OF ARTS AND SCIENCES. 169 



results are given in the table on page 173, below those of the first 

 series ; and it will be noticed that, while the average value obtained is 

 essentially identical with the previous result, the agreement of the 

 several determinations is more close, and in consequence the probable 

 error is reduced more than one half. A closer agreement under the 

 circumstances could not possibly be expected. 



Such a striking confirmation of the previous result seemed very 

 conclusive, and the very small probable error indicated a command of 

 the method which was very satisfactory. Still, it. could not be proved 

 that there might not be a constant impurity in the hydrogen used. As 

 the hydrogen had passed every possible chemical test unimpeached, 

 the only possible impurity that could be suspected was nitrogen, and 

 Mr. Richards therefore made a careful spectroscopic examination, 

 searching for the more conspicuous nitrogen lines in the spectrum 

 obtained by passing an induction current through a rarefied atmosphere 

 of the gas from the generator just described ; but not the faintest trace 

 of any of these lines could be seen. 



Still, as in the electrolytic method of preparing the hydrogen the 

 same materials, hydrochloric acid and zinc, were used as in the first 

 series of experiments, it was determined to procure hydrogen by a 

 wholly different chemical process, using the well-known reaction of 

 metallic aluminum upon a solution of potassic hydrate. 



The purest aluminum sheet that could be obtained in the American 

 market was procured for the purpose, and the apparatus represented 

 in Fig. 8 (Plate) was used for generating the gas. The generator here 

 was a simple flask holding a strong solution of chemically pure potassic 

 hydrate, aud the aluminum was introduced in small pieces through a 

 large open tube, — dipping under the surface of the solution, — the 

 liquid being maintained at a level near the open mouth of the tube by 

 the tension in the interior of the apparatus. The small strips of alu- 

 minum were carefully cleaned, and caused slowly to slide down the 

 tube ; the evolution of hydrogen from the surface began as soon as 

 the strips of metal touched the liquid, and became very active in the 

 tube before they dropped into the flask. And this action insured the 

 removal of any traces of air which might adhere to the surface. In 

 this apparatus the long caustic potash washer was not used, as being 

 no longer necessary, and the gas was passed through caustic potash 

 bulbs to remove the spray, and then through a calcic chloride tube, 

 and over sulphuric acid and phosphoric pentoxide, as before. 



With hydrogen thus prepared, the six determinations of the third 

 series in the table were made ; and it will be seen that the average of 



