THE CHEMISTRY OF WATER. HYDROGEN. 115 



and lest effervescence be violent. When the effervescence 

 slackens, more of the acid can be added. The gas passes 

 out through the tube, which, like the funnel tube, is fitted 

 in the cork, and is received in the jar standing in the pneu- 

 matic trough. The first portion of the gas must be al- 

 lowed to escape, as it has the air which is in the flask and 

 tube mingled with it, constituting an explosive mixture. 

 The explanation of the process is this: The acid makes the 

 oxygen of the water unite with the metal to form an oxide, 

 and so the hydrogen is set free and rises in effervescence. 

 This union does, indeed, take place when there is no acid 

 present, but it is very slow ; while the acid causes a rapid 

 union, and therefore sets free at once a large amount of hy- 

 drogen. But this is not all. The acid not only turns the 

 metal into an oxide, but it unites with that oxide, making 

 with it a substance called zinc sulphate, which dissolves in 

 the water. Observe the effect of this on the production of 

 the gas. If the acid merely occasioned the formation of an 

 oxide, this would make an insoluble coating over the metal, 

 preventing the acid from acting farther upon it, and so, 

 though there would be considerable gas formed at the first, 

 very soon the process would stop. But as it is, the acid 

 takes away the oxide as fast as it is formed, so that a fresh 

 surface of the metal is constantly present for it to act upon. 

 Sometimes a different sort of explanation is given; and the 

 hydrogen is considered to come from the acid rather than 

 from the water ; the zinc replaces the hydrogen of the sul- 

 phuric acid, forming zinc sulphate, and the hydrogen is set 

 free. This way of regarding the matter is shown in the 

 following equation : 



Zinc. Sulphuric acid. Zinc sulphate. Hydrogen. 



Zn + H a S0 4 ZnS0 4 + H 2 



144. Forming Water by Uniting Oxygen and Hydrogen. 

 As the oxygen and hydrogen that constitute water may be 



