Some of the crystals from Paris, Me., 

 change from white at one termination to 

 emerald green, then light green, then 

 pink, and finally are colorless at the other 

 termination. In some crystals again the 

 red passes to blue, the blue to green and 

 the green to black. 



Exactly what produces these differ- 

 ences of color is not known. It is- known 

 that black Tourmaline has an excess of 

 iron, the red and green an excess of so- 

 dium and lithium, and the yellow and 

 brown an excess of magnesium in their 

 composition. These same differences of 

 composition characterize similar colors in 

 portions of the same crystal as well as 

 separate crystals. Hence the evidence is 

 quite conclusive that the color in some 

 way depends on the composition. Many 

 transparent Tourmajines, while appear- 

 ing of a uniform color when viewed 

 in any one direction, exhibit different col- 

 ors when viewed in different directions. 

 Thus, one of the long, slender crystals 

 may appear green when held lengthwise 

 in front of the eye, but when looked at 

 from the end appears brown. Again, 

 some crystals appear perfectly transpar- 

 ent when viewed perpendicularly to the 

 sides of the prism, but when viewed from 

 the end are perfectly opaque. This may 

 be true even when the thickness is less 

 in the latter direction. Both these prop- 

 erties are due to the arrangement of the 

 molecules of Tourmaline, which is such 

 as to make the power of absorbing light 

 different in different directions. 



The form of crystals of Tourmaline is 

 usually that of a three-sided prism. The 

 sides of the prism are usually marked by 

 narrow parallel lines called striae, and the 

 prism may be more or less rounded by 

 the addition of other planes. 



If a doubly terminated crystal be ex- 

 amined carefully, it will be seen that 

 the planes on the two ends are not alike, 

 either in number or inclination. On one 

 end there may be three planes, on the oth- 

 er six, or even twelve. If the planes on 

 one end make a blunt termination, those 

 on the other may make a sharply pointed 

 one. Such a peculiarity of crystal form 

 is possessed by but few minerals. Those 

 possessing it are said to be hemimorphic, 

 i. e., half formed. In such minerals it 

 is evident that the forces of attraction by 



which the molecules were arranged dif- 

 fered in character at one end from those 

 at the other. In other words, a separa- 

 tion of the molecular forces seems to have 

 taken place, one kind going to one end 

 and the other force to the opposite end. 

 Now, it is a curious fact that most crys- 

 tals which exhibit this peculiarity of form 

 are also pyroelectric, i. e., become electric 

 on heating. It was this development of 

 electricity which caused the stones with 

 which the Dutch children played, to pick 

 up ashes, paper, etc., when the stones 

 were warmed by the heat of the sun. 

 Anyone can repeat their observation by 

 gently heating crystals or even fragments 

 of Tourmaline and applying them to bits 

 of paper. The electrical attraction will 

 often be found to be very strong, though 

 it varies with different crystals. The 

 fragments should not be overheated, the 

 electricity being most strongly developed 

 between ioo and 200 degrees Fahrenheit. 

 A study of the kinds of electricity devel- 

 oped by the heat shows that positive elec- 

 tricity is produced at one end and nega- 

 tive at the other. Now, this exactly cor- 

 responds with what we have seen of the 

 form of the crystal, and shows that the 

 polar arrangement of the molecules pro- 

 ducing different shapes at the two ends, 

 also produces corresponding electrical 

 properties. Crystal form, heat, electrici- 

 ty, and even light, are therefore seen to 

 have intimate connection, and it may be 

 that this interesting mineral will furnish 

 us a means of learning more about these 

 forces. , 



In composition Tourmaline is a com- 

 plex silicate chiefly of aluminum and 

 boron. Iron, magnesium, the alkalies, 

 and water also enter in varying amounts 

 into it. In fact, so complicated is its 

 chemical nature that perhaps no other 

 mineral has been so often analyzed or had 

 its analyses so much discussed. 



Ruskin, in his "Ethics of the Dust," 

 thus describes its composition : "A little 

 of everything; there's always flint and 

 clay and magnesia in it ; and the black is 

 iron according to its fancy ; and there's 

 boracic acid, if you know what that is, 

 and if you don't, I cannot tell you to- 

 day, and it doesn't signify ; and there's 

 potash and soda, and, on the whole, the 

 chemistry of it is more like a mediaeval 



77 



