INTRODUCTION xix 



3. Saline : as in Common Salt. 



4. Alkaline : as in Soda. 



5. Cooling : as in Saltpetre. 



6. Bitter : as in Epsom Salts. 



7. Sour : as in Sulphuric Acid. 



Odour. 



The odours of minerals may be tested by breathing strongly upon them 

 or by friction. They are of six kinds, as follows, viz. : — 



1. Alliaceous, like garlic. Arsenical Iron emits this odour by friction. 

 It may be obtained by heat from all the arsenical ores or salts, and is a sure 

 indication of the presence of arsenic in the substance from which it is 

 evolved, 



2. Horse-radish odour. The odour of decaying horse-radish is very per- 

 ceptible on heating the ores of Selenium. 



3. Sulphureous. Sulphureous odours are given off by Pyrites when it is 

 rubbed, and by many sulphides when heated. 



4. Bituminous, or the odour of Bitumen. 



5. Fetid. The odour of sulphuretted hydrogen or rotten eggs is elicited 

 by friction from Quartz and some kinds of Limestone, Anthraconite, &c. 



6. Argillaceous. The smell of moistened clay may be detected in Ser- 

 pentine, Clayslate, and some other minerals, by breathing strongly upon 

 them ; and from some, as Pyrargillite, it may be elicited by heat. 



Adhesion to the tongue 



Is in some cases a useful character, dependent on the capacity of the mine- 

 ral to imbibe moisture. Lithomarge adheres strongly to the tongue, and is 

 a good example of this character, which is also generally sufficient for dis- 

 tinguishing argillaceous from pure limestones. 



Cold. 



The cold feel caused by some minerals when taken into the naked hand. 

 Thus various kinds of Rock Crystal and gems may be distinguished from 

 glass, which may be made to imitate them closely, by their relative coolness. 



Electrieify. 



This property may be produced in certain minerals by friction or by 

 heat, the latter being called Pi/ro- electricity/. Tourmaline, Calamine, and 

 Boracite are examples of pyro-electric minerals, as are also Topaz, Axinite, 

 Scolecite, Prehnite, Electric Calamine, Sphene, Bhodizite, Rock Crystal, and 

 Barytes. 



Magnetism. 



The property of attracting the magnetic needle is most strongly exhibited 

 by Iron and some of its compounds ; but Nickel, Cobalt, Platinum, Tita- 

 nium and Palladium, have also been proved, by the experiments of 

 Faraday, to be magnetic in the sense of iron. 



