ON TEACHING CHEMISTRY. 243 



thoir oxygen by means either of hydrogen or carbon. Opportunity is 

 here afforded of explaining the manufacture of iron. 



Several dried combustible organic substances, sugar, bread and meat, 

 may now be burnt with copper oxide in a tube the fore part of which is 

 clean and is kept cool : liquid is seen to condense, while ' chalk gas ' is 

 given off; the liquid has the appearance of water, and sufficient may 

 easily be obtained to ascertain whether it is water. The presence of 

 hydrogen in organic substances is thus discovered ; its origin from water 

 may now be explained, and the double function of water in the plant and 

 animal economy may be referred to — viz., that it both enters into the 

 composition of the animal and plant structure and also acts as a solvent. 

 The combustion of ordinary coal gas, of alcohol, of petroleum, of oil and 

 of candles, may then be studied, and the presence of hydrogen in all of 

 these noted. 



Problem XI. To determine ivhetJier oxides such as tvater and chalk gas 

 may he deprived of oxygen hy means of metals. — It being found that hydrogen 

 and carbon withdraw the oxygen from some but not from all metallic 

 oxides, it follows that some metals have a stronger, others a weaker, 

 hold upon or ' affinity ' to oxygen than has either hydrogen or carbon ; 

 the question arises whether any and which metals have so much greater 

 an affinity to oxygen that they will withdraw it from hydrogen and 

 carbon. Copper and iron have been found to part with oxygen, but zinc 

 and magnesium did not, so these four metals may be studied compara- 

 tively. Steam is passed through a red-hot copper tube full of copper 

 tacks : no change is observed. The experiment is repeated with an iron 

 tube charged with bright iron nails : a gas is obtained which is soon 

 recognised to be hydrogen, and on emptying out the nails they are found 

 to be coated with black scale. Zinc and then magnesium are tried, and, 

 like iron, are found to liberate hydrogen. Chalk gas is next passed over 

 red-hot copper, and is found to remain unchanged, but on passing it over 

 red-hot iron or zinc a gas is obtained which burns with a clear blue 

 smokeless flame : this gas is not absorbed by milk of lime, but on com- 

 bustion yields chalk gas, so it evidently contains carbon, and is a new 

 combustible gas. Like hydrogen, it is found to afford an explosive 

 mixture with oxygen. Finally, magnesium is heated in chalk gas : it is 

 observed to burn, and the magnesium to become converted into a 

 blackish substance unlike the white oxide formed on burning it in air. 

 But it is to be expected that this oxide is produced, and to remove it, as 

 it is known from previous experiments to be soluble in muriatic acid, 

 this acid is added : a black residue is obtained. AVhat is this ? Is it not 

 probable that it is carbon ? If so, it will burn in oxygen yielding chalk 

 gas. So the experiment is made. These experiments in which hydro- 

 gen is obtained from water, and carbon from chalk gas, afford the most 

 complete ' analytic ' proof of the correctness of the conclusions previously 

 arrived at regarding water and chalk gas, and which were based on 

 ' synthetic ' evidence ; taken together, they illustrate very clearly the 

 two methods by which chemists determine composition. 



As hydrogen and cai'bon form oxides from which oxygen may be 

 removed by means of some metals but not by all, the question arises, 

 which has the greater hold upon or affinity to oxygen — carbon or hydro- 

 gen ? As it is the easiest experiment to perform, steam is passed over 

 red-hot charcoal: a combustible gas is obtained which yields water 

 and chalk gas when burnt, so evidently the hydrogen is deprived of its 



K 2 



