THE LEWIS BROOKS MUSEUM, 5 



natural crystals, and by artificial models. Trie six systems 

 of crystals are illustrated by glass models, showing the 

 forms of the crystals and their axes. Numerous wooden 

 colored models exemplify all the most important, simple and 

 derived forms of crystals in each system. 



Besides these, Twin crystals, Hemitropes and Pseudo- 

 morphs are fully illustrated by numerous models and natu- 

 ral forms. 



Structure, as developed iu crystalline aggregates, from 

 imperfect crystal forms, is also fully shown in many illus- 

 trative specimens. 



II. — Physical Series. 



Several hundred specimens are arranged in series, so as 

 to exhibit the character of minerals depending upon light, 

 cohesion, &c, &c. Series 1 illustrates properties depending 

 upon light, and contains many specimens exhibiting the 

 different kinds of lustre, color, and shades of color, play of 

 colors, opalescence, iridescence, tarnish, pleochroism, dtc. 



The degree of transparency, or diaphaneity , of mine- 

 rals,, is also fully displayed in this series. Series 2 illus- 

 trates by many specimens the qualities due to differences of 

 cohesion, as hardness, brittleness, .malleability, elasticity, 

 flexibility, &c, &c, with also the different kinds of frac- 

 ture, such as even, conchoidal, hackly, &c. 



The specimens composing the above mentioned series 

 present a visible and tangible glossary of all the terms used 

 in the description of the physical properties of minerals. 

 A complete and well chosen suite of minerals, illustrating 

 the physical properties of minerals, is of the highest impor- 

 tance in the study of Mineralogy, for if the student makes 

 himself thoroughly acquainted with these properties, his 

 labor in the determination of minerals will be greatly light- 

 ened, and in the case of many minerals he may, by such 

 knowledge alone, determine their character. 



