TRANSACTIONS OP SECTION B. 369 



of the calcium hydroxide liberated crystallises in large plates, and is readily 

 detected by the microscope, whilst another part is adsorbed by the silicate gel. 

 The gradual increase of strength which is characteristic of calcareous cements 

 is a consequence of the continued adsorption and of the physical charges in the 

 structure of the gel. ' ... 



The colloids formed may be examined and characterised by staining with 

 dyes, such as methylene blue, patent blue, and safranin. The principal diffi- 

 culty in the microscopical examination of cement has hitherto been the brittle- 

 ness of the material, making it impossible to grind very thin sections, resulting 

 in loss of clearness. This may be avoided by treating the cement as a metal- 

 lographic specimen, grinding and polishing one surface only, and examining 

 under vertical illumination after etching with weak acids or staining with 

 other reagents. 



(v) The Rate of Coagulation of Colloidal Copper. 

 By H. H. Paine, M.A., B.Sc. 



The copper colloidal solution studied was prepared by Bredig's electrical 

 method, a positive hydrosol being thus obtained. The coagulation of this solution 

 by means of simple salts, such as sodium sulphate or potassium nitrate, appears 

 to be a gradual process. The precipitated gel can be separated from the solution 

 by allowing it to settle ; the clear liquid containing the copper not yet coagulated 

 can then be drawn off. The amount of copper present can be determined 

 volumetrically by titrating in the warm with dilute nitric acid, the disappearance 

 of the colour of the liquid indicating the complete solution of the copper by the 

 equivalent quantity of acid. This estimation of the copper still in colloidal solu- 

 tion can be made after definite intervals of time from the addition of the salt. 

 The course of the coagulative process can thus be studied. 



The following results among others were obtained : (1) There is an initial 

 period during which the solution remains clear and no coagulation takes place. 

 (2) Using colloidal solutions of various concentrations, the rate of precipitation 

 is proportional to the square of the initial concentration for the irreversible 

 coagulation produced by salts containing a divalent anion (sodium sulphate). For 

 salts containing monovalent anions (potassium nitrate or sodium chloride) the 

 coagulation is not irreversible, and a more complicated relation holds. (3) For 

 varying amounts of the electrolyte, the rate of coagulation is proportional to some 

 power °of the concentration of the salt; i.e., if we increase the concentration of 

 the salt proportionally, the rate of the coagulation will be increased proportionally 

 also. 



2. Report on the Study of Hydro-aromatic Substances. 

 See Reports, p. 99. 



3. Report on the Transformation of Aromatic Nilroamines. 

 See Reports, p. 94. 



4. Report on Electroanalysis. — See Reports, p. 98. 



1911. 1! l! 



