123 SI i DIES IN [MMUNITY. 



flocculated on the addition of a sodium salt, precipitate with 

 much greater difficulty when citrate is present. This fact may be 

 regarded as a result of the formation of complexes between the 

 calcium fluoride or the mastic and citrate. 



We may now consider the appearance of an insoluble substance, 

 the particles of which undergo a much more marked dissemination 

 than docs barium sulphate when citrate is added. Inasmuch as the 

 particles of such substances an 1 extremely small, they may be more 

 like the component particles of a colloidal solution. 



We have known for some time that citrates of alkalis, particularly 

 the insoluble citrates of calcium salts "dissolve" certain substances 

 which are insoluble in water, it is in addition noted that the 

 citrates of alkalis prevent precipitation of a copper, aluminium, 

 or iron salt by a base. We think that this action of citrates of 

 alkalis may be comparable to the action of sodium citrate and 

 stable colloids on barium sulphate; it may be that in these cases the 

 citrate forms a complex with the precipitate of hydrate as soon 

 as it appears, so that the hydrate, instead of being flocculated, 

 remains in colloidal solution. 



We place a given amount of well-washed aluminium in increas- 

 ingly concentrated solutions of citrate, and find, in a few moments, 

 that cloudiness, caused by the aluminium, becomes more marked in 

 certain tubes and that it then decreases and the fluid eventually 

 becomes perfectly clear. We know that w T e are not dealing with 

 an ordinary dissolution phenomenon, inasmuch as the hydrate 

 gives a more marked cloud with the citrate than does ordinary 

 aluminium, which is not the case with ordinary dissolutions. And 

 what is more, no soluble citrate of aluminium has been described so 

 far as we know.* But the reason for comparing a dissolution of 

 aluminium with a suspension of barium sulphate in citrate is the 

 corresponding action of such salts as sodium chloride in both in- 

 stances. We have already noted that the complex, barium 

 sulphate plus citrate, is flocculated on the addition of sodium 

 chloride, in the same way we find the "solution" of aluminium 

 in citrate becomes troubled when sufficient amounts of sodium 

 chloride are added. 



* In Wurtz' "Dictionary of Chemistry," only an insoluble citrate of alu- 

 minium transferred by an acid into a very soluble gummy product is mentioned. 



