152 Royal Society : — 



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 polarization 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 resist- 

 ances 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=X-,(C+7)-y- ....... (l) 



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-reser- 

 voir 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 obser- 

 vation 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 measurements are 

 given of resistances varying from 1 to 10,000 units, showing that the 

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

 surements 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. 



" On the Change of Pressure and Volume produced by Chemical 

 Combination." By M. Berthelot. 



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

 combinations, viz. can the pressure be diminished in consequence of 

 a reaction, 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 ap- 

 pears at first sight, leads to general notions concerning chemical 

 combination. 



2. The pressure depends upon the temperature evolved, and upon 

 the state of condensation of the products. Let us determine this 

 quantity. 



Let t be the temperature produced by the real reaction, this 

 being effected at a constant volume, admitting that the whole of the 

 disengaged heat was employed in warming the products. 



