MINERALOGY. 



467 



frequently in roundish grain* ; and crystallized in the 

 following fibres : 



1. The rhnmboidal dodecahedron, which is the funda- 

 mental figure. 



2. The rhomboidal dodecahedron, more or let* deeply 

 truncated on all the edges. When the truncating 

 planes becomes so large as to obliterate the origi- 

 nal planes, there is formed the 



3. Levcite crt/stallization. 



4. Rectangular four-fitted prism, acuminated on both 

 extremities with four planes which are set on the 

 lateral edges. 



The fundamental crystallization alternates from very 

 Urge to very small ; the others are middle-sized, small, 

 and very small. 



The crystals are all around cryttallizcd and imbed- 

 ded. 



The turfnce of the grains is usually rough, uneven, 

 or granulated; that of the crystals u almost always 

 smooth. 



Internally it alternates from iplendent nearly to glis- 

 tening, and the lustre is intermediate between vitreout 

 and reiinou*. 



The fracture is wore or Uti perfect conckoidal ; and 

 sometimes tiff old cleavage. 



The fragments are indeterminate angular, and ra- 

 ther sharp-edged. 



It alternates from trantpnrent to translucent. 



It is so hard as to scratch quartz. 



It ii rather difficultly frangible. 



It is heary. 



Specific gravity, 4.530, Werner. 



Werner recommends the more esential characters of 

 the mineral to be printed in a different letter from that 

 of the others, in order that they may more readily strike 

 the eye, a practice which is followed in the preceding 

 description. 



CHEMICAL CHARACTERS OF MINERALS, 



Chemical The chemical characters of minerals are those we ob- 

 thirict<Ti {; by their complete analysis, and the changes in- 

 duced on them by the action of the atmosphere, water, 

 acids, and heat, by means of the blowpipe. In this 

 article, we shall not enter on the various modes follow, 

 ed by chemists in the complete analysis of minerals, 

 hut confine ourselves to a short account of the chemical 

 characters obtained by the other means just enumer- 

 ated. 



1. Action of the Atmosphere. 



Action of Many minerals, on exposure to the atmosphere, ex- 

 perience considerable changes in colour, lustre, hard- 

 ness, or decay, fall in piece*, deliquesce, change into 

 vitriol, &c. owing partly to the abstraction of the wa- 

 ter, which enters as constituent part into many spe- 

 cies, partly to the absorption of water from the atmo- 

 sphere, or to the oxidation of some of the constituent 

 parts of the mineral. 



2. Action of Water. 



Action of Water either forms a chemical combination with mi- 

 nerals, ami completely dissolves them, as is the case 

 with the mineral salts ; or it act* by simply destroying 

 their state of aggregation, when the mineral falls into 

 until pieces with an audible noise, as is observed in 

 bole; or it (alls without noise into small pieces, which are 

 soon diffused through the fluid, without either clissolv. 

 ing in it, or becoming plastic, as in fuller's earth, and 



some minerals, as unctuous clays, it renders plastic. Oryctogno- 

 Other minerals absorb water in greater or less quanti- *? 

 ty, by which their transparency, and also their colour, "V-"-' 

 are changed. 



3. Action of Acids. 



Acids act powerfully on many different minerals, and Action of 

 are the principal agents employed in their complete ac'ds. 

 analysis. When we wish, by means of acids, to ob- 

 tain some obvious characters, the dilute muriatic acid 

 is that which is generally employed. The native car- 

 bonates effervesce, and are soluble in it. In some, as 

 in agaric mineral, calcareous spar, and witherite, the 

 effervescence is brisk, and the solution rapid ; in others, 

 as in dolomite, even when pulverised, the effervescence 

 is feeble, and the solution slow. Some of the earthy 

 minerals which contain silica, water and alkali, in a par- 

 ticular state of combination, as zeolite, if pulverised, 

 and covered with an acid, are, in the space of a few 

 hours, converted into a perfect jelly. 



4. Action of the Blowpipe. 



Common blowpij*. The blowpipe is a tube of silver, Action bf 

 copper, brass, 'or of glass, for delivering a continued ll j olow - 

 stream of air. The stream being directed across a P|P *' 

 flame, turns it more or less from a vertical position, 

 concentrating it at the same time, and occasioning a 

 more powerful combustion. The air employed is ge- 

 nerally either that of the atmosphere, or air which has 

 been breathed; sometimes oxygen gas is made use of, 

 and sometimes an inflammable gas, as the vapour of 

 boiling alcohol. The continued stream of air is fur- 

 nished by some apparatus, such as a pair of double 

 bellows, a gasometer, a large Madder, or, what is most 

 convenient of all, by blowing with the mouth. 



Few Persons, Mr. Aikin remarks, arc able at first 

 to produce a continued stream of air through the blow- 

 pipe, and the attempt often occasions a good deal of 

 fatigue. The first thing to be done is, to acquire the 

 habit of breathing easily, and without fatigue, through, 

 the nostrils alone; then to do the same when the mouth 

 is filled and the checks inflated with air, the tongue 

 being at the same time slightly raised to the roof of 

 the mouth, in order to obstruct the communication be- 

 tween the mouth and the throat. When this has been 

 acquired, the blowpipe may be put into the mouth, and 

 the confined air expelled through the pipe by means of 

 the muscles of the cheeks; as soon as the air is nearly 

 exhausted, the expiration from the lungs, instead of be- 

 ing made through the nostrils, is to be forced into the 

 cavity of the mouth ; the communication is then in- 

 stantly to be shut again by the tongue, and the remain* 

 der of the expiration is to be expired through the nos- 

 tril*. The second, and all subsequent supplies of air 

 to the blowpipe, are to be introduced in the same man. 

 ner u the first. Thus, with a little practice, the power 

 may be obtained of keeping up a continued blast for a 

 quarter of an hour, or longer, without inconvenience. 

 Much depends on the size of the external aperture of 

 the blowpipe. If so large that the mouth requires very 

 frequent replenishing, the flame will be wavering, and 

 the operator will soon be out of breath ; if on the other 

 hand, the aperture be too small, the muscles of the 

 cheeks must be strongly contracted, in order to produce 

 sufficient current, and pain and great fatigue of the 

 part will soon be the consequence. An aperture, about 

 the size of the smallest pin-hole, will generally be 

 found the most convenient, though, for particular pur- 

 poses, one somewhat larger, or a little smaller, may be 

 required. 



