18 



MOTION OF THE JUICES OF THE ANIMAL BODY. 



matter of indifference whether the tube be closed with a single, Fig. 

 double, or treble layer of bladder. (*) For experiments on very thin a 

 membranes which are permeable to liquids imder a very low pressure, 

 the apparatus (Fig. 5) is obviously better adapted. For the explana- 

 tion of the phenomenon we have to distinguish 



1. The mixture of different liquids. 



2. The change in their volume. 



As to the mixture of two liquids of dissimilar nature and characters, 

 this always depends on a chemical attraction.* In a mixture of 

 alcohol and water, or of brine and water, there is in every part the 

 same proportion of particles of alcohol and water, or of salt and 

 water. If in the former, the lighter particles of alcohol lying at the 

 bottom of the vessel were not retained, in the place and arrangement 

 which they occupy, by the surrounding particles of water, they would 

 undoubtedly rise towards the surface. In like manner, the particles of 

 salt in the brine are sustained and prevented from sinking by the lighter 

 particles of water which surround them. 



Without an attraction, which all the particles of alcohol or of salt 

 must have towards all the particles of water, or all the particles of 

 water must have for all those of salt and alcohol, a uniform mixture 

 cannot even be conceived. If but one particle of alcohol were less 

 powerfully attracted than the surrounding particles, it would rise to the surface ; 

 and in like manner, the particles of salt would, in consequence of their greater 

 specific gravity, gradually occupy the bottom of the vessel, were it not that a cause 

 prevents them from rising or falling ; and this cause can be nothing but an attrac- 

 tive force, which retains them in the place where they happen to be. 



The cause which effects a change in the place or in the properties of the ulti- 

 mate particles or atoms of dissimilar substances, when these particles are in 

 absolute contact, or at infinitely small distances from each other, as well as the 

 cause which manifests itself as a resistance to such changes of place or properties, 

 we call CHEMICAL ATTRACTION ;t and in this sense the mixture of two dissimilar 

 liquids, the simple moistening of a solid body, the penetration and swelling of it 

 by a liquid, are effects in which chemical affinity or attraction has a decided share ; 

 and although we are accustomed to limit the notion of affinity to such cases as 

 exhibit a change perceptible to our senses, in the properties of the substances 

 employed, as, for example, when sulphuric acid and lime, or sulphuric acid and 

 mercury combine together, this limitation arises from the imperfect apprehension 

 of the essence of a natural force. 



Every where, when two dissimilar bodies come in contact, chemical affinity is 

 manifested. It is a universal property of matter, and by no means belongs to a 

 peculiar class of atoms, or to a peculiar arrangement of these. But chemical 

 combination is not, in all cases, the result of contact. 



Combination is only one of the effects of affinity, and occurs when the attrac- 

 tion is stronger than all the obstacles which are opposed to its manifestation. \ 

 When the forces or causes, which oppose chemical combination, heat, cohesive 

 attraction, electric attraction or whatever they may be called, preponderate, then 

 chemical combination does not take place ; and effects of another kind are 

 manifested. 



Melted silver in a crucible, surrounded with red hot coals, in a place, therefore, 

 where we should hardly anticipate the presence of free oxygen, absorbs as much 



( ! ) In these experiments membranes of all kinds may be used. With the thinner 

 membranes, such as the bladder of the calf and the pig, the experiments are more 

 rapidly completed than with the thicker, such as the gall-bladder and urinary bladder 

 of the ox. The peritoneum of the ox and calf is preferable to all others. The tube c 

 is tied with bladder under water. 



* Causes of the mixture of dissimilar liquids. 



t Chemical affinity is the chief cause of mixture. 



; Affinity is everywhere active between bodies in contact 



