104 



15. — The Physic a.l Chaeacteeisxics of Mineeils. 



By James U. Harford. 



[A Lecture delivered 11th April, 1878.] 



Absteact. 



The word Mineralogy means strictly a discourse on matter found 

 in mines ; but, like many other terms of natural philosophy, has 

 a more comprehensive application, and must be taken to include 

 the inquiry into the character and properties of all materials which 

 are found composing the crust of the earth. It addresses itself to 

 the investigation of the laws, chemical, optical, and physical, of 

 those materials. 



It will be at once seen that the inquiry is of wide extent. The 

 chemical laws prevail throughout the world of inanimate matter ; 

 thus Chemistry becomes a necessity in the complete study of the 

 mineral world. Again, minerals are found in crystallised forms : 

 so are other substances artificially produced; Crystallography is 

 therefore brought into requisition. Again, many remarkable 

 optical phenomena are exhibited by minerals ; thus the science 

 of Optics becomes necessaiy. And the same may be said in respect 

 to Electricity and Magnetism. There are two ways of looking at 

 the subject which will help us on this occasion. One is to take 

 minerals as they are and submit them to the ordeal of our ordinary 

 senses. Thus we attain to a knowledge of t\vQ\v physical properties. 

 The other is followed by chemical decomposition and the applica- 

 tion of chemical tests. Thus we determine their chemical com- 

 position. The former of these modes is followed in this lecture, 

 and probably there is not a single sense to which appeal may not 

 be made. 



Smell. — This is a characteristic of many minerals, even in their 

 natural state ; such as sulphur and some of the carboniferous 

 or bituminous group. When heat is applied, the smell of the 

 fumes or vapour becomes powerful ; but this test is beyond the 

 limit of the present subject, and must be passed by. 



Taste. — This distinguishes all minerals which are soluble, such as 

 some metallic and earthy salts. Somewhat analogous to taste is 

 adhesion to the tongue, which characterises some aluminous earths. 



Feel. — This is somewhat complex, and requires practical illustra- 

 tion, involving the feel of minerals as rough, smooth, meagre, 

 unctuous or soapy, dry, harsh, etc. ; — all furnishing distinct cha- 

 racteristics of different minerals. 



Hardness. — The hardness of minerals is a decisive characteristic, 

 and a regular scale in ten gradations is agreed on by mineralogists 

 for the purpose of ascertaining and recording this character. The 

 softest mineral of the scale is talc, which may be scratched with 

 the finger nail. The hardest known substance is the diamond. 



