4io 



NA TURE 



[March i, 1906 



is subdued to what it works in, we feel, too, that 

 that is an essential ingredient of his success, and 

 that with less complete absorption his work might 

 have been less brilliant as well as less convincing. 



R. A. S. 



ULTRAMICROSCOPIC STUDIES OF THE 

 COLLOIDS. 

 Zur Erkenntniss dcr KoUoide. Ueber irreversible 

 Hydrosole und Ultramikroskopie. By Richard Zsig- 

 mondy. Pp. vi+185. (Jena: Gustav Fischer, 1905.) 

 Price 4 marks. 



THIS work forms a valuable addition to the litera- 

 ture of the colloids, giving as it does an authori- 

 tative account of the results obtained through the 

 application of the method of ultramicroscopy to the 

 study of solutions of colloids. 



A brief account is first given of the nature and 

 properties of colloid solutions or hydrosols. At the 

 outset the author refers to the difficulty of giving a 

 satisfactory definition of the term " solution." He 

 adopts homogeneity as the most universal character- 

 istic of solutions. The definition of homogeneity will 

 naturally vary according to the delicacy of the methods 

 employed to test it. , By means of the method of ultra- 

 microscopy devised by Zsigmondy and Siedentopf, the 

 majority of colloid and even some crystalloid solutions 

 can be shown to be optically heterogeneous. Every 

 increase in the accuracy of the methods of examina- 

 tion would lead to a further limitation of the term 

 "solution." In order lo include the colloids Zsig- 

 mondy defines solutions as uniform distributions of 

 solids in fluids, which are transparent to ordinary 

 light, and not separable into their constituents by the 

 action of gravity or by filtration. 



In order to gain a clearer idea of the nature of 

 colloid solutions, it is necessary to find criteria for 

 distinguishing such solutions from those of crystal- 

 loids on the one hand and from suspensions on the 

 other. Zsigmondy only refers very briefly to the dis- 

 tinguishing characteristics of the former,- as this 

 subject has been previously treated by Bredig in 

 his monograph on " Inorganic Ferments." He deals 

 more fully with the properties of colloid solutions 

 which distinguish them from suspensions. In this 

 connection he mentions the following as the chief 

 features distinguishing colloid solutions from suspen- 

 sions : — 



(1) The particles in colloid solutions are much 

 smaller than in suspensions. In colloid solutions the 

 average diameter of the particles varies from 5 to 

 20 mm. This difference is, however, one only of 

 degree. 



(2) Many colloids are capable of undergoing irre- 

 versible changes. Separation of a metal from its 

 colloid solution may be readily brought about by the 

 withdrawal of water or the addition of electrolytes. 

 In this process the metal has undergone an irre- 

 versible alteration or coagulation. For the reforma- 

 tion of the colloid solution, chemical or electrical means 

 must be employed. In the case of suspensions, on 



NO. 1896, VOL. 73] 



the other hand, sedimentation rapidly takes place 

 under the influence of gravity, and its rate is little 

 influenced by the withdrawal of water or by the 

 addition of electrolytes. The suspension may be re- 

 formed by purely mechanical means. 



(3) Alterations in the total energy of the system are 

 frequently associated with the process of coagulation. 

 These have been measured in several cases by means 

 of the calorimeter. 



(4) Colloids in solutions are capable of undergoing 

 reactions with one another, which closely simulate 

 purely chemical reactions. 



The next section of the book deals with the classifi- 

 cation of colloid solutions or hydrosols. The classi- 

 fications of the hydrosols have been based on two 

 principles, namely, the size of the particles and the 

 reversibility or irreversibility of the hydrosol (Hardy). 

 On plate i. the author gives a graphic representation 

 of a classification of colloids founded on these prin- 

 ciples. The reversible colloids differ from the irre- 

 versible in not being readily coagulated by the addition 

 of electrolytes. It is noteworthy that irreversible col- 

 loids may be partially protected from the coagulating 

 action of electrolytes by the addition of a reversible 

 colloid to their solutions. Great quantitative differ- 

 ences are found to exist in the extent of protection 

 given by different reversible organic colloids to irre- 

 versible gold hydrosols. 



A historical account of the preparation and pro- 

 perties of irreversible colloid solutions occupies the 

 next section of the book. 



The author next gives an interesting account of the 

 development of the method of ultramicroscopy by 

 Siedentopf and himself. A full description is also 

 given of the necessary apparatus and of the method 

 of using it. 



The succeeding sections give details of the results 

 of his own researches on gold hydrosols. By means 

 of the ultramicroscope he was enabled to determine 

 approximately the average size of the gold particles, 

 their colour, and the rapidity of their movements both 

 translatory and oscillatory. The limit of size deter- 

 minable by the ultramicroscope appears to be about 

 6 MM in the case of gold hydrosols. Still smaller 

 particles (amicrones) are also present in gold hydro- 

 sols. Their presence may be proved by the coagu- 

 lation of the hydrosols on the addition of electrolytes. 



An excellent summary is also given of the results 

 obtained by other observers through examination of 

 various colloid solutions by means of the ultra- 

 microscope. 



Brief reference only is made to some points of 

 great theoretical interest, namely, the causes of the 

 stability of colloid solutions, and the mechanism of 

 their formation. 



I lu book concludes with a short summary of what 

 js known with regard to the products of coagulation 

 of colloid solutions. 



The work as a whole is to be regarded as a valuable 

 monograph indispensable for those interested in the 

 ultramicroscope and its applications. 



j. A. Milroy. 



