84 PRINCIPLES OF GENERAL PHYSIOLOGY 



limited time, the Ringer's solution is changed at intervals. Abel has already 

 obtained considerable amounts of amino-acids. To investigate the changes taking 

 place in the contents of the blood as it traverses a particular organ, the diffusate 

 from the ingoing blood can be compared with that of the outgoing blood. The 

 substances that have been identified as diffusing out from the blood are sugar, 

 urea, phosphates, amylase, and amino-acids. The name of vim-diffusion is given 

 to this method by its discoverer. 



ULTRA-FILTRATION 



Although membranes of hydrophile colloid substances do not allow water to 

 filter through at any perceptible rate under moderate pressures, it is possible, 

 by the application of pressures from two to thirty atmospheres or more, to 

 concentrate colloidal solutions and separate them from " crystalloid " admixture, 

 by forcing the liquid phase through the membrane. 



This was first done by Chas. J. Martin (1896). His filter consisted of a 

 porous clay Chamber-land candle, whose pores were filled with gelatine. This 

 was fixed in a gun-metal case, with the nozzle projecting, and the space 

 between the two was filled with the liquid to be filtered. A pressure of some 

 thirty atmospheres or more, applied to the solution, caused the water and 

 crystalloids to be driven through, while the colloids remained behind. 



Bechhold (1907) modified the apparatus so that flat sheets of various 

 membranes, differing in permeability, could be used. He also showed how to 

 make membranes of different degrees of permeability. Some of the results 

 obtained by this method will be referred to in Chapter V., on u Permeability of 

 Membranes." The name of " Ultra-filter " is due to this investigator. 



BROWNIAN MOVEMENT 



If sand be shaken with water, and the mixture then allowed to stand, the 

 sand rapidly falls to the bottom, leaving the water clear and free from grains. 

 Why, then, do the particles of gold, whose density is greater than that of sand, 

 remain suspended for an indefinite time in the colloidal state ? 



It will be obvious that this is, in some way, connected with their size ; but 

 there must also be forces active in preventing them from sticking together to form 

 grains large enough to fall rapidly, as the following consideration will show. The 

 larger the number of particles into which a given mass is divided, the greater the 

 surface energy. Now, by the principle of Carnot and Clausius, the system strives 

 to diminish this free energy, so that, unless prevented, the particles will aggregate 

 together to form larger particles and sink. 



In 1828 the botanist, Robert Brown (1828), noticed particles in microscopic 

 preparations to be in a continuous state of rapid oscillatory motion ; the smaller 

 the particles, the greater the amplitude of the movement. Various suggestions 

 were made from time to time to explain this " Brownian" movement, such as 

 inequality of temperature, electrical charge, and so forth, but none were found to 

 stand the test of experimental investigation. 



One fact, which at once disposes of any hypothesis referring the movement to any external 

 cause, is the complete independence of the direction of movement of two particles in close 

 proximity to one another. That electrification has nothing to do with the phenomenon is 

 shown by an experiment of Svedberg (1907). By gradual addition of an aluminium >alt 

 to a colloidal solution of silver, he was able, owing to facts which will be explained 

 below, to reverse the sign of the electric charge on the silver particles, thereby passing 

 through a stage of zero charge, without in any way diminishing the extent of the Brownian 

 movements. 



It is only in recent years that it has been shown, chiefly by the work of Perrin 

 (1908), that this movement is identical with that of the molecules of the liquid, as 

 postulated by the kinetic theory. 



In order to understand the nature of the proof, which has also important 

 bearings on the question of the real existence of molecules, a few words are 

 necessary on the kinetic theory and on the molecular basis of chemical science. 



