48 



ELASTICITY OF AIR. 



Fig. 4. 



is ground to a perfectly smooth and flat edge. A circular brass plate is con- 

 structed, also ground truly plane, and perfectly smooth, and its magnitude is 

 accommodated to the size of the largest receiver intended to be used ; a sec- 

 tion of this plate is represented at S, S. 



When the receiver is placed on the plate with its mouth downward, the edge 

 of the mouth and the surface of the plate should be so truly plane and smooch 

 that they may rest in air-tight contact. This may always be insured by 

 smearing the ground edge of the receiver with a little lard or unctuous matter. 

 When the receiver is thus laid on the plate, it becomes an enclosed chamber, 

 similar to R, g. 3, but with this convenience, that any substance or object to 

 be submitted to experiment may be previously placed under it, and observed 

 through it after the air has been rarefied. In the centre of the plate S, S, a 

 small aperture O, communicates with a tube T, analogous to the tube inserted 

 in the bottom of the syringe in fig. 3. This tube is furnished with a stop-cock 

 at C, which, when closed, cuts off all communication between the receiver and 

 the syringe, and when open allows the syringe to act on the receiver as al- 

 ready described. 



The syringe B, furnished with a piston P, is fixed on a firm stand, and the 

 tube T, is carried in such a direction as to open a communication with the 

 valve V, in the bottom of the syringe. To facilitate the operation, it is usual 

 to raise and depress the piston, not by the hand applied at the extremity of the 

 piston-rod, as formerly descriBed, but by a winch, D, which turns a toothed 

 wheel, W, working in corresponding teeth, formed on the edged of the piston- 

 rod E, 



It is not necessary again to describe the operation of the syringe, since it is 

 exactly what has been already explained with reference to fig. 3. The piston 

 P, is elevated and depressed by alternately turning the wheel W, in opposite 

 directions, and the piston-valve V 7 , and the exhausting-valve V, have the prop- 

 erty and work in the manner already described. This instrument, and that 

 represented in fig. 3, differ in nothing except the length and shape of the com- 

 municating-tube T, the shape of the receiver R, and the mechanical method 

 of working the piston. 



To expedite the process of rarefaction, it is usual to provide two syringes 

 worked by the same wheel, as represented in the figure, each being drawn up 

 while the other is depressed. By these means a givea degree of rarefaction 

 is produced in half the time which would be required with a single syringe. 



