February 23, 191 1] 



NATURE 



535 



Beginning with a discussion of the mathematical 

 work of Laplace on surface-tension, the author points 

 out that while it adequately expresses the effect as 

 observed. on curved surfaces, it gives no explanation 

 of tension on plane ones, since it assumes the internal 

 pressure to remain constant to the surface and to 

 react vertically to it only. A more complete theory 

 has been developed by van der Waals and his pupils. 

 In the gas-equation, (P f a/v') (7/-b) = RT, the a/r^ 

 represents the increase of pressure due to the mutual 

 attraction of the gas molecules ; and this, while only 

 a trifling correction in gases, becomes an enormous 

 pressure in liquids owing to the closeness of the mole- 

 cules. When, however, a particle lies actually on the 

 surface, it is only attracted by those below and around 

 it, and not compressed by others above, so that the 

 surface-layer is not only under a much lower pressure 

 vertically, but the surface is under actual tension 

 from the horizontal component of the attraction of 

 the particles around and below it. Since the liquid 

 particles not only exert attraction on others in the 

 surface-layer, but on those of vapour or gas imme- 

 diately above it, it is clear that the layer of rarefied 

 liquid must pass without break into a layer of com- 

 pressed vapour within the very small range of mole- 

 cular attraction. Substances, like most salts, which 

 dissolve with contraction of volume, increase of in- 

 ternal pressure, and diminution of vapour-pressure, 

 also increase the surface-tension, while volatile liquids 

 and many colloid organic substances diminish it. 



It was first pointed out by Willard Gibbs, and 

 afterwards, more fully, by J. J. Thomson, that bodies 

 which diminish the surface-tension must tend to 

 accumulate in that surface, while the reverse is the 

 case with those which increase it. Freundlich sees in 

 this the explanation of adsorption, positive and nega- 

 tive ; and as such an effect on surfaces even between 

 liquid and vapour can be shown experimentally to 

 occur, it must be accepted as one of the causes, though 

 whether it plays the important part which he assigns 

 to it must remain uncertain until means are found 

 at least of estimating its quantitative effect. Lager- 

 gren has suggested another physical theory of adsorp- 

 tion based on the idea that a surface is a region not 

 of tension but of compression, and that substances 

 which favour contraction of volume must accumulate 

 there. While it seems impossible to accept the idea 

 of a compressed layer at the surface of a liquid in 

 contact with gas, it may well be that the liquid is 

 compressed, and its surface-tension negative on solid 

 surfaces, just as van der Waals assumes that gas is 

 compressed on liquid ones, since the internal pressure 

 and surface-tension of solids must be enormously 

 higher than those of liquids. Neither theory ade- 

 quately explains many of the individualities, both of 

 absorbent surfaces and adsorbed substances, some 

 substances being adsorbed at surfaces both of positive 

 and of negative surface-tension, and one is inclined to 

 believe, what indeed Freundlich admits, that chemical 

 forces often come into play, and that adsorption re- 

 sembles in many cases a sort of contact-solution of 

 two bodies having chemical affinities, or, what is prob- 

 ably the same thing, opposite electric potentials. 



H. R. P. 

 NO. 2156, VOL. 85] 



TRAVELS IN ICELAND. 

 Island in Vergangenheit und Gegenwart, Rcise-Erinner- 

 tingen. By Paul Hernnann. Teil iii., Zweite Reise 

 quer durch Island. Pp. x + 312 + map. (Leipzig: 

 W. Engelmann, 1910.) Price 7 marks. 



THIS volume, although complete in itself, is a 

 sequel to the two parts of " Island in Vergang- 

 enheit und Gegenwart" (1907), describing Herr Herr- 

 mann's travels four years previously. As before, the 

 town of Torgau allowed him the long leave, pro- 

 vided a locum tenens (as schoolmaster), and relieved 

 him of all anxiety during the illness resulting from 

 an accident on the journey. The money was pro- 

 vided for the undertaking by the higher educational 

 authorities, and we cannot wonder when he observes, 

 "Surely few towns would act so munificently." The 

 same guide was employed — he who accompanied 

 Thoroddsen on his explorations— but although the 

 surrounding circumstances were equally favourable, 

 the book does not give quite such an impression of 

 enthusiasm for Iceland in each and every aspect as 

 on the former visit. 



The route was by sea round the eastern, western, 

 and northern coast, descriptions being given of all 

 places called at, and the scenery passed. Herr Herr- 

 mann is pleased to think his former books are used 

 as reference by tourists, and this part of the present 

 work is specially for their benefit. He thinks the 

 number of visitors will greatly increase when the 

 steamship service is improved. He complains greatly 

 of the accommodation now provided, and quotes and 

 agrees with the opinion of another traveller (O. 

 Komorowicz) "that if such were used in Germany 

 for the transport of animals the S.P.C.A. would inter- 

 fere " ! 



From Reykjavik the journey as far as Uxahryggtr 

 was over familiar ground, but a new route was struck 

 thence to Kalmanstunga — with an excursion to the 

 Surtshellir caves— and westward round the Snaefellsnes 

 peninsula, where the inhabitants were not found as 

 lacking in progressive spirit as from other accounts 

 was expected. Northward to Hrofberg with an appre- 

 ciative allusion to the agricultural school at Olafs- 

 dalur. Then eastward to Holar, the seat of an 

 ancient bishopric and present agricultural school, and 

 southward over the Kjolur to Geysir. 



When the disadvantages of storms, cold, tent-life, 

 and many minor catastrophes were overcome, and the 

 pleasant neighbourhood of Hvitarvatn reached, an 

 attempt to visit Frodardalur resulted, owing to an 

 overtaking storm, in a severe fall from the pony. As 

 a result of this accident the remaining portion of the 

 journev, bv Skalholt, GuUfoss, and round Reykjanes 

 to Reykjavik, is undertaken with less spirit. 



The author enlarges more on the geological aspect 

 of the country than in the preceding volumes; in the 

 meantime he has learnt much in this direction, but 

 wishes the scientific reader to remember always that the 

 descriptions, remarks, and conclusions are those of a 

 layman only. The more he knows of Iceland the 

 more his admiration of Thoroddsen, as a geologist 

 and explorer, grows, and this appreciation is ex- 

 pressed in many references. 



For younger geologists the entirely or partially 



