48 



NATURE 



[March io, 1921 



which, being embedded in the solution, give to 

 the whole its temporary rigidity and elasticity. 

 Many other lines of evidence support this view. 

 For instance, the optical evidence shows that the 

 structural elements in the gel are of very fine col- 

 loidal dimensions, far below the powers of the 

 microscope. This conclusion that the particles in 

 sol and gel are identical in number and nature 

 shows that nothing analogous to crystallisation 

 has taken place. 



Fig. 1.— Ultramicroscopic appearance of a soditiip soap 

 (o8iV Sod. Myristate, X 600). 



In clear contradistinction to this, curds and 

 coagula are formed by a process closely analogous 

 to crystallisation. Soap sols and gels show almost 

 nothing in the ultramicroscope with its dark 

 ground illumination, but when solidification to 

 white curd begins white fibres of barely micro- 

 scopic diameter are seen to shoot out until the 

 whole becomes a dazzling white felt of these fine 

 fibres. Fig. i (magnification 600) illustrates 



this appearance in a typical sodium soap, the 

 myristate, in this its permanent stable state. To 

 the naked eye it appears as a hard white cake 

 of soap. Fig. 2, the stearate, exemplifies the more 

 complicated behaviour of soft potassium soaps, 

 in which the fibres that first appear are extremely 

 short, and often twinned, but in which, on stand- 

 ing, true microscopic crystalline plates appear. 

 These tiny crystals undoubtedly account for the 

 "figging " which is seen in most good soft soaps. 

 Work at the Bristol University laboratory has 



Fig. 2. — Ultramicroscopic appearance of a potassium soap 

 (o's-V Pot. Stearate, X 500). 



not been confined to the elucidation of the results 

 here outlined, but an extensive programme of in- 

 vestigation of the colloid and phase-rule pheno- 

 mena involved in the typical processes of soap 

 boiling is in progress, in the expectation that the 

 precise elucidation of the behaviour of this par- 

 ticularly suitable and characteristic material may 

 lead to the better understanding of some of the 

 typical problems of the physical chemistry of the 

 colloidal state. 



Inland Waterways-^ 



T^HE outstanding feature of Mr. Minikin's book 

 -•- is the very interesting series of photographic 

 illustrations which it contains ; these impart a 

 most effective realisation of the physical character- 

 istics of the watercourses described in the text. 

 They are a most serviceable adjunct, and some of 

 the views have the additional charm of being pic- 

 turesque. We reproduce two by way of example. 

 The work consists of ten chapters, of which 

 the first is preliminary, and the second deals with 

 general considerations relating to torrential 



1 " Practical River and Canal Engineering." V>\- R. C. Royal Minikin. 

 Pp. vii+i33+i2 plates. (London: Charles Griffin and Co., Ltd., 1920.) 

 I2X. td. 



By Dr. Brysson Cunningham. 



phenomena, bends, valleys, and erosion, while 

 chap. iii. is on rainfall. The available rainfall, 

 or run-off, is said to vary between 20 per cent, 

 on permeable soils and 75 per cent, on imper- 

 meable ground. As limits, these are perhaps some- 

 what wide, and might, in this country at any rate, 

 be appreciably narrowed. From a survey of flood 

 discharges in England and Wales it has been com- 

 puted by Mr. Clayton that in average areas the 

 run-off to the sea is between 50 and 60 per cent, 

 of the total rainfall. Transpiration, as a source 

 of absorption of rainfall, receives little notice. 

 Chap. iv. deals with river surveys, and in par- 

 ticular describes methods applicable to running 



NO. 2680, VOL. 107] 



