482 



NATURE 



[March 21, 1895 



that similar organisms may very possibly be found to be 

 the exciting cause of both measles and small-pox. In 

 malaria protozoa are now accepted as the origin of the 

 disease, and a similar suggestion has been made to 

 account for yellow fever in the absence of all trustworthy 

 microbic evidence. The examination of air, water, &c., 

 for micro-organisms, is, of course, only dealt with in a 

 very cursory manner. In the small section devoted to 

 wafer-bacteriology, we are glad to find Dr. Heim calling 

 attention to the errors which may so easily occur in the 

 correct numerical estimation of bacteria in water. 



Dr. Heim has spent an immense amount of patient 

 labour on the compilation of this volume, and he ap- 

 proaches his subject invariably from the point of view 

 of a man who has worked out things and problems for 

 himself; it is thus that his book acquires an original 

 flavour, which, whilst making it more palatable to the 

 teacher, renders it less likely to find favour, or prove use- 

 ful to the student. We venture to ih'nk that it will, in 

 the concluding words of the preface, "sich den Herrn 

 KoUegen niitzlich erweisen " ; and it must in any case be 

 regarded as a responsible and noteworthy contribution 

 to the bacteriological text-books, now fairly numerous, 

 already in existence. G. C. Frankland. 



CHEMICAL ANALYSIS. 



Die wisunsihaftlichtn Crundlagcn der analytischen 

 Chemie. Von \V. Ostwald. (Leipzig : Wilhelm Engel- 

 mann, 1S94.) _ 



'TT'HE Professor of Chemistry in the University of 



Leipzig has taken so assiduously to the making of 



books that, as repaids the fruitfulnessof his pen, he may 



well take rank with even the more prolific of our writers 



of fiction. Despite this fact, it is not loo much to say 



that none of his productions will appeal to a larger 



section of the chemical public than the little volume 



under notice. For although it is professedly a work on 



general analytical chemistry, it will mainly make its 



mark as a concise exposition of qvalitalive ansilysis, 



based upon the most recent developments of chemical 



theory. 



The author first deals with the modes of recognising 

 substances- ..^//jAjnj/j- and Vorgangscigenschafteit — and 

 the modes of separating substances prior to recognition. 

 Amongst other things the theory of filtration and wash- 

 ing is discussed, together with the complication intro- 

 duced by "adsorption"— that curious property in virtue of 

 which the concentration of a solution near the surface of 

 an immersed solid is greater than the average concen- 

 tration. Separation, it is emphasised, must always be 

 mechanical, and if it cannot be effected directly by the 

 physical meihods described, such as distillation or the 

 action of solvents, chemical operations, such as form the 

 subject-matter of qualitative analysis, have first to be 

 performed. 



At this point we meet with views which are novel in an 

 analytical text-book. The theory of analysis, it is urged, 

 is now upon a new platform. Wiih van't Hoff and 

 Arrhenius, we must regard the properties of dilute 

 solutions of acids, bases, and salts as mainly those of 

 NO. 1325. VOL. 51I 



their ions: and herein lies a simplification of the 

 utmost importance, for it is possible to express in terms 

 of the properties of, say, 50 anions and 50 cathions 

 the properties of the 2500 odd salts to which they may 

 give rise. A digest of the new theory of the constitution 

 of solutions is here given, and of the mode in which the 

 law of mass action may be extended to analytical re- 

 actions by regarding the ions as individual substances. 



A precipitate in its mother-liquor we are to regard as 

 a system in equilibrium. Precipitation is never com- 

 plete. On precipitating BaSO,, for example, a small 

 amount remains in solution, and portion of this is dis- 

 sociated into the ions Ba and SO4. The precipitate is 

 in equilibrium with the soluble Ba.SO,, while it in turn 

 is in equilibrium with the ions. Now the active mass of 

 a solid is constant, hence for equilibrium the product 

 of the concentrations of the ions in the solution must 

 have a definite value — the solubility product. If the 

 product be r..ised above this definite value, more precipi- 

 tate .Tiust form ; if it be lowered, more precipitate must 

 dissolve. We have here a reason for the common prac- 

 tice of adding an excess of precipitant in order to ensure 

 complete precipitation. In the case quoted the solubility 

 product of the BaS04 will be partly kept up to the 

 necessary value by means of the Ba ions of the excess of 

 precipitant, and hence less BaSO, will require to rem^iin 

 in solution. This is a particular instance of the general 

 rule that the solubility of a substance i- diminished by 

 the presence of another having an ion V common with it. 

 We can thus formulate a theory of precipitation. It is 

 in general the result of interaction between ions ; conse- 

 quentlv, if on mixing two solutions ions are present which 

 can form a substance having a sufficiently small solu- 

 bility product, that substance is precipit.ited, and causes 

 which al'fect the solubility product are the causes affect- 

 ing the precipitation. Here are one or two instances 

 of this method of treating analytical reactions. 



The solubility of Mg(HO)._, in water is regulated by the 

 solubility product of Mg and HO ions. If hydrochloric 

 ac.d, which is almost completely dissociated, be added 

 to the water, the H ions of the acid at once unite with 

 the HO ions of the dissolved ^'gCHO), to form water, 

 because water is practically undissociated. In this way 

 the solubility product of the Mg(I10).j is diminished, .ind 

 more solid Mg(H0)2 must dissolve in order to maintain 

 the product at the value necessary for equilibrium. If 

 sufficient HCl be p.esent, all the Mg(Hu)a will pass into 

 solution. 



The following case of precipitation is more complex :— 

 If CO, be passed through a solution of Pb(N03^2, little or 

 no precipitate is obtained, whereas two-thirds of the 

 lead is thus removed from a solution of Pb(CjllaO.^).,. In 

 solution carbonic acid is but slightly dissociated into its 

 ions 2H and CO3 ; yet if Pb(N03)._, be present, the pro. 

 duct of the concentrations of the Pb and CO3 ions it 

 greater than the solubility product of PbCO, ; hence a 

 precipitate foims. At the same time, however, IlN'Oj is 

 produced, and H and NO, ions accumulate in the solu- 

 tions, since IlNOj is strongly dissociated. Now H ions 

 diminish the electrolytic dissociation of the H.jCO, just 

 as excess of NH-, or HCl diminishes the ordinary dis- 

 sociation of NH,C1. The solubility product of the CO 

 and Pb ions is thus diminished, and but a small quantity 



