284 



MB. V. H. VELEY ON THE CONDITIONS OF THE 



acquiert une force elastique suffisante, 1'action chimique s'arrete ; elle est suspendue 

 jusqu'au moment oil Ton donne issue au gaz comprimeV' BABINET states that decom- 

 position of zinc by sulphuric acid is stopped by a pressure of 13 atmospheres at a 

 temperature of 0, and at 33 atmospheres at a temperature of 20 C. ; while LOTHAR 

 MEYER* observed that a pressure of 66 atmospheres is required. Experiments made 

 on this subject were more in accordance with the statement of BABINET ; to com- 

 pletely stop the reaction at 15 C. a pressure of 28 '4 atmospheres was required, this 

 being calculated from the volume of hydrogen which should be theoretically given off 

 by the observed loss in weight of the zinc. The superincumbent pressure was not 

 however equal to this, as a considerable volume of the hydrogen was dissolved in the 

 liquid, and was given off, on releasing the pressure, as carbonic acid from soda water. 

 This subject has also been examined by CAILLETET,! as also from an electric point 

 of view by F. J. SMITH. BEKETOFF^ also noticed that solutions of certain salts of 

 silver, such as the nitrate or the chloride, in ammonia, though not reduced by zinc and 

 an acid at ordinary pressures, were readily reduced when the materials were confined 

 in a sealed tube. Experiments were made to determine the effect produced by 

 decrease of pressure on the rate at which the hydrogen is given off. 



I. Experiment on the Effect produced by Reduction of Pressure on the Rate of 



Evolution of Hydrogen. 



The method of experiment was the same as that described above ; the twin-U tube 

 method was adopted, 5 c.c. of dry air at and 760 mm. being taken as the standard. 



Weight of redistilled zinc =7 '089 grams; 55 c.c. of dilute sulphuric acid, con- 

 taining 2 grams replaceable hydrogen in 500 c.c., and thus equivalent to 7'15 

 grams zinc, were mixed with 450 c.c. water. At a pressure of 769 '5 mm., at first 

 no appreciable quantity of gas was given off; as soon as a few bubbles were being 

 evolved, observations were made in the usual manner. Temperature 26'5-28'7. 



' POOOENDOEFF, Annalen,' vol. 104, p. 189. t ' Compt. Rend.,' vol. 68, 1869, p. 395. 



J ' Compt. Rend.,' vol. 48, 1859, p. 442. 



