CHAPTER XX. 

 CONSTITUTION OF AIR. 



55. EFFECT OF HEATED METALS ON AIR. 



i. Action of heated tin on a confined volume of air. (a) Get a long 

 hard glass combustion tube, closed at one end, and in it place a long strip 

 of tin foil. Close with a rubber stopper, and place in a clamp on a retort 

 stand. Take care not to close the jaws of the clamp too tightly. Why ? Heat 

 gently at first, but strongly afterwards. Turn the tube from time to time 

 so as to expose a fresh surface of molten tin. Continue heating till no 

 further action is observed. Allow to cool. Remove the cork under water 

 and note the height to which the water rises in the tube. Transfer the 

 remaining gas to a small gas jar. Try the effect of plunging a lighted taper 

 into this gas. 



(6) Repeat this experiment using " reduced " iron or " reduced " copper. 



Air changes in properties when metals are heated in it. It has 

 been seen that certain metals increase in weight when heated, and 

 change into calces (Section 53). It was assumed that this increase 

 must come from the air. It is now found that when metals are heated 

 in a confined volume of air, that although the air does not undergo any 

 visible change, yet it is changed in a very remarkable manner. First 

 a portion of it has disappeared, as is shown by the water which enters 

 the tube, when the cork is removed under water, and the remaining 

 gas extinguishes a lighted taper. Hence it is clear that the change 

 which takes place in certain metals when heated in air is accompanied 

 by a change in the air itself. It is reasonable to suppose that the gas 

 which disappears is concerned in the formation of the calx of the 

 metal which is heated in it. During calcination the tin takes out of 

 the air that part of it which helps burning, and the portion of the air 

 which has disappeared has combined with the tin, forming a calx. 

 These results may be summarised. Tin on calcining gains in weight, 

 taking some material from the air. This material is the part of the air 

 which causes substances to burn in it, and might be called the " active 

 part." The remaining and greater portion, which does not assist burning 

 or calcination, might be called the " inactive part." 



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