The first two volumes of this set dealt with "Theory and Methods" and "Biology and Medicine" respectively. The one under review is the first half of "Technological Applications." Since "Nature pays no attention to the classifications we make for our own convenience " there are chapters in this volume which might have been in the first just as there is much of interest to the technologist in the earlier volumes.
An important factor in lubrication, painting, cleaning, and adhesion is wetting. defines adhesional, spreading, and immersional wetting and gives a method for the determination of adhesional tension. After the surface has been wet, the strength of attraction between like and unlike molecules determines whether the wetting liquid will act as an adhesive or as a hlbricant. McBain and Alexander make an effort (pp. 9-26) to clarify the subjects of cohesion and adhesion.
Although the patterns differ in character from those of crystals, X-rays give evidence of structure in colloids (pp. 27-4o ). "The formation of rubber-like and liquid-crystalline states of matter" are discussed by yon Weimarn (pp. 89-IO2) "in connection with the classification of crystals according to their vectorial fields."
Although under certain conditions drops of liquid are spontaneously dispersed (pp. 6I-4), usually it is necessary to provide mechanical aid. Colloid mills furnish very powerful shearing action but alone give rise to "turbid dispersions" (pp. I79-88). For some purposes these are preferable to materials which are truly peptized. Deflocculated graphite (pp. 547-54) is in this class although it requires much less severe mechanical treatment, as is often the case when deflocculators are added. While in many cases it is desirable to for nl colloidal particles, they often must be removed. Electrical precipitation (pp. 189-2o6) metafilters (pp. 215-24) , and supercentrifuges (pp. 2o6-i4) are means which may be employed. The flotation process (pp. 225-38 ) is used for the preferential separation of larger particles with differing surface reactions.
To one who is interested in silica and its derivatives this volume contains much of interest. The "crushing and fine grinding of quartz" may seem to be simple but a twenty page article (pp. 157-77 ) indicates that this is not necessarily true. The crystalline material may even be changed to an anlorphous form. Lenher and his co-workers found this to be more pronounced when the grinding was done in water. Hydrated silica is usually produced by the acidification of soluble silicates (pp. 289~:)6 ). If properly prepared, silica gel is an excellent adsorbent for many vapors (pp. 1 I9-34). A non-aqueous system of colloid chemistry may be found in glass (pp. 283-8 ) and still another in minerals (pp. 261-82) which leads to the colloidal effects found in geology (pp. 251-6o). Ceranfie bodies such as porcelain (pp. 297-35o) and refractories ~pp. 351-6) introduce new factors in addition to the silic~ and the relatively simple silicates while soils (pp. 559~9o) are still more complicated. Here it may be necessary to resort to colloidal analysis (pp. 591-6) to simplify matters.