2 PLAN OF THE WORK. 



with numerous important chemical facts, and until those substan- 

 ces by whose aid it must be obtained, have been exhibited. It is 

 then easy to revert both to the simple and compound bodies that have 

 preceded, and to explain the relations of chlorine to them ; and the 

 similarity between chlorine and oxygen, as supporters of combustion, 

 can then be made even more intelligible, than in the outset. 



It is obvious, that wherever chlorine may be placed, iodine must 

 follow, because of the great similarity in the properties of the two bo- 

 dies, and because, alone, iodine would be less intelligible than chlo- 

 rine. Upon this plan also, the origin of iodine from the marine 

 plants and other natural sources, admits of more intelligible explana- 

 tion. The new body bromine, from its character and affinities, nat- 

 urally comes in immediately after chlorine and iodine. It has been 

 the practice, of late years, to rank oxygen, chlorine, and iodine to- 

 gether, because they have similar electrical and chemical relations : 

 and fluorine, a principle which is, as yet, known only in name, ha? 

 been added to the list. As our evidence of the simplicity of any 

 body is merely negative, it is possible that all the bodies now re- 

 ceived as simple, may be hereafter decomposed, and every table of 

 simple bodies must be regarded as an assumption, founded on the 

 negative fact, that those bodies have not yet been decomposed. 



The natural process of acquiring knowledge is the analytical, or 

 the progress from the complex to the simple, from the whole to its 

 parts ; the shortest is the synthetic, that is, from the simple to the 

 complex ; from the parts to the whole ; and diis is the course now 

 more generally pursued in chemistry. If our knowledge were per- 

 fect, this would be not only the most obvious, but the best process ; 

 and perhaps that mode will be found to combine most advantages 

 which unites them both. With this view, I have, therefore some- 

 times adopted the one and sometimes the other, aiming to present the 

 most important elements and combinations as early as possible. 



The atmosphere and water are concerned in nearly all chemical 

 phenomena. 



2"! I have therefore introduced, after oxygen, an account of nitro- 

 gen, and then, at the next step, the composition and leading mechan- 

 ical properties of the atmosphere. 



3. Then follows hydrogen, with the composition and properties of 

 water ; and as a natural appendage, the compound or oxy-hydrogen 

 blowpipe. We are thus early put in possession of this useful and 

 splendid instrument.* 



4. The alkalies and acids are among the most important of the 

 chemical agents, and it is necessary that their properties should be 



* This instrument is in my laboratory, kept in readiness, and is used as occasion? 

 require, through the whole course. 



