ANALYSIS OF QUANTITATIVE PHENOMENA. 407 



displaced ; the third method in fact is adopted. To make 

 all the calculations in the frequent weighings requisite in 

 chemical analysis would be exceedingly laborious, hence 

 the correction is usually neglected. But when the chemist 

 wishes to weigh a quantity of gas contained in a glass 

 globe for the purpose of determining its specific gravity, 

 the correction becomes of much importance. Hence 

 chemists avoid at once the error, and the labour of cor 

 recting it, by attaching to the opposite scale of the balance 

 a sealed glass globe of exactly equal capacity to that 

 containing the gas to be weighed, noting only the dif 

 ference of weight when the globe is full and empty. The 

 correction, being exactly the same for both globes, may be 

 entirely neglected e . 



A device of nearly the same kind is employed in the 

 construction of galvanometers which measure the force of 

 an electric current by the deflection of a suspended 

 magnetic needle. The resistance of the needle is partly 

 due to the directive influence of the earth s magnetism, 

 and partly to the torsion of the thread. But the former 

 force may often be inconveniently great as well as 

 troublesome to determine for different inclinations. Hence 

 it is customary to connect together two exactly equal 

 needles, with their poles pointing in opposite directions, 

 one needle being within and another without the coil of 

 wire. As regards the earth s magnetism, the needles are 

 now astatic or indifferent, the tendency of one needle 

 being exactly balanced by that of the other. 



An elegant instance of the elimination of a disturbing 

 force by compensation is found in Faraday s researches 

 upon the magnetism of gases. To observe the magnetic 

 attraction or repulsion of a gas seems impossible unless we 

 enclose the gas in an envelope, probably best made of 



c Rcgnault s Cours Elementaire dc Ohimie, 1851, vol. i. p. 141. 



