INTRODUCTORY REMARKS. 239 



It would, of course, be entirely inconsistent with the object 

 of this book, if all the thousands of organic substances already 

 known (the number of which is continually being increased 

 by new discoveries) were to be considered or even men- 

 tioned. It must be sufficient to state the general properties 

 of the various groups of organic substances, to show by what 

 processes they are artificially produced or how they are found 

 in nature, how they may be recognized and separated, and finally 

 to point out those members of each group which claim a special 

 attention for one reason or another. 



Difference in the analysis of organic and inorganic substances. 

 The analysis of organic substances differs from that of inorganic 

 substances, in so far as the qualitative examination of an organic 

 substance furnishes in many cases but little proof of the true 

 nature of the substance (except that it be organic), whilst the 

 qualitative analysis of an inorganic substance discloses in most 

 cases the true nature of the substance at once. 



For instance : If a white, solid substance, upon examination, 

 is found to contain potassium and iodine, and nothing else, the 

 conclusion may at once be drawn that the compound is potas- 

 sium iodide, containing 39 parts by weight of potassium, and 

 126.6 parts by weight of iodine. Or, if another substance is 

 examined, and found to be composed of mercury and chlorine, 

 the conclusion may be drawn that the compound is either mer- 

 curous or mercuric chloride, as no other compounds containing 

 these two elements are known, and whether the examined sub- 

 stance be the lower or higher chloride of mercury, or a mixture 

 of both, can easily be determined by a few simple tests. 



Whilst thus the qualitative examination discloses the nature 

 of the substance, it is different with organic compounds. Many 

 thousand times the analysis might show carbon, hydrogen, and 

 oxygen to be present, and yet every one of the compounds 

 examined might be entirely different ; it is consequently not 

 only the quality of the elements, but chiefly the quantity present 

 which determines the nature of an organic substance, and in 

 order to identify an organic substance with certainty, it fre- 

 quently becomes necessary to make a quantitative determina- 

 tion of the various elements present, and this quantitative 



