ANALYSIS OF COMPLEX MATTER 125 



change is one of ordinary solution, the zinc may be recovered 

 by causing the liquid to evaporate. This is now done, but 

 instead of zinc we obtain a white crystalline body. By using 

 a double-necked vessel, fitted with a thistle tube, for 

 adding the hydrogen sulphate after the zinc has been placed 

 in the bottle, and a delivery tube, a quantity of the gas may 

 be prepared and collected over water. It will be found a 

 very light and inflammable gas. By substituting a narrowed 

 tube for the delivery tube, and taking every precaution that 

 the issuing gas is not mixed with air, it may be made to burn 

 with a steady blue flame. A cold clean glass vessel held over 

 the flame will become moist, as if steam had condensed upon 

 its surface. The gas is called hydrogen. We have there- 

 fore obtained, by the interaction of zinc and hydrogen sul- 

 phate, two new kinds of matter. 



The rise of temperature which coincides with the above 

 changes cannot yet be explained. Some definite thermal 

 change will always be observed when a chemical change takes 

 place. Besides this, however, there are other conditions, which 

 are never absent from this kind of change. 



Definite quantities only of the reacting bodies are con- 

 cerned in the change. In a case of ordinary solution there 

 may be more or less of a solid dissolved, provided a certain 

 maximum quantity be not exceeded ; but there is no such 

 variety possible in this case. If a large quantity of zinc is 

 added to a small quantity of hydrogen sulphate, most of the 

 zinc will be unchanged. It may be recovered and proved to be 

 unaltered. Some, however, has reacted with the liquid, as may 

 be shown by evaporation. On the other hand, if excess of 

 hydrogen sulphate is used, unchanged hydrogen sulphate will 

 be found after the action is completed. This may be proved 

 by using the distillation apparatus, and showing that the 

 distillate retains its power of changing the colour of litmus 

 a power which entirely disappears when excess of zinc is 

 used. 



It will be noticed that in this and in nearly every similar 

 case of chemical change, one or more of the reacting bodies 

 must be in the liquid or gaseous state. It might be supposed 



