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This study demonstrates that N-mono-and N,W-di-substituted ureas and thioureas tnay expediently be synthesized, in essentially theoretical yield, by reacting primary or secondary aliphatic, alicyclic, aralkyl, aromatic, and heterocyclic amines with silicon tetraisocyanate or tetraisothiocyanate, or the corresponding methyl-substituted analogues. The method fails with aromatic amines possessing bulky (e.g. phenyl, bromo), or strongly electronegative (e.g. trifluoroinethyl), substituents in the 2-, or 2,6-positions of the aromatic nucleus. Data on several new substituted ureas and thioureas are reported, and improved preparative procedures for the silicon pseudohalides are described. I t has long been known that organic isocyanates and isothiocyanates react readily with primary and secondary aliphatic and aromatic anlines to produce substituted ureas and thioureas. Indeed, these reactions are conlmonly employed to characterize amines and to prepare urea derivatives (1, 2). Despite the large amount of literature available on con~pounds containing the carbon-isocqanate and carbon-isothiocyanate groups, very few studies have been published on the reactions of amines with compounds containing the silicon-isocyanate (Si-NCO) and silicon-isothiocyanate (Si-NCS) groups.In 1951, Anderson (3) reported that silicon tetraisocyanate and several alkyl-and arylsilicon mono-, di-, and tri-isocyanates and isothiocyanates react readily with aniline, but the products resulting from these reactions were not characterized. Later, O'Brien (4) showed that trimethylsilicon isocl-anate in benzene solution reacts with n-, i-, s-, and t-butylamines t o produce butylureas. In 1958, Neville (5) demonstrated that silicon tetraisocyanate and isocyanates of empirical formula R,Si(NCO),-, (R = Ale, Ph; x = 0, 1, 2, 3) reacted alone or in benzene solutioil with ally-lainine, diallylamine, aniline, benzylamine, and o-and 9-toluidines to produce essentially theoretical yields of the corresponding N-mono-or K,N-di-substituted ureas. In every case the isocyanates and amines reacted in stoichiometric proportions and intermediate silylureas were not isolated. T h a t these reactions do, in fact, i~lvolve silylurea intermediates was shown by Goubeau and Heubach (6), who reacted aniline and several secondary aliphatic amines with methylsilicon isocyanate~, ;\lezSi(NCO)4-z (x = 0, 1, 2, 3), in petroleum ether." The silylureas isolated

Superficial observations on the chemical and physical properties of air were made in remote antiquity. ft, was recognized that air was in some way necessary to support fire and flame. The fact that air (or gas) possesses pressure was also clearly recognized, though not, proved. The Aristotelian notion that "nature abhors a vacuum" still persisted in the seventeenth century, and it, was this hypothesis that led to many heated discussions between the protagonists of the theory (the plenists) and the antagonists who believed that it. was possible to create a vacuum.We shall very briefly outline the experiments which led to the overthrow of the Aristotelian hypothesis.It was not until the mid-17th century that, any systematic or scientific investigations of the air were carried out.

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