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KNOWLEDGE & SCIENTIFIC NEWS. 



[April, 19O4. 



" ionisation " or dissDciation to begin with, and require 

 to be further broken up bsfore the hydrogen and hydroxyl 

 can combine. 



What exactly is to be understood by the strength of an 

 acid or a base was for a long time the subject of dispute. 

 It used to be said that sulphuric acid was stronger than 

 hydrochloric or nitric acids, because it " turned them 

 out" from their combinations with bases. Sodium nitrate, 

 for example, when distilled with sulphuric acid, gives 

 sodium sulphate and free nitric acid. The test of strength, 

 however, when applied in this manner, is not legitimate, 

 since the nitric acid is not given a fair chance ; it is re- 

 moved from the sphere of action by vaporisation. A 

 much more rational way of arranging the encounter 

 was that devised by Thomsen when he mixed, in 

 dilute solution, one eijuivalent weight of each acid 

 with one equivalent weight of base. Here there is in- 

 sufficient base to satisfy both acids, and all the substances 

 concerned, before and after the action, remain dissolved 

 together without any removal. The two acids then strive 

 for the base, and the one which gets most of it is the 

 " strongest." In this way it is possible to arrange the 

 acids in the order of their "strength," and experiment 

 showed that hydrochloric and nitric acids head the list in 

 such an arrangement. 



Sulphuric acid proves to be only about half as strong 

 as nitric or hydrochloric acids— a result altogether at 

 variance with the older ideas. 



The problem has been attacked also from various 



other points of view : it is known, for example, that the 



salts of weak acids or weak bases may undergo what is 



called hydrolysis in aqueous solution, that is to say that 



Salt + water = acid + base. 



Such an action can easily be shown in the case of 

 ferric chloride or sodium borate. It was proposed, there- 

 fore, to classify acids as weaker or stronger accordmg to 

 the extent to which their salts were " hydrolysed " by 

 water under similar conditions. 



Again, there are many chemical changes which are 

 found to be greatly accelerated by the presence of acids, 

 and if these changes happen to be sufficiently slow to 

 enable one to map out the rate of change, it is possible 

 to compare the influence of different acids. Results 

 obtained in such ways agree, on the whole, remarkably 

 well with the order of "strength" as measured by the 

 "stri\ing for base " method. 



The electric conductivity again was found to be better 

 for the stronger acid ; and the same is true with regard to 

 the deviation from the " normal " osmotic pressure abo\e 

 referred to (Article I.). Such observations were largely 

 instrumental in leading up to the new theory. If one 

 " believes in ions,"'^ it is a comparatively simple matter 

 to explain, in terms of the hypothesis, what is meant 

 by the " strength " of an acid. The strongest acids, like 

 nitric and hydrochloric acids, undergo complete, or nearly 

 complete, ionisation when dissolved in a moderate volume 

 of water, whereas the weaker acids, like acetic or hydro- 

 fluoric acids, are ionised to a less extent. In other 

 words, a moderately diluted solution of hydrochloric acid 

 contains a (relatively) large number of free hydrogen ions 

 in a given volume, and an equivalent quantity of acetic 

 acid contained in the same volume gives rise to (rela- 

 tively) few free hydrogen ions. 



But ionisation increases as dilution increases, so that 

 we arrive at the conclusion, which sounds paradoxical at 



first, that when their aqueous solutions are infinitely 

 diluted all acids would be equally " strong " \ 



The strength of an acid, then, depends, according to 

 these ideas, upon the concentration of the free hydrogen 

 ions which is attained when an equivalent weight of the 

 acid is dissolved in water and the solution made up to a 

 given volume. But how are we going to measure this, 

 or compare it, say, in the case of two given acids ? 



The complete explanation of the way in which this can 

 be done would perhaps be out of place in a brief sketch 

 like the present one; we will merely attempt here to give 

 a very rough indication of the principle. 



The electric conductivity of an acid in solution depends 

 upon the number of free ions present in a given volume 

 and upon the speed with which they move. If we 

 determine (directly or indirectly) the molecular con- 

 ductivity of a given acid (i) when the solution is 

 moderately dilute, and again (2) when it is infinitely 

 dilute, it can easily be shown that the first number 

 divided by the second will tell us the extent to which the 

 acid is ionised in the moderately dilute solution. We 

 can then make similar experiments with other acids 

 under the same conditions, and so compare the extent to 

 which each is ionised. Assuming for simplicity that 

 each acid splits up into two ions, one of w hich, of course, 

 is hydrogen, it is evident that the one which is most 

 ionised is the strongest under the given conditions, i.e., 

 there will be more free hydrogen ions in a given volume 

 of solution. 



The extent of ionisation of the acid can also be arrived 

 at from other considerations, such as the deviation from 

 the normal osmotic pressure (see Article I.); but the 

 electric conductivity method is the most generally applic- 

 able. 



The Problem of Cancer 



By Felix Oswald, B.A., B.Sc. 



• At the present time il is well to look upon this ionic explana- 

 tion as a very efficient and complete working hypothesis, and not 

 to regard it, as is often done, in the light of a creed or dogma. 



The failure of bacteriologists to discover a cancer- 

 bacillus has facilitated future investigation regarding 

 a probable cure for cancer by narrowing the issue 

 and disposing of a fruitless line of research. On the 

 other hand, the recent important discovery of Professor 

 Farmer and his colleagues, that cancer-cells agree with 

 reproductive cells in only containing half the number of 

 chromosomes in the nucleus after nuclear division, re- 

 calls the experiments of Galeotti,'" in 1693, with regard 

 to the unsymmetrical and irregular nuclear division in 

 cancer-cells. It appears probable that the efficacious 

 preventive treatment of cancer is to be sought in the 

 direction indicated by these experiments, which have 

 hardly received the attention they deserve. Briefly 

 stated, Galeotti treated acti\ely dividing, epithelial cells 

 of salamanders with dilute solutions of drugs such as 

 antipyrin, chloral, quinine, cocaine, nicotine, potassium 

 iodide, &c. The action of these substances caused 

 asymmetrical and tripolar division of the nucleus, 

 exactly similar — as the accompanying figures will show 

 — to the asymmetrical and tripolar division which takes 

 place in cancer-cells in a human subject. The remark- 

 able similarity between these pathological occurrences 



* Beitr. zur patholog. .\natomie und zur allgem. Pathologie, 

 XIV. 2 ; Jena, 1S93- 



