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NA TURE 



[January 2! 



which, so far as I see, are not questioned by Mr. J. W. R. The 

 newer theory of solutions, in its entirety, is only a development 

 of the consequences of these facts ; and if errors are present 

 therein, they can only be errors in the application, since the 

 premisses are correct : for the proof of such errors we can, of 

 course, in the interest of science, be only thankful. 



It cannot possibly be used as an objection to the newer theory 

 of solutions that it concerns itself at present with those pro- 

 perties of solutions which depend only so far upon the nature of 

 the substance in question as one constant — the molecular weight 

 ^is concerned ; and which properties, like the relation holding 

 for gases between pressure, volume, and temperature, I have 

 proposed to term colligative properties. For Mr. J. W. R. 

 agrees with me that the number and variety of the conclusions 

 which have been drawn from this fact of the existence of these 

 properties is already very great, and it seems to be open to no 

 doubt that the possible applications are by no means exhausted. 

 It will certainly be the task of the future to take also into con- 

 sideration those properties of solutions which depend upon the 

 individual nature of substances ; and this has, indeed, already 

 to a certain extent been done in a particularly important case — 

 that of the change with the solvent of the molecular weight of a 

 dissolved substance, and especially that of the specific property 

 of water to form electrolytic solutions. But I do not believe 

 that we can be justly reproached for having endeavoured to first 

 solve the relatively more simple problems before turning to the 

 more complex ones. It will be here that what is termed by Mr. 

 J. W. R. the "chemical theory" will take its proper place. 



I beg Mr. J. W. R. to recall the history of the rivalry between 

 these two " theories." Van 't Hoffand his successors developed 

 the laws of solutions entirely without polemical strife, because, 

 since the fundamental ideas of van 't Hofif's theory were entirely 

 new, there was nothing at all in its territory to combat, as till then 

 there was nothing there. The attacks upon van 't Hofif's theory 

 were begun by an investigator who had until then directed his 

 attention exclusively to the phenomena which 1 have above 

 characterized as individual, and who was evidently unprepared 

 to deal with such colligative properties. The defence had to 

 consist in an unceasing clearing up of misconceptions. Now, 

 the greatest of these misconceptions is, that both "theories" 

 are rivals. The existence and form of the laws founded by 

 van 't Hofif and his successors stand at present beyond question ; 

 if the totality of these laws be termed the physical theory of 

 solutions (which I should not do), there is nothing to be 

 objected to this. But what has until now been known as the 

 hydrate theory has not been in a position to give any infor- 

 mation whatever in regard to these laws ; none of them have 

 been discovered with its aid, and since it has for its subject not 

 the colligative but the individual properties of solutions this 

 will not be otherwise in the future. In fact, the existence of the 

 colligative laws, or van 't Hofif laws, is entirely independent of 

 whether hydrates exist in solutions or not, and all attempts, 

 successful or otherwise, to demonstrate the existence and com- 

 position of such hydrates, lead conversely in no wise to the 

 van 't Hofif laws. 



It will possibly not be superfluous to emphasize that with the 

 new theory of solutions the question is not one of hypotheses, 

 but of facts, of numerical relations. Whether one can form a 

 conception as to the cause of the colligative laws of solutions 

 with the aid of the kinetic molecular hypothesis or in any other 

 way, is, for the actual existence of these laws, just as much a 

 matter of indifiference as it is for the existence of the laws of 

 gases. In my book the question is this one of facts, and 

 although I have therein made more use of molecular considera- 

 tions than I should at present hold to be proper, yet I have done 

 so only to render more clear the actual relations, and never to 

 prove quantitative laws. 



The theory of solutions which I represent and defend con- 

 sists, accordingly, of a certain number of laws, i.e. of exact 

 relations between measurable quantities belonging to solutions. 

 I cannot see that that which as "chemical theory" is set in 

 opposition thereto contains anything similar to this. The latter 

 contains in actual material certain methods — very doubtful ones 

 to my mind — for demonstrating the existence of compounds 

 between dissolved substance and solvent. The result of these 

 endeavours remains, however, in any event, entirely without in- 

 fluence upon those colligative laws. The same may be said of 

 the future answer to the question, whether an interaction, and 

 what, occurs between dissolved substance and solvent. In 

 order to render this apparent, I need only recall the fact, re- 



NO. II61, VOL. 45] 



cently observed by Goldschmidt, that the depression of the freez- 

 ing-point in a basic solvent, such as paratoluidine, caused by 

 dissolving in it an acid, has practically the same value as that 

 efifected by the equal molecular quantity of an indififerent sub- 

 stance, although in the first case a chemical compound is formed 

 and none in the second. 



So, in the presentation of laws of solutions, as known up to 

 the present, and which form the subject of my book, the so- 

 called hydrate theory or chemical theory, did not enter into 

 the question, because it had discovered no law of anything like 

 general character. And further, that the methods and hypo- 

 theses of this " theory " cannot be yet looked upon as reason- 

 ably supported scientific results is known to the Knglish-reading 

 scientific public from a number of papers published in recent 

 numbers of the Philosophical Magazine. W. Ostwald. 



Leipzig, January 4. 



A Simple Heat Engine. 



At the last soiree of the Royal Society, a beautiful experiment 

 was shown by Mr. Shelford Bidwell, illustrative of the fact that 

 nickel ceases to be magnetic at a certain definite temperature. A 

 piece of nickel was arranged as the bob of a pendulum. As long 

 as the nickel was below a certain temperature it was held on one 

 side by a magnet, but when it was heated by a spirit lamp-flame 

 beyond a certain temperature it ceased to be attracted, and 

 passed out of the range of the flame ; it then cooled, and almost 

 instantly returned to its first position, again to be released by the 

 heat of the flame. It occurred to me that if a disk of nickel 

 were placed in a certain position in a magnetic field, and then 

 heated, it would continuously revolve. This on trial I found to 

 be the case. 



The experiment was arranged thus : — The nickel disk BC, 

 5 cm. in diameter and r mm. thick, was mounted on an axle 

 passing through A ; it was held in a frame so that the faces of 

 the two poles, N, s, of an electro-magnet were at right angles to 

 one another ; the heat was applied at H ; the disk revolved in the 



direction shown. A great many different positions were tried, 

 but the one indicated gave far the best results. The rotation 

 began when the part of the disk above the flame reached 290° C. 

 The distribution of the lines of force passing through the disk 

 from edge to edge was examined by placing a sheet of a non- 

 magnetic metal above the disk when cold and when heated, and 

 sifting iron filings on to it. When cold, the lines of force were 

 uniformly disposed across it ; but when the disk was heated, only 

 a few appeared to pass through the heated region. From this it 

 is evident that the fall on the heated side of the disk is always 

 less than that on the side which is not so hot, and to this the 

 rotation is due, Frederick J. Smith. 



Trinity College, Oxford, December 31, 1891. 



The Migration of the Lemming. 



Dr. Andrew Wilson, in referring to my letter on the 

 Norwegian lemming, is under the very natural misapprehension 

 — which I formerly shared — that "only a miserable remnant of 

 the original swarm " reach the sea. Now, although it is true 

 that throughout their pilgrimage they are exposed to the attack 

 of every rapacious bird, beast, and biped, and that even the 

 Rendyr, which is by no means rapacious, never misses the 

 opportunity of obtaining a bonne bouche of grass a la satir krout 

 from their paunch, yet so prolific are the lemmings at this time 

 that their numbers increase despite their enemies ; some of 

 which, be it remembered, do not dare to follow them when they 

 leave the fjeld. During the last great migration I noticed that 

 the little pilgrims became much more numerous as they ad- 



