chemists to the value of these compounds as agents, and that the use which Hofmann in turn made of them is traceable to this circumstance; all he tells us, however, is that "the compounds of these radicles (methyl, ethyl, &c.) with chlorine, bromine and iodine appeared to be the most appropriate substances" for the purpose he had in view. Their introduction as substituting agents, however, clearly dates from this time, as indeed, Hofmann himelf pointed out a few years afterwards "These successive substitutions (of hydrogen in ammonia) were accomplished by the action of ammonia upon the bromides of the alcohol radicles, which since that time have become most valuable agents of substitution in the hands of chemists" (Proc. Roy. Soc., 1858, 9, 151). Aniline was without difficulty converted into ethyl- and diethylaniline, beyond which stage the substitution could not be carried.* As the passage of ammonia into aniline had been accomplished, as already pointed out, in the course of the experiments Laurent and he had previously made, in which phenol was acted on by ammonia at a high temperature— "The formation of aniline, ethylaniline, and diethylaniline appeared to have established in a sufficiently satisfactory manner the point of theory in question." Such are the words in which Hofmann recognises the bearing of his results the calmness and apparent indifference with which he views his success is almost aggravating. : A true man of action, he at once goes on to remark— "Still I thought desirable the acquisition of additional facts in support of the position to which this inquiry has conducted me. Thus I have been led to study the action of bromide of ethyl upon several of the derivatives of aniline, and to try whether other alcohol radicles, such as methyl and amyl, would have a similar action; lastly, in order to complete the investigation, I was obliged to leave the amidogen bases altogether, in order to submit the typical ammonia itself to examination.”† land, in the introductory remarks prefacing the first of these papers in his volume of Experimental Researches (p. 65), in fact states: "In these experiments the iodides of the alcohol radicles were employed as reagents, I believe for the first time." As we know that, when Frankland's paper was communicated to the Society, "Dr. Hofmann exhibited a specimen of the zinc methyl he had obtained from Dr. Frankland, and demonstrated its spontaneous inflammability," it is evident that his attention had been very specially directed to the work. See also Frankland (J. Chem. Soc., 3, 48). * It was afterwards recognised that the products obtained on this occasion were not pure substances. The preparation of pure methylaniline and the relative proportions of methyl- and dimethyl-aniline formed on methylating aniline by means of methyl chloride, bromide and iodide were fully gone into by Hofmann many years later (Ber., 1874, 523; 1877, 591). The action of ethyl chloride on an alcoholic solution of ammonia was examined The action of ethyl bromide on chlor- and bromaniline and nitraniline was first studied; the results obtained were important as throwing light on the constitution of these substitution bases. To quote Hofmann's words "The possibility of introducing into these substances* two equivalents of ethyl, shows that they must contain the same amount of basic hydrogen (an expression by which I may be allowed to represent briefly the hydrogen of the ammonia-skeleton) as aniline itself, and hence it is evident that it was the hydrogen of the phenyl which was replaced by chlorine, bromine, and hyponitric acid in the transformation of aniline into its chlorinated, brominated, &c., relatives. The transformation is due to a secondary substitution, affecting the hydrogen in the radicle which replaced the original ammoniahydrogen." This seems to have been the only occasion on which the hydrocarbon radicle in aniline in any way engaged Hofmann's attention or excited his curiosity-his interest in aniline seems to have been centred so entirely on its basic properties, his interest in bases to have been so great, that it was long before other problems in any way attracted him. After preparing a variety of substituted amines from aniline, and showing that it was possible to displace the three hydrogen atoms in ammonia by three different hydrocarbon radicles, Hofmann showed that ammonia could be directly converted, not only into the ethylamine already prepared by Wurtz, but also into a di- and a triethyl derivative; and at once foreseeing the probability "that arsenietted and phosphoretted hydrogen, which imitate to a certain extent the habits of ammonia," would afford similar derivatives, he announces his intention of extending his researches to these hydrides, a promise which was amply fulfilled in after years. Hofmann, in the first instance, speaks of the three kinds of base derived from ammonia as amidogen, imidogen and nitrile bases respectively. The terms primary, secondary and tertiary, to which we are so accustomed, appear to have been first used in a lecture which he delivered to the Chemical Society in June, 1858 (J. Chem. Soc., 11, 255), in which the classification of the various derivatives of ammonia-both amines and amides-then known is discussed at great length, reference being also made to the basic derivatives of phosphorus, arsenic and antimony. The closing words of this lecture are worth quoting in Hofmann's laboratory by C. E. Groves in 1861 (J. Chem. Soc., 13, 331). He obtained the monethylated compound almost exclusively. Hofmann, working on a large scale, subsequently obtained a mixture of mono-, di-, and tri-ethylamine containing a slight excess of the secondary amine (Ber., 3, 776). * The statement here made that nitraniline could be methylated and ethylated was corrected in a later paper (Roy. Soc. Proc., 1862, 12, 639). "In the preceding paragraphs I have endeavoured to give an outline of the chemical history of the type ammonia, such as it exhibits itself at the present moment. No department of chemistry, perhaps, reflects in a more salient manner the rapid progress of science during the last quarter of a century. Nearly all the bodies mentioned in this paper were discovered during that period; nine-tenths, in fact, the very compounds which have most assisted in the elaboration of our theoretical views, are the fruits of the last 10 years. Though much has been achieved, more is left to be done. But even now, while so many known substances remain to be finally grouped and classified, and while countless groups and classes remain to be discovered, yet the general features of the system are distinctly perceptible, and the time is rapidly approaching when, losing sight of the chaotic mass of overwhelming detail, the eye may rest with complacency upon the simple beauty of the law which governs the construction of the bodies belonging to the type of ammonia." When we recollect how entirely we owe to Hofmann the establishment of the "simple beauty of the law" governing the constitution of amines generally, this passage may justly be selected as a fit epitaph to attach to his work. The monamines were largely used by him in subsequent investigations, as will appear in the sequel, and therefore, in concluding the account of his researches of the monamines, it will only be necessary to refer here to the process which he devised of separating the ethylamines, and also to his proposals for effecting the diagnosis of amines. Although they differ considerably in boiling point, it is impossible to separate the ethylamines by fractional distillation. The "simple and elegant process," as he himself originally termed it, devised by Hofmann (Proc. Roy. Soc., Nov., 1860, xi, 66) is based on the observation that when submitted to the action of ethylic oxalate, ethylamine is converted into diethyloxamide, a crystalline substance, only sparingly soluble in water; whilst diethylamine yields ethylic ethyloxamate, a liquid boiling at a high temperature; triethylamine remaining unchanged. Hence, after subjecting the mixture to the action of the oxalate, it is possible to separate the tertiary base by distillation, and to mechanically separate the crystalline amide from the oily oxamate; the amide may then be purified by recrystallisation from boiling water, and distilled with alkali, the oxamate being similarly treated after separating the dissolved oxamide by cooling to 0°, and then fractionally distilling the liquid. * The boiling points, &c., of the various methyl- and ethyl-amines were determined by Hofmann in 1889 (Ber., 699), with the aid of large quantities of purified material placed at his disposal by Kahlbaum. In describing his results he states that the experience gained in working on the large scale had proved that these amines were best prepared by his original method. It was subsequently pointed out (ibid., p. 526) that a simpler and more perfect separation of the oxamide and oxamate might be effected by submitting the mixture at once to the action of boiling water, when diethyloxamide dissolves, the oxamate remaining as an insoluble layer floating upon the hot solution. Hofmann re-examined the process ten years later, when the opportunity occurred to prepare the ethylamines on a large scale with the aid of ethyl chloride obtained as a bye-product in the preparation of chloral (Ber., 3, 109, 776). The criticisms to which the method had been subjected were considered, and a full account was given of the manner of operating which experience had shown to be the best. Provided that sufficient material be at disposal, the diagnosis of the amines is easily effected, following the indications afforded by Hofmann's inquiry, by ascertaining the number of methyl or ethyl groups which can be introduced in place of hydrogen, but the method involves the analysis of the product. A qualitative method of perfectly general application in the case of primary, but of restricted. application in the case of secondary amines, was proposed by him in 1870 (Ber., 3, 767), after he had discovered the isonitriles. All primary amines yield the corresponding isonitrile when warmed with alcoholic potash and chloroform, and these nitriles are all possessed of intolerable odours. On the other hand, both primary and secondary paraffinoid amines form thiocarbamates with carbon bisulphide, and these are converted by mercuric chloride into mustard oils, which are also characterised by their evil odours. By reversing the test, the primary amines become remarkably delicate tests of chloroform; it is possible, for example, to detect with certainty 1 part of chloroform in 5000 to 6000 parts of alcohol by means of aniline. In the second memoir, entitled "Researches into the Molecular Constitution of the Organic Bases," published in the Phil. Trans. of the Royal Society (1851, ii, 357; Abstr., J. Chem. Soc.), Hofmann considers "the history of a new group of alkaloïds, which, although intimately connected (with the amines) by their origin, differ from them altogether by their properties, and especially in not being volatile." After having succeeded, he tells us, in replacing, step by step, one, two, or three equivalents of hydrogen in ammonia by a corresponding number of compound atoms, the question arose whether the agent by which these changes were effected would exert a still further influence on the last product of the action. In order to decide this question by experiment, he submitted two nitrile bases, diethylaniline and triethylamine, to the prolonged action of bromide of ethyl. These experiments did not lead to perfectly decisive results. It was evident that in these cases an action ensued. No doubt, however, could be entertained as to this action being altogether different from the preceding stages of the process. A mixture of an aqueous solution of triethylamine and bromide of ethyl sealed up in a tube, solidified after several hours ebullition, the crystals consisting chiefly of the fibrous hydrobromide of triethylamine, with which, however, a certain quantity of white, opaque, granular crystals was invariably mixed. Anhydrous materials were therefore used, in the hope of increasing the quantity of this latter substance, but the progress of the action was found to be retarded in a most remarkable manner by this variation, days elapsing before the conversion was completed. But the difficulty thus met with was soon overcome by using iodide instead of bromide of ethyl, action taking place almost instantaneously on warming a mixture of the anhydrous substances, the product-as every chemical tyro now knows-being an iodide formed by the direct combination of the triethylamine with the ethyl iodide. The properties of the iodide thus produced are so remarkable, there is still so much that requires explanation in connection with such compounds, that the account Hofmann originally gave may be quoted almost verbally with advantage, both because it is historically full of interest, and in order that attention may be directed to a subject which is rarely more than superficially noticed in our textbooks. It is in no way surprising that he should remark— "I confess I had not anticipated the possibility of the existence of (such) a compound; it appeared at the first glance in direct opposition to the theoretical view repeatedly referred to. For if it was possible to decompose the new iodide by potassa, just as the preceding iodides, .. if it was possible to separate from this substance a new volatile base analogous to the preceding ethylated alkaloïds, there was no reason why the process of ethylation should cease with the fourth equivalent of ethyl. On the contrary, it then appeared more than probable that the formations of ethylamine, diethylamine, and triethylamine, far from being due to the successive replacement of the various equivalents of hydrogen in ammonia, were rather to be considered as special instances of a far more general tendency of carbon and hydrogen to accumulate in organic substances. "The eagerness with which I submitted this question to the test of experiment may be readily imagined "-Indeed, it may !" On adding potassa to a solution of the iodine compound in water, the liquid solidified at once into a crystalline mass, which I took of course for the base existing in the iodide; analysis, however, soon showed that the substance thus precipitated is nothing but the iodide itself, which is less soluble in an alkaline solution than in pure water. The iodide was now distilled with a very concentrated solution of potassa for several hours; but during this time nothing but pure water passed over. The solution in the retort solidified on cooling to crystals of the unchanged iodide. When the solution of the iodide in |