Tlic authors in their preface tell us that the prominence which has been given t o tlie use of bituminous road eniulsions cluring the past eight or ten years is due to their intlependencc of watlicr conditions, tlir case with which the crnulsioils can lie applied, the succcss with ivhicli they h a w lieen employed, :ind t Iic satisfactory service they giro when properly cniployctl. But, as is ivell knoivn, not a11 attempts to use bituininous road emulsions have been succcssful : i d tlic aut Iiors think it still '* sufc to say that even now tlic hrge tiiujority of those road engineers using emulsions (lo not thorougltly untlcrstund either their pliysical properties or inct hods of mnnufacturc." l'resuni:ilJly, therefore, tlie present volume is intended to IJring i1 clearer undcmtnndirig of the inany problems iiivolvetl in this sort of road work to those concerned wit11 tlio proper care of our liigliwiys. On tlic whole, the iiutliors have discliargcd tlicir task ably.
I n nine chapters they treat of colloid cliciiiistry, the merits of IJitunicn and allicd sulistni~ccs used in road work with tlie nictliocls of testing tlicsc, c2tar:ictcristics of cniulsions and emulsifying agents, processes of tilanufacture, dcscription of emulsion niainhcturing works, tests for bituniinous emulsions, uses of such cinitlsiont, on roadways, atlvnntagcs and disadvaiitages of hot :ind cold emulsions respectively, concluding with a cliupter on specifications iind costs.
The principal criticism wllich may be made is that the authors are not very fniiiiliur with ~v l i i i t may be ternied the purely cheiiiical aspects of the subject. This is vcry upparent in tlic first chapter dealing with colloid chemistry. IIerc the autliors Iiave made frequent use of Wolfgang Ostwaltl's book on the subject, but in spite of this they use the terms '* colloid '* and " colloidal " very indiscriminately. Moreover, :L