NATURAL PHILOSOPHY. 103 



pose, either by a slight artificial corrosion or by roughening the surface 

 of the crystals by means of friction." 



Brewster observed, that when he used water, muriatic acid, nitric 

 acid, etc., as a corrosive, the figures changed according to the corrosive 

 used. When, however, the surface of the crystal was prepared, by 

 rubbing on a stone or by means of a file, similar figures were produced, 

 but not clearly and distinctly formed ; and, what was most extraordi- 

 nary they appeared in a position directly the reverse of those produced 

 by corrosion. 



Brewster examined minutely crystals of the cubic, pyramidal, and 

 hexagonal systems only ; with those of the rhombic and the klinic sys- 

 tems he could obtain no determined results by means of artificial corro- 

 sion. 



It may be well here to give a few hints as to the method to be ob- 

 served when experimenting with crystals whose surfaces are to be thus 

 etched. An essential necessity is that such surfaces be smooth and 

 polished, and that a beginning be made with only a slicjlitly corroding 

 power. For salts easily soluble in water, Prof, von Kobell adopted 

 the following mode of procedure. He wetted a piece of woollen cloth 

 (broadcloth) with water, leaving one part of it dry ; then laying the 

 crystal evenly on this dry part, he glided it forwards to the part which 

 was damp and back again immediately. This was repeated according 

 to circumstances. Xhe crystal should be held by the thumb and fore- 

 finger of each hand close to the taper, and low down, in order that the 

 rays may fall upon it as perpendicularly as possible. The eye is to be 

 brought as close to it as may be, and the crystal then slowly turned 

 till the appearance of the reflection of the light be clearly seen on its 

 surface. It will be well to have a sheet of dark paper on the table, at 

 the spot where you hold the crystal. 



If the transparency of the crystal allows the observation of transmit- 

 ted light, it may then be held between thumb and finger close to the 

 eye, and the flame of the taper gazed at through it. A side-light is to 

 be carefully guarded against ; it may be remarked, too, that the bright 

 figure is generally seen distinctly when you stand two or three or more 

 steps distant from the candle. In judging of the figure thus seen, you 

 must not forget to observe whether only one surface has been etched, 

 or its parallel also ; for the latter will often give the figure of the 

 former reversed. Thus, for example, if three radiating lines of light 

 are seen when one surface has undergone corrosion, a star with six 

 rays would be observed, should the parallel surface have also been 

 etched. 



Such figures are most easily observed in, and produced by, alum. 

 If a damp piece of cloth be passed once or twice over the smooth sur- 

 face of an octahedron, and it then be wiped with a dry one, a three- 

 rayed star at once appears. If wetted thus several times, then a 

 change takes .place in the central part, and three additional short lines 

 of light appear in the space between the rays. The star, however, 

 will be instantlv turned into one with six ravs of li^ht, if the surface of 



> ~ / 



the crystal be wetted in the manner described above, with a solution 

 of muriatic or nitric acid. If again wetted with pure water and dried 

 on a bit of cloth, the six-rayed star instantly changes back into one 

 with three rays. Brewster also asserts that such an etched or corroded 



