372 



SCIENCE. 



[N. S. Vol. XXIV. No. 612. 



the features of Alaska, the Alps, Norway 

 or New Zealand. In face of the fact that 

 such peculiar, strongly marked and abnor- 

 mal features always occur concordantly in 

 regions known to have been extensively 

 glaciated, any other hypothesis than that 

 they are due directly or immediately to 

 ice action of some kind seems gratuitous 

 and impertinent. To set up such trifles 

 as a patch of residual soil, an island in the 

 lake, a shore cliff, etc., as competent to 

 negative the evidences of glacial erosion is 

 to swallow gnats and strain out a camel. 

 A gnat may be as difficult to explain as a 

 camel, but it is of relatively little conse- 

 quence whether he is explained or not. As 

 a means of progress across the desert of 

 hypothesis to the oasis of conclusion, the 

 camel may be relied on and the gnats that 

 annoy him disregarded. The various ses- 

 sions and excursions of the section were 

 attended by thirty different members of 

 the association. 



The section finally adjourned at about 

 4 P.M., Tuesday, July 3. 



Edmund Otis Hovey, 



Secretary. 



SCIENTIFIC BOOKS. 

 Zur ErJcenntnis der Kolloide. By Eichard 



ZsiGMONDY, Jena, Gustav Fischer. 1905. 



Pp. vi + 185. 4 Marks. 



The names of Siedentopf and Zsigmondy 

 have become familiar to the whole scientific 

 world through the brilliant researches which 

 resulted in the development of the ' ultra 

 apparatus ' ; their device for observing par- 

 ticles so small as almost to reach the hypothet- 

 ical molecular dimensions.* 



In this monograph, ' Zur Erkenntnis der 

 Kolloide,' Zsigmondy has given a careful ac- 

 count of his own and Siedentopf's methods 

 and results, a brief historical review of many 

 other noteworthy researches and a short dis- 



^ The original articles appeared in the Zeitschrift 

 filr Elektrochemie, Vol. VIII., pp. 684-687 (1902) 

 and in Drude's Annalen, Vol. X., pp. 1-39 (1903). 



cussion of the bearing of the ' ultra ' methods 

 upon certain theoretical issues. 



He states that the main purpose of his 

 researches on colloids has been to determine 

 whether or not the polarization and disper- 

 sion of light in Tyndall's experiment is an 

 essential characteristic of all hydrosols or col- 

 loidal solutions. He has answered this ques- 

 tion in the affirmative in numerous cases, 

 having demonstrated that the dispersion of 

 light is due to the presence of the same par- 

 ticles to which are to be ascribed the other 

 remarkable properties of the liquids contain- 

 ing them. 



It may not be amiss to state briefly the prin- 

 ciple of Tyndall's experiment and of the ' ultra 

 apparatus,' and to recount some of the inter- 

 esting facts described in the book. A beam of 

 light is sent through the space under observa- 

 tion, and the observer looks at the space in a 

 direction perpendicular to the course of the 

 beam. If dust, or other fine particles, are 

 present in that space, they polarize, disperse 

 and reflect the light, and the beam is seen. 

 If no such particles are present, the beam is 

 not seen, and the space is said to be optically 

 empty. 



With all the resources of the famous firm 

 of Carl Zeiss at Jena at their command, Sie- 

 dentopf and Zsigmondy constructed their ap- 

 paratus to send a beam of great intensity, a 

 minute image of the sun, into the medium 

 under investigation, and observed through the 

 best of microscopes. Under these conditions, 

 the particles of metallic gold in a colloidal 

 gold solution appear as brilliant sources of 

 light, but their shape can not be determined. 



Siedentopf and ZsigTnondy counted the num- 

 ber of these bright spots in a space of known 

 dimensions, they knew the concentration of 

 the gold solution (the weight of gold per unit 

 volume) ; they assumed that all the gold 

 present was visible, that it had the same spe- 

 cific gravity in this finely divided state as 

 when massive, and that the particles were 

 cubes. From these values they easily cal- 

 culated the linear dimensions of the indi- 

 vidual particles. It will be seen at once that 

 these dimensions can not be regarded as as- 

 certained, because of the assumptions, but 



