APPARATUS FOR PREPARING HYDROGEN. 227 



the bottle I ; upon this layer the zinc is placed ; the bottle a is filled to 

 about three-fourths with dilute acid, and then placed upon a support 

 consisting of small wooden blocks. Such blocks are frequently used 

 for supporting and adjusting the heights of apparatus, and a number 

 of them, 10 to 16 cm square, and varying in thickness from 1 to 4 cm , 

 should be obtained from a joiner. 



As long as the stop-cock h is closed, no acid can pass from a to 

 &, because the space in b is filled with air (art. 2) ; but if the stop- 

 cock is opened, the air can escape, acid from a reaches the zinc in 6, 

 and hydrogen is generated. If the stop-cock is again closed, either 

 completely or so much that less gas escapes than is generated, then 

 the pressure of the gas in b will drive the liquid back into a. When 

 \ the liquid in b has sunk below the lateral orifice, then gas passes 

 through the connecting tube into a, and if the zinc were placed im- 

 mediately upon the bottom of 6, the generation of gas would go on 

 1 and the generated gas would be wasted by escaping through the 

 bottle a ; this is prevented by the layer of pebbles which is inter- 

 posed between the acid at the bottom of the bottle and the zinc ; 

 as soon therefore as b is filled with gas, the further generation of 

 i it ceases. The zinc remains somewhat moistened by the acid, and 

 hence for some time after the liquid in b has sunk below the tubule, 

 bubbles of gas escape with a loud gurgling sound through the tube 

 into a ; but only a little gas is thus wasted and the apparatus affords 

 a much more economical means of preparing the necessary quantity 

 of gas than the apparatus shown in fig. 154. The zinc is dissolved 

 by the acid and combines with it to form a salt, the so-called ' white 

 vitriol '(zinc sulphate) ; if the saturated solution be allowed to eva- 

 porate in the air, the zinc sulphate separates from it in crystals. 

 When the acid is saturated, that is when no more zinc is dissolved 

 by it, it must be replaced by\i fresh supply. 



Small balloons are made of goldbeater's skin and latterly also often 

 jof ' collodion.' If gun-cotton is dissolved in ether and a thin layer 

 of the solution be spread over a surface of glass and dried, the gun- 

 cotton appears as a thin film ; to the solution and to films formed by it 

 the name of collodion has been given. If a glass flask is rinsed 

 with collodion the film formed after drying appears in the form 

 'of a small balloon, but it is not advisable to attempt making a, 

 | balloon in this manner. The collodion must be prepared with 

 i special care and thoroughly free from water, or else the balloon 

 cannot be got out of the flask without tearing ; the common 

 commercial collodion is not fit for making balloons, and it is 

 | better to buy them. To fill a balloon with hydrogen, it is carefully 

 pressed between the palms of the hands, in order to remove the air 



Q 2 



