220 C. E. Linebarger — Nature of Colloid Solutions. 



silver is so anomalous in its behavior that the results of experi- 

 ments carried out with it cannot be well applied to other col- 

 loid substances. 



Exner* has made an examination of finely-divided bodies 

 suspended in a liquid with a view to explaining the phenom- 

 ena of the Brownian movements. He prepared emulsions of 

 gamboge by adding water to a solution of gamboge in alcohol. 

 In this way so fine an emulsion was obtained that under ordi- 

 nary conditions it kept as such indefinitely. Exner found that, 

 if a little of this "suspension" be put in a vessel and water 

 carefully run down over it so as to form a distinct surface of 

 separation between the two liquids, after some days this sur- 

 face separating the liquids became obliterated, in other words, 

 diffusion took place. Accordingly, finely-divided particles held 

 in suspension in a liquid would seem to have the power of doing 

 work against gravity ; for it is only by the expenditure of 

 energy that a particle would pass from a lower to a higher 

 position in the liquid. But Exner found again that, if the ex- 

 periment be conducted in a room of constant temperature, after 

 three days not only no particles had passed from the lower 

 liquid to the upper, but that, on the contrary, a portion of the 

 gamboge had settled down as a deposit at the bottom of the 

 vessel. The diffusion phenomena observed in the first experi- 

 ment must then have been due to changes of temperature and 

 jarring. According to the second experiment, emulsions 

 do not possess the property of diffusion ; they are only 

 subject to the action of gravity. Muthmann's second experi- 

 ment also affords additional proof for this conclusion. But 

 solutions of colloids diffuse, as has been shown by Graham in 

 the case of albumin, etc. Here is then a fundamental distinc- 

 tion between a true solution and an emulsion. Diffusion, as 

 has been shown by Nernst,f is clue to osmotic pressure. There- 

 fore, we would expect that a solution that diffused, would have 

 an osmotic pressure. Pfeffer, in the course of his researches 

 on osmosis^: has found the osmotic pressure of a 1 per cent solu- 

 tion of gum arabic to be 7"l cm of mercury at 15°. This is 

 the only determination of the osmotic pressure of a colloid that 

 I know to have been made. Gum arabic, from a chemical 

 standpoint of view, is hardly suited for an investigation 

 of this character. Too little is known of the constitution 

 of its molecule, and it is not always a chemical unity. 

 What is wanted for an investigation of the osmotic pressure 

 of colloids is a colloid as simple and stable as possible. The 

 colloid modifications of tnngstic and molybdic acids fulfill 

 these conditions very well. A solution of colloid tungstic acid 



* Wiener Ber. \ Zeitschr. 1 phys. Ch , ii, 613, 1838. 



% Osmotische Untersucliiingen, Leipzig, 1877. 



