 veias or yias, veia or via ; vfiiamos or viamos, veiais or viais, veian or vian. 



