( 42 ) 



tbc modus operandi (concentrution of the gelatine and of the salts) 

 greatly different dimensions may be expected. Picton and Linuner 

 who among others have proved the extremely small conductivity of 

 the aqueous colloidal solution of arsenious sulphide, admit four diffe- 

 rent „grades" of division of this substance i). It will be found impos- 

 sible however to make clearly defined distinctions in this respect, 

 a gradual change in the size of the particles being much more probable. 



The study of the phenomenon of the internal light-reflexion and 

 fluorescence, that shows itself more or less distinely in several sub- 

 stances formed in gelatine, may perhaps lead to further insight; 

 these optical phenomena are also found in bodies as milky colored 

 glass, that also contains invisible, extremely small particles. Others 

 however of the colloid solutions in gelatine are not only perfectly 

 transparent, but do not show any reflection of the light or fluores- 

 cence; it may therefore be admitted that the division of matter in 

 gelatine approaches that of bodies soluted in water. In view of 

 phenomena as those stated here, the idea of being soluted seems to 

 lose its sharpness of outline. 



Equally interesting are the chemical reactions, which are the eftbct 

 of dift'usion, on the substances formed in gelatine and in which one 

 may be passed into the other. If a Na Cl-gelatine is surrounded by 

 a kgi Cr O4 gelatine the Na CI diffuses from the former gelatine into 

 the latter, the colloidal Agg Ci- O4 being not (or very little) diffusable, 

 while it is known (f. i. by experiments of Graham and Hugo de 

 Vries) that the salts dissociated into ions diffuse as quickly in 

 gelatine as in water. Now when the Na CI ions penetrate in the 

 bright red Agg Cr 0+ the latter is converted into Ag CI which, equally 

 colloidal in solution, remains transparent and spreads in the form 

 of a ring. 



Astronomy. — On the motion of the Pole of the Earth according 

 to the observations of the years 1890—1896. By Dr. E. F. 

 VAN DE Sande Bakhuyzen. (Communicated by Prof. H. G. 



A'AN DE SaNDE BaKHUYZICN.) 



1. In treating the meridian-observations made at Leiden in the 

 last 20 years, it was necessary to arrive at a knowledge as exact 

 as possible of the change in latitude during that period and, before 

 investigating the Leiden observations in this respect, it seemed de- 



») Joi ;. Chem. Soc. «1, 137, «», 63, St, 568, 



