48 ON A PIECE OF CHALK 



a more familiar example, the fur on the inside of a 

 tea-kettle is carbonate of lime; and, for anything 

 chemistry tells us to the contrary, the chalk might be 

 a kind of gigantic fur upon the bottom of the earth- 

 kettle, which is kept pretty hot below. 



Let us try another method of making the chalk tell 

 us its own history. To the unassisted eye chalk looks 

 simply like a very loose and open kind of stone. But 

 it is possible to grind a slice of chalk down so thin that 

 you can see through it until it is thin enough, in 

 fact, to be examined with any magnifying power that 

 may be thought desirable. A thin slice of the fur of 

 a kettle might be made in the same way. If it were 

 examined microscopically, it would show itself to be 

 a more or less distinctly laminated mineral substance 

 and nothing more. 



But the slice of chalk presents a totally different 

 appearance when placed under the microscope. The 

 general mass of it is made up of very minute granules ; 

 but, imbedded in this matrix, are innumerable bodies, 

 some smaller and some larger, but, on a rough average, 

 not more than a hundredth of an inch in diameter, 

 having a well-defined shape and structure. A cubic 

 inch of some specimens of chalk may contain hundreds 

 of thousands of these bodies, compacted together with 

 incalculable millions of the granules. 



The examination of a transparent slice gives a good 

 notion of the manner in which the components of the 

 chalk are arranged, and of their relative proportions. 

 But, by rubbing up some chalk with a brush in water 

 and then pouring off the milky fluid, so as to obtain 

 sediments of different degrees of fineness, the granules 

 and the minute rounded bodies may be pretty well 

 separated from one another, and submitted to micro- 



