RECENT ADVANCES IN SCIENCE 105 



of electrolyte formed by the oxidation of the metal at the 

 high temperature reached locally in the sparking process. 

 The presence of chloride, bromide, iodide, and hydroxyl ions 

 has a marked stabilising effect upon these colloidal metals even 

 when the electrolyte is at extreme dilution (o"oooi normal), 

 and this observation affords a convenient method for preparing 

 such colloids. Even when no electrolyte has been added to 

 the medium beforehand, it is well known that these colloidal 

 metals possess a negative charge and this charge is essential 

 to their stability. The inference to be drawn from the above 

 experiments is, that even in those cases in which the medium 

 is quite pure, the stability of the colloid produced is dependent 

 upon its power of adsorbing or combining with some anion — ■ 

 probably a complex anion containing the metal itself. If this 

 be accepted, it follows that the most minute trace of metallic 

 anion is sufficient to stabilise the colloidal metal, for all attempts 

 to detect gold in the filtrate after the coagulation of a colloidal 

 gold solution gave negative results. It must not be thought, 

 however, that all anions are equally effective. Thus fluorides, 

 nitrates, chlorates, and sulphates appear to be inactive. The 

 problem therefore appears to have slightly shifted ground, but 

 is by no means satisfactorily answered as yet. Granted that 

 traces of electrolytic ions, combined somehow with the colloidal 

 particles, are the source of the charge exhibited by the particles 

 and likewise of their stability, we are still faced with the cause 

 of the selective effects manifested by different colloids in 

 respect of certain anions. The investigation has reached a very 

 interesting stage. 



In connection with the preparative side of colloidal solutions 

 a problem of some importance is the production of colloidal 

 carbon. Bredig does not mention carbon among the colloids 

 in water prepared by the electrical method, but Svedberg 

 mentions the production of colloidal carbon in certain liquid 

 organic media, the colloid being evidently produced by the 

 action of the spark upon the medium itself. More recently a 

 stable colloidal carbon has been obtained in dilute aqueous 

 sodium hydroxide (but not in pure water) by Thomae, whilst 

 Sabbatani has succeeded in producing colloidal carbon by the 

 action of sulphuric acid upon sugar, the resulting product being 

 poured into water. The production of colloidal carbon derives 

 its chief technical importance at the present time on account 



