280 Prof. Challis on a Theory of the Force of Electricity. 



the pump regulates the production of gas, and drives it into a 

 receiver placed in a mixture of crystallized chloride of calcium 

 and ice. In this way many litres of liquid ammonia may be 

 prepared in a few hours. 



In order to expose an apparatus to a stationary low tempe- 

 rature, it is hermetically adjusted in the receiver; this receiver is 

 placed in the freezing mixture, and ammoniacal gas liquefied in 

 it. When it is sufficiently full of the liquid gas, the freezing 

 mixture is removed, and the receiver connected with one of my 

 large reservoirs of air, by which the'pressure can be kept perfectly 

 constant, either greater or less than that of the atmosphere. 

 The ammonia distils in this manner at as low pressures as can be 

 desired ; and it is easy to keep them perfectly constant provided 

 the ammoniacal gas is prevented from reaching the air-reservoir. 

 For this purpose a cylindrical vessel containing pieces of ice is 

 placed before the reservoir ; the ice in liquefying almost entirely 

 redissolves the ammonia, and the little which escapes is caught 

 in a second vessel full of pumice soaked in acid. 



By this arrangement I hoped to obtain low temperatures per- 

 fectly stationary, but have been disappointed, for the reasons 

 stated above (page 2Y6). A certain regularity can only be 

 obtained when a continual stream of small air-bubbles is passed 

 into the liquid ammonia through a rose, which continually 

 agitates the liquid, and destroys its viscosity. An air-thermo- 

 meter ought to be connected with the apparatus. By means of 

 a regulating screw, the supply of air-bubbles can be regulated 

 so as to keep the thermometer stationary. 



XXXV. A Theory of the Force of Electricity. 

 By Professor Challis*. 



A FTER an interruption occasioned by close occupation with 

 -*** astronomical work, I propose now to resume the expla- 

 nation of my theory of the physical forces. The principal 

 hypothesis of the theory is, that the physical forces are all 

 consequences of the motions and pressures of a uniform and 

 highly elastic medium pervading space. The variations of 

 the pressure of the medium are supposed to be proportional 

 to variations of its density; and this supposition forms the 

 basis of a mathematical investigation of the relations between the 

 motions and the pressures. Further, it is assumed that the 

 medium acts immediately by pressure on the ultimate atoms of 

 bodies, which are all supposed to be spheres of invariable mag- 

 nitudes, and of the same intrinsic inertia. According to these 

 hypotheses, the different phenomena and properties of bodies 

 * Communicated by the Author. 



