ANALYSIS OF COMPLEX MATTER 12l 



heated gradually, and allowed to cool gradually, lest it be 

 weakened in the process. 



The tube is then placed over a small Bunsen flame, and 

 shielded to ensure safety from possible danger of the tube break- 

 ing. The enclosed solid liquefies as before, and commences 



c 



Fig. 41. 



after a time to give off a dark-red vapour, which fills the tube. 

 It may remain for some time at a temperature much higher 

 than that at which it is completely decomposed, when the 

 vapour escaped into the air. The flame is removed and the 

 tube allowed to cool. The vapour will be noticed to apparently 

 diminish in quantity from the colour becoming much lighter. 

 On breaking the tube, a small quantity only of vapour escapes ; 

 and an examination of the solid in the tube shows very little 

 trace of silver. If the solid is again placed in the flame, with 

 free outlet, the decomposition rapidly proceeds, and silver is 

 readily obtained. Similar results may be obtained by using 

 other substances, and we learn that chemical changes do not 

 depend on temperature alone. The precise alteration produced 

 by this change of conditions has not been shown by this ex- 

 periment, which merely indicates the necessity of studying 

 every aspect of the system undergoing change. In the first 

 case the vapour contained in the silver nitrate was free to diffuse 

 away into the air ; in the second case it was confined to a 

 small space, unable to escape, although undoubtedly exercising, 

 together with the air, so great a pressure upon the inside of 

 the tube as to make it liable to burst. The results have been 

 very different. Further investigations would be required to 

 show the exact nature and extent of the modifications of 

 which this is an illustration. 



An important observation, resembling the one just described, 

 may be made by placing some zinc, magnesium, or similar 

 body in a specially thick tube to which a good cork has been 

 carefully fitted. Dilute acid is added, and the cork replaced so 



