30 MINERALS AND GEOLOGY 



(g) The test-fragment may fuse, either wholly or only at the point and 

 edges ; and the fusion may take place quietly, or with bubbling, and with or 

 without a previous "intumescence "or expansion of the fragment into a cauli- 

 flower-like mass. Most of the so-called Zeolites, for example, (minerals abun- 

 dant in Trap rocks) swell or curl up on exposure to the blowpipe, and then 

 fuse quietly ; but some, as Prehnite, melt with more or less bubbling. 



(h) The test-fragment may remain unchanged. Example, Quartz and various 

 other infusible minerals. 



(2) Treatment iu the Flask or Bulb-tube (The Water Test): 

 Minerals are frequently subjected to a kind of distillatory process by 

 ignition in small glass tubes closed at one end. These tubes are of 

 two general kinds. One kind has the form of a small flask, and is 

 commonly known as a " bulb-tube." Where it cannot be procured, a 

 small-sized test-tube may supply its place. It is used principally in 

 testing minerals for water. The other kinds consist simply of narrow 

 pieces of glass tubing, closed and sometimes drawn out to a point at 

 one extremity. They are chiefly employed in testing for mercury and 

 arsenic (see below). Our present description refers solely to the use 

 of the bulb-tube. Many minerals contain a considerable amount of 

 water, or the elements of water, in some unknown physical condition. 

 Gypsum, for example, yields nearly 21 per cent, of water. As the 

 presence of this substance is very easily ascertained, the water test is 

 frequently resorted to, in practice, for the formation of determinative 

 groups, or separation of hydrous frbrn anhydrous minerals. The 

 operation is thus performed. The glass is first warmed gently over 

 the flame of a small spirit-lamp to ensure the absence of moisture, and 

 is then set aside for a few moments to cool. This effected, a piece of 



the substance under examination, of about 

 the size of a small pea, is placed in it and 

 ignited over the spirit-lamp, as shown in 

 the annexed figure. If water be present 

 in the mineral, a thin film, condensing 

 rapidly into little, drops, will be deposited 

 on the neck or upper part of the tube. 

 As soon as the moisture begins to show 

 itself, the tube must be held in a more 

 horizontal position, otherwise a fracture 

 may be occasioned by the water flowing 

 FlG 34< down and coming in contact with the hot 



