380 Mr. R, T. Lewis on the Changes in the 



membrane^ and it forms the osmose. The contact of the saline 

 thud is thus attended by a continuous catalysis of the gelatinous 

 hydrate, by which it is resolved into a lower gelatinous hydrate 

 and free water. The inner surface of the membrane of the 

 osmometer contracts by contact with the saline solution, while 

 the outer surface dilates by contact with pure w T atcr. Far from 

 promoting this separation of water, the diffusion of the salt 

 throughout the substance of the membrane appears to impede 

 osmose, by equalizing the condition as to saline matter of the 

 membrane through its whole thickness. The advantage which 

 colloidal solutions have in inducing osmose, appears to depend 

 in part upon the low diffusibility of such solutions, and their 

 w r ant of power to penetrate the colloidal septum. 



The substances fibrine, albumen, and animal membrane swell 

 greatly when immersed in water containing minute proportions 

 of acid or of alkali, as is well known. On the other hand, when 

 the proportion of acid or alkali is carried beyond a point peculiar 

 to each substance, contraction of the colloid takes place. Such 

 colloids as have been named acquire the power of combining with 

 an increased proportion of water, and of forming superior gela- 

 tinous hydrates, in consequence of contact with dilute acid and 

 alkaline reagents. Even parchment-paper is more elongated in 

 an alkaline solution than in pure water. When so hydrated and 

 dilated, the colloids present an extreme osmotic sensibility. 

 Used as septa, they appear to assume or resign their water of 

 gelatination under influences apparently the most feeble. It is 

 not attempted to explain this varying hydration of colloids with 

 the osmotic effects thence arising. Such phenomena belong to 

 colloidal chemistry, where the prevailing changes in composition 

 appear to be of the kind vaguely described as catalytic. To the 

 future investigation of catalytic affinity, therefore, must we look 

 for the further elucidation of osmose. 



LIV. On the Changes in the Apparent Size of the Moon. 

 By Richard T. Lewis. 



To Professor Tyndall. 



1 Lowndes Terrace, Knightsbridge, S.W., 

 Sir, March 25, 1862. 



THE enlarged appearance of the sun and moon at rising or 

 setting has long been a subject of controversy; and although 

 astronomers and others have proved by micrometrical measure- 

 ments that the apparent diameter of those bodies is actually 

 largest when they are on the meridian (since they are nearest to 

 us then), the popular mind refuses to believe that the illusion is 

 only mental, and hitherto no illustration has been found suffi- 



