2 94 



NA TURE 



[yan. 27, 1 88 7 



these densities and that of water at ioo\ i.e. their specific 

 gravities at 100°. But for any particular instrument, the 

 different values obtained with different fatty matters 

 exhibit the same differences as those obtained with any 

 other instrument ; whilst the indications of any two 

 instruments are obviously comparable, provided that the 

 mode of graduation and the coefficient of expansion of 

 the plummet are the same ; which practically is the case 

 if a glass plummet be always used, as recommended. In 

 a recent communication to the Anxiyst (January 18S7, 

 ip. 18), the author suggests that the term '''■indicated 

 filummet-gravity " should be employed to represent the 

 apparent values obtained at such and such a temperature 

 by means of the plummet-balance ; which is clearly pre- 

 ferable to the use of either of the terms " spscific gravity " 

 or " density " in such cases. It may be noticed in passing 

 that the " indicated plummet-gravities " of fats and oils at 

 100° or thereabouts by no means necessarily follow the 

 same order as the so-called specific gravities obtained at 

 lower temperatures, not only on account of difference of 

 physical state, but also through the different rates of ex- 

 pansion possessed by the various substances. 



Another point o;i which the author has worked with 

 results of some interest is the determination of the amount 

 of glycerol yielded by fats and oils on saponification. He 

 concludes that there is no experimental basis for the 

 suggestion put forth some time ago by Wanklyn and Fox 

 that isoglycerides are present in such substances, these 

 bodies yielding on saponification propionic (or other 

 homologous) acid and water, instead of glycerol. Such 

 a view is opposed not only to the author's laboratory 

 experience, but also to that of manufacturers, who 

 frequently recover 7 '5 to 8'o pirts of glycerol per 

 100 of fatty matter, instead of less than 5 as stated by 

 Wanklyn and Fox. Making allowance therefore for 

 deficient saponification and loss of glycerol by evapora- 

 tion during recovery, the theoretical amount of glycerol 

 obtainable is satisfactorily accounted for, instead of 

 being largely in excess of that actually produced. It is 

 noticeable that, whilst the author obtained results reason- 

 ably concordant with the permanganate process for the 

 determination of the glycerol produced during saponifica- 

 tion as compared with the other ordinary methods in 

 many instances, this was not always the case, the former 

 process sometimes yielding figures far in excess, indi- 

 cating the presence of other substances besides glycerol 

 capable of forming oxalic acid by treatment with per- 

 manganate. An extreme case was afforded by linseed oil 

 dried up to elastic skin, which gave 4'9 per cent, of 

 impure glycerol directly isolated, and i5'5 per cent, by 

 the permanganate process. 



The author considers that the usually received mole- 

 cular weight of linoleic acid, generally represented as 

 indicated by the formula CjuHosOj, is incorrect, as the 

 mean equivalent of the acids obtainable from linseed oil 

 on saponification has been found by him to be con- 

 siderably higher than that thus indicated. The formula 

 Ci8H3202 agrees better with his results, and moreover is 

 not at all incompatible with the analytical data obtained 

 by previous investigators. The analogo rs determination 

 of the mean molecular weight of the acids produced on 

 saponification (by means of alcoholic potash and phenol- 

 phthalein) of fatty, waxy, and oily matters, and of the 



fatty and resinous acids contained in soaps, is justly re- 

 garded by the author as a valuable criterion in judging of 

 the nature of such substances, especially when taken in 

 conjunction with other data {e.g. in the case of butter-fat, 

 the amount of volatile acids capable of being subsequently 

 distilled off along with water, working under particular 

 constant conditions as recommended by Reichert ; and 

 the proportion between acids soluble and insoluble in 

 water, &c.) ; and a large amount of experimental work 

 has been done by him in connection with such valuations. 

 He has also made an excellent digest of recent researches 

 in connection with fats, oils, waxes, soaps, and analogous 

 bodies. In connection with toilet-soaps, he regards the 

 addition of sugar to produce transparency as " simply 

 diluting the true soap-material as so much water would 

 do, without communicating any compensating property 

 of value." This is a very mild way of putting the case, 

 the fact being that soaps containing sugar are liable to 

 produce, in sensitive subjects, a great amount of irritation 

 of the skin (in fact, a mild kind of '" grocer's eczema," 

 traceable to the same cause), even though free from 

 causticity and otherwise unobjectionable : and numerous 

 persons are, to the reviewer's knowledge, unable to use 

 certain widely-advertised soaps, in consequence of the 

 large admixture of sugar therein contained, although the 

 composition would otherwise be quite uninjurious. 



The sections devoted respectively to hydrocarbons and 

 phenols are equally comprehensive, including descriptions, 

 necessarily in some cases somewhat brief, of the leading 

 points in the petroleum, coal-tar, and shale-oil industries, 

 and of the technical examination of the various products 

 obtained on fractional distillation and subsequent further 

 treatment of these and allied raw materials, e.g. wood-tar ; 

 and the commercial examination of turpentines and resins, 

 essential oils, camphors, and various miscellaneous sub- 

 stances, such as cantharidin and cbolesterin. In these, 

 as in the previous section, the value of the various precis 

 given of other observers' work is frequently further en- 

 hanced by the comparative experiments and examinations 

 made in connection with different analytical methods, &c., 

 by the author himself Bone tar, obtained as a by-product 

 of the animal charcoal manufacture, is not described, 

 probably on account of the limited uses to which, hitherto, 

 it has been put ; whilst, for similar reasons, but little is 

 said of blast-furnace and co'ce-oven tars. 



C. R. Alder Wright 



OUR BOOK SHELF 



Practical Dynamo-Building for Amateurs. By Fred. 



VV. Walker, M.E. (London: Ihfife and Son, 1886.) 

 Would that all books for amateur guidance were written 

 with as full a knowledge of proper principles as this 

 unassuming little work. We are not saying that the 

 machine which the author recommends amateurs to con- 

 struct is the equal of the commercial dynamos of the current 

 date. His field magnet cores are of cast iron, and not of 

 the best form ; his armature might be improved by getting 

 greater cross-section of iron into it. But there is nothing 

 wrongly done. All instructions about the essential 

 details of proper insulation and testing of the work in 

 progress are accurate and full ; and an appendix on 

 alternative constructions of field-magnets supplies in some 

 degree the deficiencies of the earlier text. 



