1871.] 



M. Berthelot on Chemical Combination. 



445 



The third part of the paper is descriptive of an instrument for measuring 

 electrical resistance without the aid of a magnetic needle or of resistance 

 scales. It consists of two voltameter tubes fixed upon graduated scales, 

 which are so connected that the current of a battery is divided between 

 them, with one branch including a known and permanent resistance, and the 

 other the unknown resistance to be measured. The resistance and pola- 

 rization being equal, and the battery being common to both circuits, these 

 unstable elements are eliminated by balancing them from the circulation ; 

 and an expression is found for the unknown resistance X in terms of the 

 known resistances C and y of the voltameter, including the connecting- 

 wires, and of the volumes V and V of gases evolved in an arbitrary space 

 of time within the tubes, viz. : — 



x=J,(C+r)-y (i) 



Changes of atmospheric pressure affect both sides equally, and do not 

 therefore influence the results ; but a reading at the atmospheric pressure 

 is obtained at both sides by lowering the little supply-reservoir with dilute 

 acid to the level indicated in the corresponding tube. The upper ends of 

 the voltameter tubes are closed by small weighted levers provided with 

 cushions of India-rubber ; but after each observation these levers are raised, 

 and the supply-reservoirs moved so as to cause the escape of the gases until 

 the liquid within the tubes is again brought up to the zero-line of the scale, 

 when the instrument is ready for another observation. A series of mea- 

 surements are given of resistances varying from 1 to 10,000 units, showing 

 that the results agree within one-half per cent, with the independent 

 measurements obtained of the same resistances by the Wheatstone method. 



The advantages claimed for the proposed instrument are, that it is not 

 influenced by magnetic disturbances or the ship's motion if used at sea, 

 that it can be used by persons not familiar with electrical testing, and that 

 it is of very simple construction. 



The following communications were read : — 



I. M On the Change of Pressure and Volume produced by Chemical 

 Combination." By M. Berthelot. Communicated by Dr. 

 "Williamson. Received April 25, 1871. 



1. A singular question has arisen in the study of the gaseous com- 

 binations, viz. can the pressure be diminished in consequence of a reac- 

 tion, at the moment it is accomplished, at constant volume, without loss 

 of heat, so that the phenomenon of explosion comes from the excess of 

 atmospheric pressure upon the inner pressure of the system, instead of 

 coming from the inverse excess of the inner pressure ? The discussion of 

 this question, however special it appears at first sight, leads to general 

 notions concerning chemical combination. 



