

152 SECTION PHYSICS. 



apparatus I have without measurement gone as far as pressures of 500 

 atmospheres, which correspond to a depth of 16,500 feet below 

 the sea. I mention the subject in this popular way so as to 

 give an idea of the general conditions corresponding to those which 

 can be seen in a few moments with the apparatus before you. With 

 regard to that apparatus itself I will very shortly describe it (Professor 

 Andrews then explained the apparatus by means of a diagram). The 

 lower ends of the glass tubes containing the gases dip into small 

 mercurial reservoirs formed of thin glass tubes, which rest on ledges 

 within the apparatus. This arrangement has prevented many failures 

 in screwing up the apparatus, and has given more precision to the 

 measurements. A great improvement has also been made in the 

 method of preparing the leather-washers used in the packing for the 

 fine screws, by means of which the pressure is obtained. It consists 

 in saturating the leather with grease by heating it in vacua under 

 melted lard. In this way the air enclosed within the pores of the 

 leather is removed without the use of water, and a packing is obtained 

 so perfect that it appears, as far as my experience goes, never to fail, 

 provided it is used in a vessel filled with water. It is remarkable, 

 however, that the same packing, when an apparatus specially con- 

 structed for the purpose of forged iron was filled with mercury, always 

 yielded, even at a pressure of 40 atmospheres, in the course of a few 

 days. The carbonic acid gas, which is now under a pressure of 

 about 40 atmospheres, can be liquefied by a few turns of the screw, 

 and then you can go on visibly compressing the liquid, because liquid 

 carbonic acid is much more compressible than water. Here is a single 

 apparatus, but the diagram shows a double one a communication 

 being made between the two tubes, so that although we have two 

 screws we can operate with either or with both. One tube is filled with 

 air and serves as a manometer, and the other is filled with the gas to 

 be examined. In making observations of this kind it is necessary not 

 merely to make them at ordinary temperatures, but to examine the 

 properties of the gases under different temperatures, and for that 

 purpose there are two modes. You may work with ordinary glass 

 cylinders, but experiments of great accuracy can never be made in this 

 way, because the inequalities in the glass produce an error of some- 

 times half a millimetre, and therefore it is necessary to make observa- 



