SCIENTIFIC CULTURE. 589 



on reweighing tlie flask, after the chemical change has taken place, 

 and the bottom of the flask covered with the white oxide formed, there 

 will be no change of weight, and this experiment may be made to en- 

 force the truth that, in this example of combustion at least, the chemi- 

 cal process is attended with no loss of material. Open now the flask, 

 and air will rush in to supply the partial vacuum, proving that in the 

 process of combustion a portion of the material of the air has united 

 to fona the white product. 



Make now a third experiment as an application of the general prin- 

 ciple which has been illustrated by the previous experiments. Bum 

 some finely divided iron (iron reduced by hydrogen) on a scale-pan, 

 and show that the process is attended by an increase of weight. What 

 does this mean ? Why, that some material has united with the iron to 

 form the new product. Whence has this material come ? Obvious- 

 ly from the air, for it could come from nowhere else. And thus, 

 besides illustrating the first of the above laws, this experiment may 

 be made to furnish an instructive lesson in regard to the relations of 

 the oxygen of the atmosphere to chemical processes. 



The second law declares that in every chemical process the Aveights 

 of the several factors and products bear a definite proportion to each 

 other. This law must next be made familiar by experimental illustra- 

 tions. A weighed amount of oxide of silver is placed in a glass tube 

 connected with a pneumatic trough. The tube is gently heated until 

 the oxide is decomposed and the oxygen gas collected in a glass bottle 

 of sufficient size. The metallic silver remaining in the tube is now re- 

 weighed, and the volume of the oxygen gas in the bottle measured, and 

 from the volume of the gas its weight is deduced. The measurement 

 is easily made by simply marking with a gammed label the level at 

 which the water stands in the bottle. If, now, the bottle is removed 

 from the pneumatic trough and the weight of Avater found which fills 

 the bottle to the same height, the weight of the water in grammes will 

 give the volume of the gas in cubic centimetres, and, knowing the 

 weight of a cubic centimetre of oxygen, we easily calculate the weight 

 of this gas resulting from the chemical process. We have now the 

 weights of the oxide of silver, the silver, and the oxygen, the one fac- 

 tor and the two products of the chemical process, and, by comparing 

 the results of different students making the same experiment, the con- 

 stancy of the proportion will be made evident to the class. 



For a second illustration of the same law, the solution of zinc in 

 dilute sulphuric acid, yielding sulphate of zinc and hydrogen gas, may 

 be selected, and the weight of the hydrogen, estimated as in the pre- 

 vious example, shown to sustain a definite relation to the weight of the 

 zinc dissolved. 



Again, silver may be dissolved in nitric acid, and the weight of the 

 nitrate of silver obtained shown to sustain a definite relation to the 

 weight of the metal. 



