104 SCIENCE PROGRESS. 



cover is given by von Sachs. 1 According to this author, 

 whose views are in agreement with those of Nageli on this 

 subject, it is necessary for the explanation of certain pheno- 

 mena exhibited by organic substances that we should assume 

 the existence of combinations of molecules which form very 

 large numbers of small particles or micellae as Nageli calls 

 them. One of the most important of these phenomena is 

 the imbibition of water. Dry organic substances, such as 

 gelatine, when placed in water, imbibe it and increase in 

 volume to a very considerable extent. The increase in 

 volume produced by the swelling up in water is almost 

 equal to the volume of water which has been absorbed. 

 The imbibition of water in such a case is something very 

 different from the imbibition of water by a porous inorganic 

 body, such as gypsum, unglazed porcelain, etc. The latter 

 substances are full of small visible and invisible cavities or 

 pores, which in the dry state contain air. The water passes 

 into these cavities or pores according- to the laws of capil- 

 larity, and in so doing displaces the air, which is forcibly 

 expelled and can be collected and measured ; there is no 

 pushing asunder of solid parts, as is shown by the fact that 

 the porous body is not perceptibly enlarged by the water 

 which has penetrated into it. But the water penetrating 

 into gelatine expels no air, it does not enter by capillarity 

 into spaces previously existent, but forces its way between 

 the particles of the dry substance, pushing these asunder, 

 and so causing the considerable increase in volume. The 

 particles thus pushed asunder are the micellae, and although 

 they are pushed further apart from one another, they do 

 not completely lose their connection. Each micella may be 

 regarded as being surrounded by an envelope of water 

 when in the moist state ; in the dry state the micellae com- 

 posing the substance are in mutual contact. This familiar 

 phenomenon of the swelling of organic substances by the 

 imbibition of water is contrasted by von Sachs with the 

 process of solution of a salt. In the latter case the water 



X J. von Sachs, Lectures on the Physiology of Plants, translated by 

 H. Marshall Ward. Oxford, Clarendon Press, pp. 205 and sq., 1887. 



