590 THE POPULAR SCIENCE MONTHLY. 



Or, still further, as an experiment of a M^holly different class, a 

 known weight of chloride of barium may be dissolved in water, and, 

 after precipitation with sulphuric acid, the baric sulphate collected by 

 filtration and weighed, when the definite relation between the weight of 

 the precipitate and the weight of the chloride of barium will appear. 



For a last experiment let the student neutralize a weighed amount 

 of dilute hydrochloric acid with aqua ammonia, noting approximately 

 the amount of ammonia required. Let him now eva23orate the solu- 

 tion on a water-bath, and weigh the resulting saline product ; taking 

 next the same quantity of hydrochloric acid as before, and, having 

 added twice the previous quantity of ammonia, let him obtain and 

 weigh the resulting sal-ammoniac as before. A third time let him 

 begin with half the quantity of hydrochloric acid, and, adding as much 

 ammonia as in the first case, again rejDeat the process. It is obvious 

 what the result of these experiments must be, but, without telling the 

 student what he is to expect, it will be a good exercise to ask him to 

 draw his own inferences from the results. Of course, he must pre- 

 viously have so far been made acquainted with the proi3erties of hydro- 

 chloric acid and ammonia as to know that the excess of either would 

 escape when the saline solution was evaporated over a water-bath. But 

 with this limited knowledge he will be able to deduce the law of definite 

 proportions from the experimental results thus simply obtained. 



The third of the fundamental laws of chemistry stated above (gen- 

 erally known as the law of Gay-Lussac) declares that, when two or 

 more of the factors or products of a chemical process are aeriform, the 

 volumes of these gaseous substances bear to each other a very simple 

 ratio. Here, again, numerous experiments may be contrived to illus- 

 trate the law. "Water, when decomposed by electricity, yields hydro- 

 gen and oxygen gases whose volumes bear to each other the ratio of 

 two to one. When hydrochloric-acid gas is decomposed by sodium 

 amalgam, the volume of the original gas bears to that of the residual 

 hydrogen the ratio also of two to one. When ammonia is decomposed 

 by chlorine, the volume of the resulting nitrogen gas is one third of 

 that of the chlorine gas employed. 



Having illustrated these three general laws, attention should be 

 directed to the fact that the nature of a chemical process and the laws 

 which it obeys are results of observation and involve no theory what- 

 soever. On these facts the science of chemistry is built. The modern 

 system of chemistry, however, assumes what is known as the molecu- 

 lar theory, and by means of this theory attempts to exj^lain all these 

 facts and show their relation to each other. Here the distinction be- 

 tween fact and theory must be insisted upon, and also the value of 

 theory for classifying facts and directing observation. 



A molecule is now defined, and, if the student has not studied phys- 

 ics sufficiently to become acquainted with the outlines of the kinetic 

 theory of gases, this theory must be develojDcd sufficiently to give the 



