5i8 THE POPULAR SCIENCE MONTHLY. 



Nature bides often leads through a far denser and a more bewildering 

 forest than this; but then there is not infrequently a hlaze on the 

 trees which points out the way, although it may require a sharp eye 

 in a clear head to see the marks. And Davy was well enough trained 

 to observe a circumstance which showed that he was now on the risfht 

 path and heading straight for the goal. On examining the alkali 

 formed in this last experiment, he found that it was not, as before, a 

 fixed alkali, soda or potash, but the volatile alkali ammonia. Evi- 

 dently the fixed alkali came from the impurities of the water, and 

 when, on repeating the experiment with pure water in agate cups or 

 glass tubes, the same results followed, he felt assured that so much at 

 least had been establislied. There was still, however, the production 

 of the volatile alkali and of nitrous acid to be accounted for. As these 

 contain only the elements of air and water, Davy thought that possi- 

 bly they might be formed by the combination of hydrogen at the one 

 pole and of oxygen at the other with the nitrogen of the air, which 

 was necessarily dissolved in the water. In order, therefore, to elimi- 

 nate the effect of the aii*, he again repeated the experiment under the 

 receiver of an air-pump from which the atmosphere had been ex- 

 hausted, but still the acid and alkali appeared in the two cups. 



Davy, however, was not discouraged by this, for the blazes on the 

 trees were becoming more numerous, and he now felt sure that he 

 was fast approaching the end. He obsei'ved that the quantity of acid 

 and alkali had been greatly diminished by exhausting the air, and this 

 was all that could be expected, for, as Davy knew perfectly well, the 

 best air-pumps do not remove all the air. He therefore for the last 

 experiment not only exhausted the air, but replaced it with pure hy- 

 drogen, and then exhausted the hydrogen and refilled the receiver 

 with the same gas several times in succession, until he was perfectly 

 sure that the last traces of air had been as it were washed out. In 

 this atmosphere of pure hydrogen he allowed the battery to act on 

 the water, and not until the end of twenty-four hours did he discon- 

 nect the apparatus. He then dips his test-paper into the water con- 

 nected with the positive pole, and there is no trace of acid ; he dips 

 it into the water at the negative pole, and there is no alkali ; and you 

 may judge with what satisfaction he withdraws those slips of test- 

 paper, whose unaltered surfaces showed that he had been guided at 

 last to the truth, and that his perseverance had been rewarded. 



The fame of Sir Humphry Davy rests on his discovery of the 

 metals of the alkalies and earths which first revealed the wonderful 

 truth that the crust of our globe consists of metallic cinders ; but 

 none of these brilliant results show so great scientific merit or such 

 eminent power of investigating Xature as the experiments which I 

 have just detailed. I have not, however, described them here for the 

 purpose of glorifying that renowned man. His honored memory needs 

 no such office at my hands. My only object was to show you what is 



