CRYSTALLIZATIONS. 101 



glittering star-like clusters. It is only those substances which crystallize rapidly 

 whose beautiful forming figures can be seen by the microscope, and in order to 

 render them visible, the following process is employed : The salts are first dis- 

 solved in water until the liquid is thoroughly saturated, and then, when it is de- 

 sired to view the crystallization, a drop or two of the solution is spread over a 

 clear strip of glass, which has been previously warmed. The watery film is then 

 placed near the focus of the object-glass of the microscope, and if the solar 

 microscope is employed, a large image of the liquid on the glass is seen upon 

 the screen. As soon as the fluid is sufficiently evaporated, the dissolved salt 

 is beheld changing rapidly from the fluid to the solid state, and branching over 

 the whole screen in crystals of the most exquisite forms ; a single crystal, in 

 certain cases, often apparently shooting the length of six or eight feet, in the 

 course of half a minute. In the present chapter we shall describe some of the 

 most interesting crystallizations. 



NITRATE OF POTASH, OR SALTPETRE. When this salt is dissolved in water, 

 and a few drops thinly spread over a glass slide, crystals are beheld shooting in- 

 ward from the edges of the fluid, upon the application of a gentle heat. The 

 crystals are very transparent, and their primitive form is that of six-sided prisms. In 

 drawing 151 many varieties of crystals are delineated, as they appear under 

 the microscope in a crystallized film, moderately magnified. If the crystals 

 form with great rapidity, long arrow-headed shafts, like that portrayed at C, 

 are seen shooting swiftly along and throwing out lateral spurs from one side, 

 forming a figure like B. These lateral branches run parallel to each other, and 

 from their sides secondary branches likewise emanate, spreading over the sur- 

 face in lines of crystal network. If the process of crystallization advances with 

 less freedom, the lateral branches are not formed ; but the main shoot appears 

 arrow-headed, with jagged sides, as in the figure C ; the sides or teeth being the 

 rudiments of the lateral spurs. In the field of view other forms are seen like 

 those in the group D, most of which resemble the variety C in their incipient 

 formation. Another kind with regular faces is seen at A ; the breadth of this 

 crystal at the end A, as measured by the micrometer, was found to be one-two- 

 hundred and seventy -seventh part of an inch. 



In India, saltpetre forms upon the surface of the ground in silky tufts and 

 slender prismatic crystals ; especially when the abundant rains of this tropical 

 region are succeeded by hot weather. These delicate filaments aro swept from 

 the surface of the soil into large heaps, which are then leached like ashes, and 

 the liquid thus obtained, after being suffered to settle, is evaporated, when the 

 nitre remains in a crystallized form. 



In certains regions of India the lower part of the mud-walls of the houses 

 becomes wet and black each morning during the dry season, from February to 

 May ; and portions of the mud crumble down into a fine powder. This dust is 

 swept up every day, and contains about one-fifth of its weight of saltpetre. It 

 is stated by the natives that the supply is abundant during those years when the 



