ON TEACHING CHEMISTRY. 235 



served to take place when iron, copper, lead, zinc, &c., were heated in air, 

 are then recalled ; as the metals were found to increase in weight it 

 would appear probable that in these cases of change also the air was 

 concerned. 



These results at once suggest the question. What is air ? So much 

 having been learnt by studying the change which iron undergoes ia 

 rusting, other changes which happen in air therefore are next studied. 



PuOBLEM II. To determine the nature of the changes which take place 

 on burning substances in air. — The use of phosphorus is introduced by 

 reference to a match. Phosphorus is then burnt under a bell jar over 

 water and the result noted : the disappearance of some of the air again 

 shows that the air is concerned. The fact that phosphorus smokes when 

 taken out of the water in which it is always kept suggests that some 

 change is going on, so a stick of phosphorus is exposed in air as in the 

 previous experiment with iron : soon one-fifth has disappeared and the 

 phosphorus then ceases to smoke. The quantitative similarity of the two 

 results suggests that iron and phosphorus behave alike towards air and 

 vice versa, and serves to confirm the idea that some constituent of the air 

 present only to the extent of about one-fifth is active. But nothing is to 

 be taken for granted, so iron is exposed in the phosphorus-air residue 

 and phosphorus in the iron-air residue : as no change occurs there is 

 no room left for doubt. Recalling the experiments in which vai'ious 

 metals were burnt in air in order to determine whether in these cases 

 the same constituent of the air was concerned in the change, air from 

 which the active constituent has been removed by means of iron is 

 passed through a heated tube containing bits of the metals : no change 

 is observed, so it is evident that as a rule, if not always, one and the 

 same constituent of air is concerned. The experiments with iron and 

 phosphorus, although they show that the air is concerned in the changes 

 which are observed to take place, do not aSbrd any information whether 

 or no the water which is also present is concerned in the change. 

 Phosphorus is therefore burnt in a ' Florence ' flask closed with a rubber 

 stopper : on removing the stopper under water some water enters, and 

 by measuring this and the amount of water which will fill the flask the 

 same result is obtained as in the previous cases. To be certain whether 

 in this case anything enters or escapes from the flask it is weighed before 

 and after the phosphorus is burnt. There is no change in weight. But 

 does nothing escape ? Yes, much heat ; whence it follows that heat is 

 not material: that, although some of the air disappears, it is mei'ely 

 because it has become aflSxed to or absorbed by something else. This 

 has been proved in the case of the rusting iron and the burnt metals. 

 To obtain indisputable evidence in the case of the phosphorus this is 

 burnt in a cun-ent of air in a tube loosely filled with asbestos to retain 

 the smoke : the weight is found to increase. The observation that the 

 phosphorus ceases to burn after a time suggests the introduction of a 

 burning taper into the residue left by iron, &c. ; it is found to be extin- 

 guished. Then a candle and subsequently a gas flame may bo burnt in 

 a bell jar full of air over water. Reversed combustion may then be 

 demonstrated in order to fully illustrate the reciprocal character of the 

 phenomena. Thus it is ascertained that all ordinary cases of combustion 

 are changes in which the air, and not the air as a whole but a particular 

 constituent, is concerned, and no doubt remains that the same con- 

 stituent is always active, but active under diflerent conditions ; it is 



