Page:The American Cyclopædia (1879) Volume XII.djvu/492

 478 NOMENCLATURE has thus been unquestionably prevented, and numerous other alterations obviated. There is a numerous class of compounds termed com- pound radicles which, though they contain sev- eral elements, nevertheless comport themselves like simple substances toward the elements. For example, cyanogen, a compound of two equivalents of carbon and one equivalent of ni- trogen, exhibits properties closely analogous to those of the element chlorine. In general the term radicle is applied to any substance which by uniting with an element can give rise to an acid or a base. Many of the more simple com- pounds formed by the union of compound rad- icles with elements or with other compound radicles are classed with the chlorine salts ; as cyanide of potassium, chloride of ethyl, &c., the radicle ethyl being composed of four equiva- lents of carbon and five of hydrogen. Some of the compound radicles acting as metals can unite with oxygen to form bases or acids, which, when combined with each other, pro- duce salts which are completely analogous to the oxygen salts of inorganic chemistry. Thus the radicles ethyl and benzoyl, after combina- tion with oxygen, are respectively a base and an acid, which by uniting form benzoate of the oxide of ethyl (benzoate of ethyl). In general terms it may be said that the nomen- clature of organic acids, bases, and salts is similar to that of analogous inorganic sub- stances. It is among the radicles themselves, and the numerous neutral or indifferent com- plex bodies of organic chemistry, that the sys- tem is at fault. Organic compounds are usu- ally divided into natural families or groups, the generic names of which are furnished in each case by the name of some one substance which happens to be familiarly known, and to which each member of the group is in some way allied. Thus the term alcohol is applied to a large class of bodies analogous to com- mon alcohol, each separate member of the class being designated by prefixing its specific name ; as methyl alcohol (wood spirit), ethyl alcohol (common alcohol), &c. In like manner ether is the generic name of a large class of bodies of which common ether is the type. In compounds produced by substitution (see CHEM- ISTRY), that is, in those cases where one or more of the equivalents of an element are re- placed in a compound by equivalents of other elements or of compound radicles, names are formed by prefixing to the name of the origi- nal compound that of the element or elements which have been newly introduced. The pre- fixes fo', ter, &c., or di, tris, tetra, &c., if the replacing substances possess basic prop- erties, are used to denote those cases where two or more equivalents of any one element are substituted. Thus, acetic acid in which one equivalent of hydrogen has been replaced by an equivalent of chlorine is called chlor- acetic acid (or monochloracetic acid) ; when three equivalents of hydrogen are thus re- placed, it is called terchloracetic acid. When one equivalent of hydrogen in ammonia is replaced by ethyl, the resulting compound is called ethyl-ammonia, or shortly, ethylamine; when two equivalents of hydrogen are thus re- placed, it becomes diethyl-ammonia (diethyl- amine), and so on. But each of the three equivalents of hydrogen in ammonia may be replaced by separate radicles, as in ethyl- methyl-amyl-ammonia. In similar cases ex- ceedingly complicated names are often un- avoidable ; but such are rarely expressed with- out their written formulas. Two or three common prefixes, besides those already men- tioned, occur. Thus, pyro (abbreviated by Guyton de Morveau from the term empyreu- matic, much used by the alchemists) is a dis- tinguishing appellation of many acids obtained by means of dry distillation, as pyrogallic acid, &c. Para (Gr. napa, near to) was proposed by Berzelius to indicate a strong resemblance between two compounds, as tartaric and para- tartaric acids, which, though very much alike, must nevertheless be regarded as distinct sub- stances. Meta (Gr. //er<i, signifying change) is also used in a somewhat similar manner. Thus, when aldehyde is kept for a long time in a close tube, it gradually changes into two com- pounds, both isomeric with the original sub- stance ; one of these is called metaldehyde, and the other paraldehyde. Many chemists have sought to apply names terminating alike to all the members of a given class. Although it is not easy to carry out this principle in de- tail, it has nevertheless been successfully ap- plied to the names of several classes of com- pound radicles which terminate in yle, or sim- ply yl, as ethyl, methyl, &c. The names of the alkaloids also, and in general of bases which are not radicles like ethyl, terminate in ine, as strychnine, morphine, and the like. In the above sketch we have given the nomenclature employed during the last 75 years in the best treatises on technology, and analytical, physio- logical, pharmaceutical, and general chemistry. But as language is the instrument used to ex- press the existing knowledge on any subject, and as the domain of science has been im- mensely increased and the theoretical views entertained by chemists have been materially modified, a new nomenclature has been found necessary. It would be impossible to notice in this place the various systems of nomencla- ture devised as substitutes for the language in- vented by Lavoisier. One of them, proposed by Prof. Samuel D. Tillman of New York, dis- played great ingenuity and a rare adaptation to the progress of chemical science ; but as it has not been generally adopted, an analysis of it would be out of place. A greater change has taken place in the notation than in the nomen- clature of modern chemistry. Many of the atomic weights have been doubled; graphic formulas for the expression of the molecular constitution and of the doctrine of substitu- tion in organic chemistry have been found ne- cessary ; and the student of the present day is