C. GENERAL PHYSICS. 177 



eons states, says that these investigations have occupied him 

 continuously since 1869. In these he has experimented with 

 gases under a pressure of 500 atmospheres. Of course, 

 great difficulties have been experienced by him in measur- 

 ing such pressures with accuracy ; but the previous difficul- 

 ties that he has experienced have been, or shortly will be, 

 entirely overcome. His recent experiments fully confirm 

 the conclusions published by him six years ago with reference 

 to carbonic-acid gas, viz., that its contraction under great 

 pressure is greater than it would be if the law of Boyle holds 

 strictly good. Under a pressure of 223 atmospheres, this 

 gas is reduced to -^^ of its volume under one atmosphere, 

 being slightly less than one half the volume it ought to oc- 

 cupy if it were a perfect gas, and contracted in accordance 

 with Boyle's law. He infers, by analogy, that the critical 

 points of the greater number of gases not hitherto liquefied 

 are probably far below the lowest temperatures yet attain- 

 ed ; and these substances are not likely to be seen, either as 

 liquids or solids, until we can obtain much lower tempera- 

 tures than those produced by liquid nitrous oxide. Again, 

 the law of Gay-Lussac, like that of Boyle, is true only within 

 certain limits and conditions of gaseous matter; in fact the 

 co-efficient of expansion changes rapidly with the pressure, 

 and if the pressure remains constant the co-efficient changes 

 with the temperature. In reference to the law of Dalton, 

 which is that the particles of one gas possess no repulsive 

 nor contractive power with regard to the particles of anoth- 

 er, Dr. Andrews's experiments show conclusively that this 

 is not true ; and that the so-called critical point is, for in- 

 stance, lowered by the admixture of carbonic-acid gas with 

 a non-condensible gas. The law also entirely fails when one 

 of the gases is at a temperature not greatly above its critical 

 point ; it only holds good when these gases are at feeble 

 pressures, and at temperatures greatly above their critical 

 points. 



OX TUE INFLUENCE UPON THE MOVEMENT OF A PENDULUM 

 OF A FLUID CONTAINED IN ITS SPHERICAL BOB. 



The illustrious Bessel, in prosecuting his investigations 

 into the force with which the earth attracts various bodies, 

 employed a pendulum having a hollow cylinder of brass as 



112 



