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 
zine-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. ‘his gives a 
Pa sufficient column of liquid to force the gas through the five 
rt 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 
E carbon dioxide, which is usually generated from c.p. hydro- 
j 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 (ro 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 
sa the first wash-bottle must not be cut off, otherwise the small 
amount of carbon dioxide in the wash-bottle will soon be 
Fig. 51.* 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, z.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 + 1 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 zroz 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 (1:9). 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. 
