PLAN OF THE WORK. 3. 



understood as early as possible. It is perhaps not quite obvious 

 which should be first presented to the student. Here, however, as 

 well as in every other arrangement, it is as desirable, as it is difficult* 

 to avoid anticipatipn : begin where we may, something must be 

 brought into view that has not been explained ; the only proper 

 course is, to anticipate as little as possible, and when it is unavoida- 

 ble, to give, at the moment, the explanation necessary to render the 

 step intelligible ; or to refer to the proper source whence it may be 

 obtained. In teaching, I have, with respect to the priority of acids 

 and alkalies, tried both methods, and have concluded, that the alka- 

 lies are presented first, with most advantage. The earths, of course, 

 follow in the train of the alkalies. 



I have not thought it advantageous to break up the natural classes 

 of alkalies and earths, and place them among the metallic oxides.* 

 Strict logic would justify, perhaps require such a method ; but the 

 convenience of teaching and learning, is in my view, decidedly 

 against it ; and there is in fact, no more difficulty in learning the pro- 

 perties of potassium and sodium under potassa and soda, man of the 

 latter under the former. Still, when the list of the metals is given, 

 these two metals and others of a similar character can be included, 

 and a proper reference can be made to the places where the de- 

 scription of them will be found. f 



In teaching, the great object should be, to find our way into the 

 mind of the pupil^ and to fix there> the knowledge that we present 

 to him. He is, ordinarily, no judge of our theoretical views with re- 

 gard to classification and arrangement ; he will, in most cases, even fail 

 to understand us, when we discuss them ; and he will be best sat- 

 isfied, with that course which, in the most interesting and intelligible 

 manner, presents to him the greatest amount of useful knowledge. 

 Both in my public courses of lectures, and in the present work, I have 

 therefore, considered this object as paramount in importance to ev- 

 ery other. 



5. The simple , non-metallic combustible bodies are next intro- 

 duced, both because their history is remarkably interesting and in- 

 structive, and because they are the bases of the most important acids, 

 whose history is easily and naturally developed, in connexion with 

 that of these combustibles. Hydrogen, already described along 

 with water, comes again into view as the basis of muriatic , acid. 

 Nitrogen,f although, in a popular sense, strictly a non-combustible ; 



* Since the metallic oxides include bodies of such widely different properties, I 

 can see no impropriety in distributing them into classes. I am supported in this ar- 

 rangement by the late edition of Murray. 



t The new vegetable alkaline principles are so peculiar in most of their properties, 

 that there would be no advantage in classing them with the alkalies commonly so 

 called. 



t Also before described in coanexion with the history of the atmosphere. 



