428 THE PRINCIPLES OF SCIENCE. [CHAP. 



element as a substance which cannot be decomposed ly 

 any 'known means ; but it seems almost certain that some 

 series of elements, for instance Iodine, Bromine, and Chlo- 

 rine, are really compounds of a simpler substance. We 

 must look to the production of intensely high temperatures, 

 yet quite beyond our means, for the decomposition of these 

 so-called elements. Possibly in this age and part of the 

 universe the dissipation of energy has so far proceeded 

 that there are no sources of heat sufficiently intense to 

 effect the decomposition. 



Interference of Unsiispccted Conditions. 



It may happen that we are not aware of all the conditions 

 under which our researches are made. Some substance 

 may be present or some power may be in action, which 

 escapes the most vigilant examination. Not being aware 

 of its existence, we are unable to take proper measures to 

 exclude it, and thus determine the share which it has in 

 the results of our experiments. There can be no doubt 

 that the alchemists were misled and encouraged in their 

 vain attempts by the unsuspected presence of traces of 

 gold and silver in the substances they proposed to trans- 

 mute. Lead, as drawn from the smelting furnace, almost 

 always contains some silver, and gold is associated with 

 many other metals. Thus small quantities of noble metal 

 would often appear as the result of experiment and raise 

 delusive hopes. 



In more than one case the unsuspected presence of 

 common salt in the air has caused great trouble. In 

 the early experiments on electrolysis it was found that 

 when water was decomposed, an acid and an alkali were 

 produced at the poles, together with oxygen and hydrogen. 

 In the absence of any other explanation, some chemists 

 rushed to the conclusion that electricity must have the 

 power of generating acids and alkalies, and one chemist 

 thought he had discovered a new substance called electric 

 acid. But Davy proceeded to a systematic investigation 

 of the circumstances, by varying the conditions. Changing 

 the glass vessel for one of agate or gold, he found that far 

 less alkali was produced ; excluding impurities by the use 

 of carefully distilled water, he found that the quantities of 



