314 Royal Society : — 



beaucoup pres la totalite de la lumiere qui tombe sur leur surface ; 

 les corps imparfaitement transparens, et meme les plus diaphanes, 

 quand ils sont assez epais, absorbent aussi (pour me servir de 

 P expression usitee) une quantite notable de la lumiere incidente ; 

 mais il n'en faut pas conclure que le principe de la conservation 

 des forces vives, n'est plus applicable k ces phenomenes; il 

 resulte au contraire de Fidee la plus probable qu'on puisse se 

 faire sur la constitution mecanique des corps, que la somme des 

 forces vives doit toujours rester la meme (tant que les forces 

 acceleratrices qui tendent a ramener les molecules & leurs posi- 

 tions d'equilibre n'ont pas change d'intensite), et que la quantite 

 de forces vives qui disparait comme lumiere est reproduite en 

 chalexir" 



Should you consider the contents of this letter suitable for 

 the pages of the Philosophical Magazine, I should feel honoured 

 by its publication in that journal. 



Giessen, August 14, 1864. 



XXXVII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 240.] 

 June 16, 1864. — Major-General Sabine, President, in the Chair. 



THE following communication was read : — 

 " On the Properties of Silicic Acid and other analogous Col- 

 loidal Substances." By Thomas Graham, F.R.S., Master of the 

 Mint. 



The prevalent notions respecting solubility have been derived chiefly 

 from observations on crystalline salts, and are very imperfectly appli- 

 cable to the class of colloidal substances. Hydrated silicic acid, for 

 instance, when in the soluble condition, is properly speaking a liquid 

 body, like alcohol, miscible with water in all proportions. We have 

 no degrees of solubility to speak of with respect to silicic acid, like 

 the degrees of solubility of a salt, unless it be with reference to silicic 

 acid in the gelatinous condition, which is usually looked upon as des- 

 titute of solubility. The jelly of silicic acid may be more or less 

 rich in combined water, as it is first prepared, and it appears to be 

 soluble in proportion to the extent of its hydration. A jelly contain- 

 ing 1 per cent, of silicic acid, gives with cold water a solution con- 

 taining about 1 of silicic acid in 5000 water ; a jelly containing 5 

 per cent, of silicic acid, gives a solution containing about 1 part of 

 acid in 10,000 water. A less hydrated jelly than the last mentioned 

 is still less soluble ; and finally, when the jelly is rendered anhydrous 

 it gives gummy-looking white masses, which appear to be absolutely 

 insoluble, like the light dusty silicic acid obtained by drying a jelly 

 charged with salts, in the ordinary analysis of a silicate. 



