May 3, 1906] 



NA rURE 



LETTERS TO THE EDITOR 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of ^AtVRS.. 

 No notice is taken of anonymous communicalions.] 



Osmotic Pressure. 



The publication of a paper by Mr. Spans in vol. Ixxvii. 

 Proc Rov. Soc, p. 234, in which he criticises a relation be- 

 tween the vapour and osmotic pressures of a solution which 

 Mr iiartley and I had deduced (see same volume), seems to 

 be an opportune moment for directing the attention of 

 physical chemists to the necessity for an agreement as to 

 what is meant bv the term osmotic pressure. 



Mr Spens following Duhem, points out that the osmotic 

 pressure defined as ^he difference between the pressure 

 on the solvent and the pressure applied to a solution to 

 keep it in equilibrium with the solvent, when the two are 

 separated by a semi-permeable membrane, varies according 

 to the nressure on the solvent. He suggests using a 

 definite pressure on the solvent, say its vapour pressure, 

 as the standard. , „ . . 



I would point out that, by accepting this definition, one 

 i.* necessarily bound to compare two solutions when they 

 are under different conditions— not only on account of the 

 different vapour pressures of different solvents, but also on 

 account of the different pressures on the solutions them- 

 selves. 



The following consideration will, I think, make .this 

 clear, and at the same time will suggest a more scientific 

 standard. 



There seem to be two methods of examining directly 

 the osmotic phenomena of a solution. 



(1) One which 1 may call the osmotic " force " method, 

 depends essentially on the determination of the rate at 

 which the solvent will flow through a semi- permeable 

 membrane into an infinite mass of solution when there is 

 no pressure on the latter. 



It is evident that if one knew the frictional resistance 

 to the flow, the heat developed, &c., one could calculate 

 the osmotic " force " in absolute units. 



1 would mention, in parenthesis, that Mr. Harilev and I 

 liave made some comparative experiments in this direction 

 with results w^hich were not entirely unsatisfactory. 



(2) .All other direct methods give what may be called 

 equilibrium pressures ; thev depend on the measurement of 

 the pressure necessary to bring about a balance between 

 the solution and the solvent. These equilibrium pressures 

 cannot, on account of the compression of the solution, be 

 measured under the same conditions. 



An example will show this plainly. The equilibrium 

 pressure between a solution of 540 grams of cane-sugar 

 in the litre of solution and the solvent (water) under atmo- 

 spheric pressure is, in round numbers, 70 atmospheres. 

 The equilibrium pressure for 750 grams in the litre is 

 134 atmospheres. In the actual measurements each solution 

 had been compressed, in one case by 71 atmospheres and 

 in the other by 135 atmospheres. The conditions were 

 therefore not comparable. 



If we could measure the osmotic " force" of these two 

 solutions as in (i) then comparable results would be 

 obtained, for in both cases the solution and the solvent 

 would be under the same pressure (gravitational). 



L'p to the present, so far as I am aware, no serious 

 attempts to measure the osmotic " force " have been 

 made, but I would suggest that, pending these, the re- 

 lation between the vapour and osmotic pressures of a 

 solution as deduced bv Mr. Hartley and myself may be 

 useful for the purpose of comparing the osmotic pressures 

 of dilTerent solutions. 



Thi^ relation gives the osmotic pressure of a solution 

 when it is under no pressure but its own vapour pressure. 

 A knowledge of the vapour pressure, together with the 

 density of the solvent, is all that is required for calculating 



NO. IQO5, VOX.. 74I 



that pressure ; while to apply the standard that Mr. Spens 

 proposes, it is necessary to determine the increment in 

 volume of the solution when unit mass of solvent enters it, 

 and in some cases it may be necessary to obtain the 

 coefficient of compression of the solution. 



The experimental work saved by the adoption of the 

 standard here proposed is apparent when it is remembered 

 that, owing to the want of suitable semi-permeable mem- 

 branes, the measurement of equilibrium pressures is con- 

 fined to but a few substances dissolved in water. 



Foxcombe, near Oxford. Berkeley. 



The Eruption of Vesuvius. 



Yesterday I ascended the cone of Vesuvius up to the 

 crater, being, I suppose, one of the first climbers after the 

 eruption. The ascent was made from Torre Annunziata 

 witnout any difficulties, but care had to be taken to avoid 

 the courses of the avalanches of stones and ashes rush- 

 ing from the cone and spreading over the slopes more than 

 half a mile from the foot of the cone. 



1 estimated the new crater to have a diameter of about 

 3000 feet ; the bottom was not visible, but the walls could 

 be seen to a depth of about 1000 feet. The inner walls 

 are nearly perpendicular, partly overhanging, and I saw 

 pieces of the very narrow crater edge breaking down, in 

 this way still enlarging the crater. The very regular 

 stratified construction of the crater walls was visible. The 

 height of the crater edge is very dilTerent from what it was 

 before the eruption, being greatest on the west side, and 

 diminishing in irregular steps to the north and east. At 

 the point to which I ascended the aneroid showed an 

 elevation of 3760 feet. From this point, which was on the 

 southern side, the Somma was clearly visible over the lower 

 northern edge of the crater. This shape of the crater may 

 account for the fact that the showers of lapilli and other 

 fragmentary products which destroyed the villages of Otta- 

 jano and San Giuseppe were given a direction to the north 

 and east over the Somma. 



The crater now closely corresponds to the descriptions 

 of the great crater formed in 1822, and described by Forbes 

 and Scrope. From the throat of the crater I heard a 

 constant roaring, and saw that white clouds of vapour 

 filled the huge hollow, but I did not see any ejections 

 of stones or dust. 



On descending I visited the points where the lava streams 

 started from the foot of the cone. The first lava reached 

 the surface on the morning of April 4 a little west of 

 the Casa Firenze, but it soon stopped. Another stream 

 started from Casa Firenze, destroying the buildings, and 

 flowed half the way toward Bosco-Trecase. The lava which 

 damaged a part of Bosco-Trecase started on April 6 a little 

 lower on the slope, and divided into two parallel branches. 

 The quantity of lava during this eruption was on the whole 

 comparatively small. No lava came from the crater. The 

 general characteristics of the eruption are the immense 

 amount of volcanic ash, lapilli, and other fragmentary 

 material ejected, and this makes the eruption of ."Vpril, 1906, 

 very similar to that of the year 70 A.D. 



Visiting the destroyed village Ottajano on April 19, 1 

 made the following curious observation. A great number 

 of the window glasses are broken, but among the others 

 there are many regularly penetrated or pierced by circular 

 holes one or two inches in size. These holes are as 

 common on the northern and eastern sides of the houses 

 as on the other sides, and they can therefore not have been 

 caused bv the showers of lapilli, which only came from the 

 south-west. Some people ascribed these holes to the very 

 heavy lightning which accompanied the fall of the lapilli, 

 but I am not aware that electrical discharges may produce 

 such effects. 



It may be of interest to note that when visiting the 

 volcanic ' vents of the Phlegraan Plain to investigate if 

 any kind of volcanic activity was shown in connection with 

 the eruption of Vesuvius I heard that the emanation of 

 steam from the Solfatara diminished greatly during the 

 days of the strongest eruption of Vesuvius : normal con- 

 ditions set in later. Hj. Sjogren. 

 Naples, April 23. 



