October 2, 1896.] 



SCIENCE. 



467 



the distinction between reactions of ions 

 and reactions of bodies which do not un- 

 dergo ionic dissociation. The former take 

 place in solutions at ordinary temperatures 

 and so instantaneously that the time factor 

 <3annot be measured ; the latter frequently 

 require an elevated temperature and are 

 sometimes very slow. The distinction is, 

 perhaps, a practical, rather than a strictly 

 logical one, for theoretical considerations 

 lead us inevitably to the conclusion that 

 only additive reactions, and in many cases 

 not even those, can take place without a 

 previous dissociation of some sort. In this 

 view the distinction between ionic reac- 

 tions and others is that in solutions of elec- 

 trolytes a considerable portion of the com- 

 pounds have undergone dissociation; and 

 as any ion is removed by precipitation, or 

 otherwise, the remainder of the compound 

 of which it is a part undergoes rapid disso- 

 <3iation, owing to the resulting dilution of 

 the solution. In such cases the dissocia- 

 tion appears to take place almost exclusive- 

 ly at one point in the compound, and the 

 reactions are clean and practically quanti- 

 tative. In what may be called non- ionic 

 reactions, on the other hand, the initial 

 dissociation appears to be trifling and, 

 notably with organic compounds, may take 

 place at several points ; the reactions be- 

 tween the resulting parts must be slow and 

 may give rise to a variety of compounds. 



In accordance with the theory, only those 

 elements or groups which exist as inde- 

 pendent ions in a solution enter readily 

 into combination with other ions. Hence 

 an atom which forms a part of a complex 

 ion as the iron of ferro- or ferri- cyan ides 

 and the chlorine of chloro-platinic acid and 

 of potassium chlorate cannot be detected 

 by the ordinary reagents for these elements. 

 This principle is of fundamental importance 

 for analytical chemistry and has, of course, 

 in its empirical form, been long recognized. 



In the case of analytical chemistry. 



especially the new theories of physical 

 chemistry seem destined to transform what 

 has been, hitherto, an almost exclusively 

 empirical science and raise it to a higher 

 plane. Two illustrations of practical ap- 

 plications of the theory in this field may be 

 of interest. 



The first is with regard to the indicators 

 used in acidimetry. It has long been known 

 that the same indicator is not equally satis- 

 factory in all cases, but the reason has 

 never been clearly stated till recently. The 

 principles on which the discussion depends 

 are these : an acid solution is characterized 

 by the presence of firee hydrogen ions, a 

 basic solution by the presence of free hy- 

 droxyl and free metallic ions ; in the case 

 of a strong acid or base the number of hy- 

 drogen or hydroxyl ions is large in propor- 

 tion to the quantity of the acid or base pres- 

 ent, while in the case of a weak acid or 

 base the number of ions is small ; in other 

 words the difference between strong and 

 weak acids and bases is that the dissociation 

 factor of the former is very much the larger. 

 The indicators in use are relatively weak 

 acids or bases for which the free ions possess 

 a different color from that of the pure acid 

 or base. Thus phenol phthalem is colorless, 

 while its ion is red; litmus is red, while its 

 ion is blue. In the presence of hydrogen 

 ions the dissociation of each of these sub- 

 stances is diminished in accordance with 

 the well known law of dissociation that the 

 presence of one of the products of dissocia- 

 tion decreases the dissociation of the com- 

 pound. Hence in acid solutions these 

 bodies are so little dissociated that the color 

 of the compound, and not that of the ion, 

 appears. In alkaline solutions, however, 

 the color of the ions is developed, since the 

 potassium and sodium salts, even of very 

 weak acids, are largely dissociated in dilute 

 solutions. 



There is, however, a very considerable 

 difference in the dissociation factors for 



