588 THE POPULAR SCIENCE MONTHLY, 



match, is rapidly converted into white sulphide of zinc, with appear- 

 ance of flame. Another example, a mixture of sulphur and fine iron- 

 filings, which, when moistened with a little water, rapidly changes 

 into a black sulphide of iron. Then some copper filings, which, when 

 heated on a saucer in the open air, slowly change into black oxide of 

 copper. Then a bit of phosphorus, burned in dry air under a glass 

 bell, yielding a white oxide. Next, some zinc, dissolved in diluted sul- 

 phuric acid, yielding hydrogen gas and sulphate of zinc. Then, a solu- 

 tion of chloride of barium added to a solution of sulphate of soda, 

 giving a precipitate of sulphate of baryta, and leaving in solution 

 common salt, which can be recovered by evaporating the filtrate. 



In all these examples the student should be made to see and handle 

 all the factors and all the products of each process, and the experi- 

 ments should be selected so that he may become familiar with the dif- 

 ferent conditions under which substances appear, and with various 

 kinds of chemical processes. He should also be made clearly to dis- 

 tinguish between the essential features of the process and the different 

 accessories, which may be more or less accidental — such, for example, 

 as the water used in determining the combination of iron and sulphur, 

 or the flame which accompanies combustion. 



After a clear conception has been gained of a chemical process, 

 with its definite factors and definite products, we are prepared for the 

 next important step. Every chemical process obeys three fundamental 

 laws : 



The Law of Conservation of Mass. 

 The Law of Definite Proportions. 

 The Law of Definite Volumes. 



According to the first law, the sum of the weights of the products 

 of a chemical process is always equal to the sum of the weights of the 

 factors. This law must now be illustrated by experiments, and ap- 

 proximate quantitative determinations should be introduced thus early 

 into the course of study. All that is required for this purpose is a 

 common pair of scales, capable of weighing two or three hundred 

 grammes, and turning with a decigramme. We use in our laboratory 

 some platform-scales, made by the Fairbanks Company, which are in- 

 expensive, and serve a very useful purpose. 



A very satisfactory illustration of the law of conservation of mass 

 can be obtained by inserting in a glass flask a mixture of copper fil- 

 ings and sulphur in atomic proportions. The glass flask is first bal- 

 anced in the scale-pan ; then removed and gently heated until the 

 ignition which spreads through the mass shows that chemical combina- 

 tion has taken place. The flask is lastly allowed to cool, and on re- 

 weighing is found not to have altered in weight. 



For a second experiment, a bit of phosphorus may, with the aid of 

 some simple contrivance, be burned inside a tightly corked glass flask, 

 of sufficient volume to afford the requisite supply of oxygen. Of course, 



