ON WAVE-LENGTH TABLES Ol-' THE 8PE0TBA OK THE ELEMENTS. 201 



Lanthanum — continved. 



Colloid Clieiaistry. Bij H. R. Puogter, M.Sc. 

 [Ordered by the General Committee to be printed in extenso.] 



MoDEBN colloidal chemistry may be said to begin with the work of 

 Graham about 18G1, though the colloids had long been known to 

 chemists, and Faraday ^ had prepared and described colloidal gold 

 solutions. Graham first pointed out the radical differences between 

 the colloid and crystalloid state, and introduced most of the nomenclature 

 cow in use, such as ' sol ' and ' gel ' for the apparent solution and the 

 precipitate or jelly, and distinguished their various liquid media by the 

 prefixes 'hydro-,' 'alco-,' Arc. It would be impossible to refer in detail 

 to the various workers who have since contributed to its advance, but 

 the following may be mentioned as marking distinct steps in its progress. 

 Wiedemann ^ showed that the absorption of water by solid colloids is in 

 its earlier stages accompanied by considerable evolution of heat, actual 

 solution and the fusion of jellies by abiorption, while in gelatinisation 

 heat is evolved. He later published researches on electrical osmose and 

 cataphoresis which liave important beir^ngs on colloidal theory. The 

 work of Van Bemmelen, 1888 et seq., on i^olloidal jpHies and precipitates 

 is of permanent value, and his theory of thp network or cellular structure 

 of jellies has been very generally accepted by later workers, though, in 

 your reporter's opinion, on insufficient evidence. In 1889 M. Carey Lea 

 published extensive researches on ' allotropic ' (colloidal) silver,-* and in 

 1892 Picton and Linder's work on ' Solution and Pseudo-solution ' ^ opened 

 new ideas on the nature of colloidal solution, and perhaps of solution 

 in general. In 1893 Siedentopf and- Zsigmondy, building on the earlier 

 work of Tyndall on the scattering and polarisation of light by non- 

 homogeneous media, invented the ultra-microscope, in which, by a special 

 form of dark-ground illumination, particles which were much too small 

 to give an actual microscopic image were rendered visible as points of 



• P?ia. Trans., i. 1857, pp. 145-152 ; Phil. 2Iag. (4), 14. ^01-417, .'■>12-530, 



- Verh. d. 2)hysili. Ges. zu Berlin, 18S4, p. 44. ' 



■■* SiUiinan's Jour. (3), 37, 476-491, 38, 47-50, 1SH9. 



^ Chcm. Xews and Jour. Ch. Soc, 1893. 



