94 PRINCIPLES OF GENERAL PHYSIOLOGY 



If the charge on particles is neutralised or reversed by the adsorption of ions 

 of opposite sign, it follows that these ions must be carried down with the 

 precipitate. This has been shown to be the case. Linderand Picton (1895, p. 66) 

 found that when arsenious sulphide is precipitated by barium chloride, the Ba- 

 ion goes down with the precipitate, while the liquid becomes acid from the hydro- 

 chloric acid set free. This Ba' ion is held fast to the precipitate by electrostatic 

 forces, since it cannot be removed by mere washing with water, although it can 

 be replaced by another cation, when washed with a solution of a salt of this latter. 

 In connection with this fact, an observation by Paine (1912, p. 62) is of interest. 

 Colloidal copper is electro-positive (probably due to a coating of hydroxide) and the 

 precipitating ion is naturally the anion. When this is Cl', by repeated washing 

 of the precipitate it can be removed and the colloidal solution formed anew. When 

 bivalent, as SO 4 ", mere washing will not remove it ; but, if first treated with 

 sodium chloride in excess, so as to replace the SO 4 " by Cl', then water will restore 

 the original colloidal solution. This illustrates the more powerful action of the 

 bivalent ion. 



In connection with this reversible coagulation, it is important to note that it has given the 

 opportunity to Oden and Ohlon (1913) to investigate the dimensions of the aggregates before 

 precipitation and after resuspension. Hydrosols of silver or of sulphur, after aggregation by 

 ammonium nitrate or by sodium chloride, can be resuspended by washing with water. 

 Investigated by the ultra-microscope, these new solutions are found to consist of particles of 

 the same dimensions as the original ones. It would appear, therefore, that in the process of 

 aggregation, no actual fusion takes place ; otherwise it is difficult to understand how 

 separation into particles of the same size as before could be ensured. 



The carrying down of the precipitating ion with the precipitate is explained by Linder and 

 Picton (1905, p. 1914) as due to salt formation. That this is not so is shown by quantitative 

 relations, e.g., Perrin (1905, p. 69) finds that one atom of lanthanum, as nitrate, will precipitate 

 425 atoms of arsenic, as sulphide. Further evidence of the same nature is given by Hopkins 

 and Savory (1911) in the case of the Bence- Jones' protein and will be referred to under the 

 head of proteins. 



The actual number of ions carried down is of interest. Burton (1906) 

 estimated the number of aluminium ions adsorbed by a particle of a certain 

 preparation of colloidal silver to be 2 x 10". It is unfortunate that an aluminium 

 salt was chosen, because these salts are hydrolytically dissociated ; lanthanum 

 should have been used. But an approximate idea of the number of atoms in a 

 colloidal particle can be obtained by combining this value of Burton's with that 

 of Perrin given above. One La--- ion precipitates 425 atoms of arsenic in the 

 sulphide, so that the number of atoms in such a colloidal particle is somewhere 

 about 425 x 2 x 10 7 or 8-5 x 10 9 . Of course this only refers to one individual 

 hydrosol. The dimensions of the particles vary very widely. In the case of the 

 free acid of Congo red, I found (1909, p. 283) by an ultra-microscopic method 

 that the mass of each particle was approximately 2*3 x 10" 11 mg. Taking the 

 mass of the hydrogen atom to be l-6xlO~ 21 mg., that of the molecule of 

 the acid (molecular weight = 652) is 1'04 x 10~ 18 ; so that there would be 2 x 10 7 

 molecules in each particle on the average. Each molecule contains 70 atoms, so 

 that there would be 70 x 2 x 10 7 atoms in each particle, or about one sixth the 

 number of those in the particle of arsenious sulphide. 



Although it may be possible to represent by a chemical formula a long chain, say uf 

 400 ferric Irydroxine molecules with one of ferric chloride at the end, all united by bom Is. 

 I am unable to see what advantage is gained. It seems rather to obscure the essential 

 nature of chemical combination, as attended by change of properties, since such colloids 

 behave chemically as mixtures only. Moreover, these ferric hydroxide colloids must be 

 regarded as completely hydrolysed, since it is possible to remove all the chlorine by diah sis, 

 although great instability results. If a compound is completely hydrolysed in solution, 

 how does it differ from a mixture? Again, it is difficult to believe that an atom of chlorine 

 at the end of a long chain can have a chemical effect on molecules 400 places away. 



If the electric charge on colloidal particles is due to surface ionisation, the 

 greater will be this charge the finer the particles into which a given mass is 

 divided. So that, given equal solid content of two solutions, that one which 

 contains the smaller particles will require more precipitating electrolyte to 

 neutralise the charge and cause aggregation. This has been found to be the case 

 by Sven Oden (1912, p. 123) for hydrosols of sulphur and of silver. A specimen 



