GAS-GENERATORS. 



55 



that acid on which zinc has reacted has a greater specific gravity than unused acid 

 and diffuses downward through the whole fluid when it is forced back from the 

 zinc-chamber into the top of the acid-tank. 



Another form of hydrogen generator is shown on plate 7. When in use the 

 lower bulb is filled with acid and also the stem of the upper one. This gives a 

 sufficient column of liquid to force the gas through the five 

 wash-bottles. All the joints should be coated with Darwin's 

 wax-mixture, set together firmly, and wired in place. Exces- 

 sive liberation of hydrogen sulphide is avoided by standing 

 the generator in ice water. The ruler is 12 inches long. 

 The same style of apparatus may be used for the generation 

 of carbon dioxide. 



These gases must, of course, be carefully washed to 

 remove accidental poisonous impurities, by passing them 

 through wash-bottles containing various solutions. For the 

 carbon dioxide, which is usually generated from c.p. hydro- 

 chloric acid, diluted with twice its volume of boiled water, 

 and marble chips (which should be boiled in advance), it is 

 sufficient for many purposes to pass it through strong solu- 

 tions of sodium hydrate (10 per cent), potassium permanga- 

 nate (10 per cent), and water, arranged in the order indicated. 

 Most of the oxygen may be removed by passing through 

 three wash-bottles containing a mixture of pyrogallol and 

 strong caustic-potash water or caustic-soda water (10 per 

 cent). When in use the stopcock between the generator and 

 the first wash-bottle must not be cut off, otherwise the small 

 amount of carbon dioxide in the wash-bottle will soon be 

 absorbed by the soda and fluids will be forced over (back- 

 ward) from the other bottles by inequalities in the gas-pressure. The place to cut 

 off the gas-flow is close to the Novy jar or other receptacle. 



For testing the purity of the gas, /. e., its freedom from air, 100 cc. may be 

 drawn off into a Hempel burette (fig. 51), equalized with the air-pressure and run 

 into the simple Hempel pipette for liquid reagents (fig. 52), the bulb of which is 

 filled with strong potash water (2 water -f i potassium hydroxide). If any gas 

 remains after thorough exposure to the potash, it may be measured by passing it 

 back into the burette. One should get with the pipette an iron stand and about 

 2 yards of capillary glass tubing. 



The scrap-zinc used for generating the hydrogen should contain some lead, but 

 should be free from arsenic, antimony, and phosphorus, and the sulphuric acid should 

 be chemically pure. For use the acid is diluted largely with water (i : 9). Hydro- 

 gen generated with zinc, especially if the evolution is rapid so that the solution is 

 warmed, contains considerable hydrogen sulphide and may contain phosphureted 



*Fic. 51. Hempel's burettes for gas-analysis. Height, 25 inches. 



Fig. 5\. 



