ANALYSIS OF COMPLEX MATTER 129 



We have now observed another example of decomposition 

 taking place through contact with another kind of matter. 

 Both hydrogen sulphate and hydrogen chloride have been 

 decomposed by contact with magnesium and also by contact 

 with aluminium. 



That different quantities as well as different volumes of the 

 gas are set free may be proved by using the apparatus illus- 

 trated (fig. 44). A known quantity of the metal is placed in 

 the tube A, while the tube B contains the liquid. The whole 

 is then weighed by suspending it with thin wire from the hook 

 of the pan. Air is now gently blown from bellows into B, so 

 that a portion of the liquid passes over into A, whereupon gas 

 escapes through the liquid in B. When the change is complete, 

 weigh again. The apparatus should be constructed of the 

 lightest material, or the loss will be a very minute fraction of 

 the whole quantity, and the corks must fit perfectly air-tight 

 and, for safety, be coated with paraffin. It must not be for- 

 gotten that in this case, and in all similar cases, no account is 

 taken of the gas which remains dissolved in the liquid. Some 

 gases might be dissolved in large quantity. Also we are 

 dependent on the purity of the bodies used. 



85. The Relative Quantities of Chemically Reacting 

 Matter. The Relative Quantities of Silver Nitrate and Sodium 

 Chloride chemically reacting always the same. A known 

 quantity of pure dry sodium chloride is dissolved in a known 

 quantity of water ; a known quantity of pure silver nitrate is 

 likewise dissolved in a known quantity of water. Graduated 

 vessels are used for the purpose. A known fraction, which 

 should be small, of the silver nitrate solution is placed in a 

 beaker ; and a graduated burette is filled with some of the 

 sodium chloride. The sodium chloride is then added gradually, 

 until the white precipitate which is at first given ceases to 

 appear. In order to know when this stage is reached, great 

 care is needed. The liquid is boiled and stirred for some time. 

 On account of this, the precipitate collects together and sinks to 

 the bottom ; and any further precipitate is readily seen in the 

 clear liquid above. By making several preliminary experi- 

 ments, the approximate quantity of sodium chloride solution 



K 



