434 ACTUAL WORKING IN ORIGINAL RESEARCH. 



If two connected phenomena go through the same of 

 similar series of quantitative changes at the same time, 

 the one is probably related to the other as cause and effect 

 or as coincident effects of the same cause ; for instance, 

 the number and magnitude of spots on the sun and the in- 

 tensity of terrestrial magnetism exhibit such changes. In 

 many cases, a quantitative effect is the result of two causes 

 acting oppositely. In investigating a physical or chemical 

 phenomenon, we should try to discover not only the cause, 

 but also the kind and direction of the effect, the quanti- 

 tative proportion of one to the other, and the law of their 

 quantitative variation. 1 



In many cases, the effect to be discovered is so small 

 in comparison with the magnitude of the interfering cir- 

 cumstances, which cannot be separated, that it cannot be 

 detected at all by means of a single observation, and in 



two poles of a galvanic pile are plunged each into a vessel of water, 

 and the two vessels united by means of wet asbestos, or any other con- 

 ducting substance, an acid appeared round the positive wire, and an 

 alkali round the negative wire. The alkali was said by some to be 

 soda, by others to be ammonia. The acid was variously stated to be 

 nitric acid, muriatic acid, or even chlorine. Davy demonstrated, by 

 decisive experiments, that in all cases the acid and alkali are derived 

 from the decomposition of some salt contained either in the water or 

 in the vessel containing the water. Most commonly the salt decom- 

 posed is common salt, because it exists in water, and in agate, basalt, 

 and various other stony bodies which he employed as vessels. When 

 the same agate-cup was used in successive experimeni s, the quantity 

 of acid and alkali evolved diminished each time, and at last no appre- 

 ciable quantity could be perceived. When glass vessels were used, 

 soda was disengaged at the expense of the glass, which was sensibly 

 corroded. When the water into which the wires were dipped was per- 

 fectly pure, and when the vessel containing it was free from every 

 trace of saline matter, no acid or alkali made its appearance, and 

 nothing was evolved except the constituents of water, namely, oxygen 

 and hydrogen, the oxygen appearing round the positive wire, and the 

 hydrogen round the negative wire.' 



J See also p. 422. 



