128 



NA TURE 



[July 28, 1923 



art of geographical embroidery. We might even repeat 

 Mr. Philby's remark that some of tlicir statements 

 " bear no ponderable relation to fact." 



" The Heart of Arabia " must be reckoned as a most 

 valuable addition to the literary efforts that the 

 mysteries of Arabia have called forth. There is always 

 the danger in a work of this sort of descriptive narrative 

 lapsing into the style of the oflftcial route report. This 

 is most skilfully avoided by the author in his story of 

 everyday happenings in a society which is as old as 

 that of the patriarchs, and still exists in its patriarchal 

 form. Mr. Philby is much to be congratulated on his 

 remarkable experiences and his marmer of telling them. 



T. H. HOLDICH. 



Our Bookshelf. 



Flavouring Materials : Natural and Synthetic. By A. 

 Clarke. (Oxford Technical Publications.) Pp. xxi 

 + 166. (London : Henry Frowde and Hodder and 

 Stoughton, 1922.) 85. 6d. net. 



The manufacturer of foods and beverages, whose 

 demands have created the infinite variety of flavouring 

 materials now available, is a person with a remarkably 

 catholic taste, since he appears to take into account 

 anything with a flavour, from aloes to lemons, as 

 possible materials for making his wares attractive. 

 While laying the rose under contribution, he is appar- 

 ently not averse from keeping scatole in his mind's 

 eye as a possible means of titillating pleasantly the 

 olfactory nerves of his clients. It is quite clear when 

 such unpromising materials as some of these, not to 

 mention colocynth and senna, which the average man 

 regards as particularly nauseous drugs, can be seriously 

 considered as ingredients in foods and beverages 

 designed to be pleasant, that flavouring has become an 

 art which requires its own experts and its own litera- 

 ture. 



Mr. Clark's contribution, which he modestly describes 

 as notes accumulated during a number of years' work 

 in a technical capacity in the foodstufif and beverage 

 trades, is a good beginning, and gives within small 

 compass a mass of useful information regarding spices 

 and condiments, the methods used in determining their 

 quality, and the best ways of baulking the wily sophisti- 

 cator who substitutes ground date or olive stones for 

 powdered cinnamon or ginger. But spices in their 

 natural state are no longer the only materials on which 

 the flavouring expert can draw, and a considerable part 

 of the book is devoted to a summary of the characteristics 

 of purely chemical substances, extracted from essential 

 oils or made in the factory. The particular part they 

 can play in compounding flavours is described, and the 

 things they may or may not be blended with are duly 

 recorded. 



The statements regarding each product are reasonably 

 complete, and where further information is required 

 references to original literature are given. Altogether 

 the book is a good example of what technical literature 

 of this kind should be, and though it would be easy to 

 find points in it that are objectionable from a purely 



NO. 2804, VOL. 112] 



scientific point of view, they are not likely to misk 

 the reader for whom the book has been cf>mpik*d. 



! ^, 



The Theory of Emulsions and Kmulsif.catwn. By 

 William Clayton. (Text-books of Chemical 

 search and Engineering.) Pp. viii + i6o. (London:^ 

 J. and A. Churchill, 1923.) 95. Gd. net. 



Dr. Clayton's book is a fitting sequel to earlier 

 issues of the series of " Text-books of Chemical Re- 

 search and Engineering " to which it belongs. These 

 have included volumes on molecular physics, the 

 physics and chemistr>' of colloids, surface tension, 

 catalysis, and catalytic hydrogenation. The skilful 

 blending of advanced theory with advanced practice 

 which characterises Dr. Clayton's book is therefore 

 by no means a novel feature of these text-books. 

 The author claims that his chief aim has been *' to 

 follow a logical line of development based on modern 

 physico-chemical principles," and that " technical 

 applications of emulsions have only been introduced 

 either as illustrating some particular laboratory method 

 on a large scale or because some important theoreti '' 

 point is involved." 



While, however, much of the book is actually de- 

 voted to theor)', the practical aspects of emulsion- 

 making and emulsion - breaking are ver\' far from 

 being neglected. Indeed, one of the most striking 

 features of the later chapters is the large number of 

 references to patents covering processes for carr}-ing 

 out these contrary operations. One of the most 

 important applications of the process of emulsion- 

 making is the homogenising of milk and cream. A 

 process whereby new milk of 4 per cent, fat content 

 acquires the appearance of a cream containing 8 per 

 cent, fat, while a 15 per cent, cream becomes a good 

 substitute for a 25 per cent, cream, has obvious attrac-l 

 tions. The opposite process of breaking emulsions : 

 is an important operation in the initial treatment ofj 

 crude mineral oils; but it is also important in the; 

 de-oiling of condensed steam, as well as in the more 

 familiar operation of separating cream from milk and 

 converting it into butter. The book contains a 

 bibliography of nearly 200 papers dealing with emulsions 

 and marks a new era in the scientific study of a subjert 

 which has very important practical applications. 



T. M. L. 



Mathematics for Students of Agriculture. By Prof. 

 S. E. Rasor. Pp. viii + 290. (New York: The 

 Macmillan Co. ; London : Macmillan and Co., Ltd., 

 1921.) 16s. net. 



Every indication that mathematics is assuming a 

 more prominent position in the curriculum of students 

 of agriculture is very welcome. The mathematical 

 requirements of the agricultural student are roughly 

 twofold. First, he requires a knowledge of simple 

 calculations applicable to the routine problems of 

 fertilisers, feeding stuffs, surveying, buildings, book- 

 keeping, etc. Secondly, he very urgently requires an 

 elementar)^ understanding of statistical methods and 

 probabilities applicable to the interpretation of ex- 

 perimental results. Prof. Rasor's book deals mainly 

 with the first of these requirements. The value of the 

 book to agricultural students approaching mathematics 



