790 PEEPAEATION OF FOSSIL SECTIONS. 



of the cylinder, which evidences the good condition of the vacuum of 

 the chuck. 



After the operation of slitting, the plate must be removed from 

 the spindle of the lathe, and the flat lead lap substituted. The 

 pneumatic chuck is now to be reversed, and the specimen placed in 

 contact with the grinder. By giving a slightly tortuous motion to 

 the specimen, that is, using the motion of the various joints, the 

 object is groimd perfectly flat when the length of both arms of the joints 

 is perfectly equal. Should the leg of the first joint on the right hand 

 side be the longer, the specimen will be ground hollow ; if shorter, it 

 will be ground convex. But if, as before stated, they are of equal 

 length, a perfectly parallel surface will be obtained. 



In operating on sUiceous objects, I have found soap and water 

 quite as speedy and efficacious as oil, which is generally used ; while 

 calcareous fossUs must be slit by a solution of common soda in water. 

 This solution of soda, if made too strong, softens the India-rubber on 

 the face of the pnuematic chuck, and renders a new piece necessary ; 

 but if care is taken to keep the solution of moderate strength, one 

 piece of India-rubber may last for six months. The thinner and 

 flatter it becomes the better hold the glass takes, until a puncture 

 occurs in the outer portion, and a new piece is rendered necessary. 



The polishing of the section is the last operation. This is per- 

 formed in various ways, according to the material of which the organ- 

 ism is composed. If siliceous, a lap of tin is to be used, about the 

 same size as the grinding lap.i Having turned the face smooth and flat, 

 a series of very fine notches are to be made aU over the surface. This 

 operation is accomplished by holding the edge of an old dinner-knife 

 almost perpendicular to the surface of the top while rotating; this 

 produces a series of criddhs, or slight asperities, which detain the 

 polishing substance. The polishing substance used on the tin lap is 

 technically called lapidaries' rot-stone, and is applied by slightly 

 moistening the mass, and pressing it firmly against the polisher, 

 care being taken to scrape off the outer surface, which often contains 

 grit. The specimen is then to be pressed with some degree of force 

 against the revolving tin lap or polisher, carefully changing the plane 

 of action by moving the specimen, in various directions over the 

 sm-face. 



To polish calcareous objects, another method must be adopted, as 

 foUows : — 



A lap or disc of wiUow wood is to be adapted to the spindle of the 

 lathe, 3 inches in thickness, and about the diameter of the other 

 laps (10 inches), the axis of the wood being parallel to the spindle of 

 the lathe, that is, the acting surface of the wood is the end of the 

 fibres, or transverse section. 



This polisher must be turned quite flat and smoothed by a plane. 



