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SUPPLEMENT 



one case facilitating and in the other retard- 

 ing the formation of the successive layers. 

 Fig. 48 is all irregular formation, in which, 

 however, the different laminae exhibit them- 

 selves with much distinctness. Figs. 47 and 

 52 are likewise crystalline structures of com- 

 mon salt ; but their integral parts are too 

 minute to be particularly noted; they are 

 drawn exactly as they appear under a super- 

 ficial magnifying power of 90,000. Fig. 46 

 shows the crystals of saliva, under the same 

 amplifying power ; these, as might naturally 

 be expected, bear strong resemblance to, indeed 

 are nearly identical with, fig. 52. 



Our limits will not permit us to enter ex- 

 tensively into the subject of crystallization ; 

 but we deem it sufficient to have called atten- 

 tion to the fact that the microscope is an inva- 

 luable assistant in detecting the process by 

 which crystalline structures are matured. For 

 the instruction of young experimentalists, we 

 may add, that the salts whose crystals they 

 wish to examine must be dissolved in water ; 

 metallic substances, and those which have me- 

 tallic bases, on being heated, give out 

 crystals as they gradually cool; these latter 

 exhibit beautiful conformations when immer- 

 sed in solutions that act upon them chemically. 

 We conclude with an extract, relevant to our 

 subject, from Gould's Companion to the Mic- 

 roscope, a useful little treatise, by a talented 

 and practical man: 



" Another important field of inquiry by the 

 microscope is chemical action. This opens an 

 endless subject of investigation of a very rich 

 order. The mode of pursuing this class of 

 observations is very simple. Prepare a few 

 pieces of thin and perfectly flat glass, of a size 

 suitable to the stage of the microscope ; some 

 of these 'should be the eighth of an inch less 

 every way than the others. A variety of che- 

 mical agents should be provided in small glass 

 bottles with stoppers, and a few glass rods 

 about four inches long, with a rounded end, 

 for the purpose of taking up a small drop of 

 liquid. In experimenting, at the outset adapt 

 the focus of the microscope to one of the glass 

 plates. Then, upon one of the larger plates, 

 spread thinly any chemical substance with 

 which you wish to work. Lay the glass upon 

 the stage, and examine leisurely the character 

 and form of the substance. Then, upon one 

 of the smaller glasses, lay, with a glass rod, a 

 small drop of any acid or other fluid with which 

 you wish to act ; and, having spread it on 

 the glass, lower it down nearly upon the other 

 glass, so that the edges of the upper glass 

 shall not reach beyond the under one ; as the 

 upper glass, when once brought in contact, 

 should never be disturbed. The two substan- 

 ces will thus, by pressure, be reduced to one 

 uniformly thin film ; and the action which 



takes place may be very accurately investiga- 

 ted. 



" For instance, let the first glass receive a 

 very minute portion of the carbonate of cop- 

 per, and the upper glass a drop of nitric acid ; 

 on contact, the carbonic acid of the carbonate 

 will be seen coming away in globules, whilst 

 the carbonate of copper breaks down and dis- 

 appears. The field is gradually occupied with 

 a solution of nitrate of copper ; and this is seen 

 to crystallize in minute rhombic crystals. 

 Lift up the upper glass, and add a small drop 

 of ammonia, and slip down the glass again ; 

 the crystals of the nitrate disappear ; a new 

 combination takes place; and you see the 

 beautiful foliations of the nitrate of ammonia 

 interspersed with groups of the still more beau- 

 tiful prisms of the deep blue ammoniuret of cop- 

 per. Or, instead of the ammonia, add muriatic 

 acid ; the nitrate is changed into a grass-green 

 solution of the muriate of copper, which crys- 

 tallizes in bundles of spears shooting in all di- 

 rections across the field of vision. 



" These phenomena may be multiplied to 

 any extent by the use of the numerous chemi- 

 cal agents to be obtained. Iodine presents an 

 endless variety of aspects in combination with 

 different agents. The crystallization of hy- 

 driodate of potass is very beautiful, if a drop 

 of the solution is put on a glass. Put on the 

 other glass sulphuric acid. When brought in 

 contact, the acid takes the potass, and forms 

 crystals of the sulphate of potass, and the iodine 

 is developed both in solution, which passes off 

 by evaporation, and in the crystalline structure 

 of the substance itself. Again : combine solu- 

 tion of iodine with solution of sulphate of 

 soda. The alcohol, in the solution of iodine, 

 takes part of the water from the soda, and the 

 rest of the sulphate of soda crystallizes in 

 prisms. The iodine, deprived of the alcohol, 

 is developed in cherry-red drops of liquid and 

 in dark rhombic crystals. 



" To these observations may be added, the 

 examination of various crystallizations in po- 

 larized light. Between the reflecting mirror 

 and the stage of the microscope insert a plate 

 of tourmaline, so that the condensed light of 

 the mirror be thrown upon the object through 

 the tourmaline. This polarizes the light re- 

 ceived. Then above the eye-piece lay another 

 plate of tourmaline, or, what is better, an ana- 

 lysing prism of Iceland spar. This enables 

 the observer to perceive the changes produced 

 by the polarized light. Many crystals so ob- 

 served, as sulphate of potass, &c. exhibit the 

 most beautiful colouring, according as the up- 

 per plate of tourmaline is turned round on its 

 axis. Some crystals, as nifrate of potass; ex. 

 hibit these colours without the upper, or ana- 

 lysing plate. The laminae of the crystal itself, 

 as is the case with the mineral called iolite or 



