SCIENTIFIC CULTURE. 587 



before ? Thus. Chloride of sodium symbol, NaCl. Crystallizes in 

 cubes. Soluble in water. Solubility only slightly increased by heat. 

 Generally obtained by evaporation of sea- water in pans. Also found 

 in beds in certain geological basins from which it is extracted by min- 

 ing. "When acted upon by sulphuric acid, hydrochloric acid is evolved 

 and sodic sulphate is formed, according to the following reaction, 

 and so on. I have known a student to recite all this and a great deal 

 more, without ever dreaming that he had been eating chloride of sodi- 

 um on his food, three times a day at least, since he was bom. 



Now, the rational system of teaching chemistry is first to present 

 to the scholar's mind the phenomena of Nature with which the science 

 deals. Lead him to observe these phenomena for himself ; then show 

 him how the conclusions which together constitute that system of 

 knowledge we call chemistry have been deduced from these funda- 

 mental facts. My plan is to develop this system in the lecture-room 

 in as much detail as the time allotted will permit ; to illustrate all the 

 points by experiment, and in addition to explain more in detail care- 

 fully selected fundamental experiments, which the student subse- 

 quently repeats in the laboratory himself. Thus I make the lecture- 

 room instruction and the laboratory demonstration go hand in hand as 

 complementary parts of a single course of teaching. 



To begin with the subject-matter of chemistry. In the broad 

 fields of Nature what portion does this science cover ? Natural phe- 

 nomena may obviously be divided into two great classes : First, those 

 changes which do not involve a transformation of substance ; and, 

 secondly, those changes whose very essence consists in the change of 

 one or more substances into other substances having distinctive prop- 

 erties. The science of physics deals with the phenomena of the first 

 class ; the science of chemistry with those of the last. Any phenom- 

 enon of Nature which involves a change of substance is a chemical 

 change, and in every chemical change one or more substances, called 

 the factors, are converted into another substance or into other sub- 

 stances called the products. The first point to be made in teaching 

 chemistry is, that a student should realize this statement, and a num- 

 ber of experiments should be shown in the lecture-room and repeated 

 in the laboratory illustrating what is meant by a chemical change. 



Here, of course, arises a difliculty in finding examples which shall 

 be at once simple and conclusive, for in almost all natural phenomena 

 there is a certain indefiniteness which obscures the simple process. 

 The familiar phenomena of combustion are most striking examples of 

 this fact, and men were not able to penetrate the mist which obscured 

 them until within a hundred years. To find chemical processes whose 

 course is obvious to an unpracticed observer, we are obliged to resort 

 to unfamiliar phenomena. 



A very simple example of a chemical process is a mixture of sul- 

 phur and zinc in atomic proportions, which, when lighted with a 



