Ol'TOliEK 6, 1910J 



NATURE 



447 



essential points may be rendered in tlie statement iliat 

 when a substance on solution in a fluid lowers the surface 

 tension of the latter, the concentration of the solute is 

 greater in the surface layer than elsewhere in the sohi- 

 tioit ; but when the substance dissolved raises the surface 

 tension of the fluid, the concentration of the solute is 

 least in the suiface layers of the solution. 



It is tiius seen liow in a system lilce that of a drop of 

 water with different contact surfaces the surface tension 

 is affected, and how this alters tlie distribution of solutes. 

 It is further to be noted that for most organic solutes the 

 action in this respect is the very reverse of that of 

 inorganic salts. Consequently, in a living cell which con- 

 tains both inorganic and organic solutes, and in which 

 there are portions of different composition and density, 

 the equilibrium may be subject to disturbance constantly 

 through an alteration of the surface tension at any point. 

 Such a disturbance may be found in a drop of an emulsion 

 of olive oil and potassium carbonate in the well-known 

 experiments of Biitschli. When the emulsion is appro- 

 priately prepared, a minute drop of it, after it is sur- 

 rounded with water, will creep under the cover-glass in 

 an amoeboid fashion for hours, and the movement will 

 be more marked and rapid when the temperature is raised 

 to 40° to 50° C. All the phenomena manifested are due 

 to a lowering of the surface tension at a point on the 

 surface, as a result of whicli there is protrusion there of 

 the contents of the drop, accompanied, Biitschli holds, by 

 streaming cyclic currents in the remainder of the mass. 



Surface tension also, according to J. Traube, is all- 

 important in osmosis, and he holds that it is the solution 

 pressure (Haftdruck) of a substance which determines the 

 velocity of the osmotic movement and the direction and 

 force of the osmotic pressure. The solution pressure of 

 a substance is measured by the effect that substance 

 exercises when dissolved on the surface tension of its 

 solution, or, to put it in Traube 's own way, the more a 

 substance lowers or raises the surface tension of a solvent 

 (water), the less or greater is the solution pressure {Haft- 

 druck) of that substance. This solution pressure, Traube 

 further holds, is the only force controlling osmosis through 

 a membrane, and he rejects completely the bombardmen*: 

 effect on the septum postulated in the van 't Hoff theory 

 of osmosis. 



The question as to the nature of the factors concerned in 

 osmosis must remain undecided until the facts have been 

 more fully studied from the physiological point of view, 

 but enough is now known to indicate that surface tension 

 plays at least a part in it, and the omission of all con- 

 sideration of it as a factor is not by any means a negligible 

 defect in the van 't Hoff theory of osmosis. 



The occurrence of variations in surface tension in the 

 individual cells of an organ or tissue is difficult to demon- 

 strate directly. We have no methods for that purpose, 

 and, in consequence, one must depend on indirect ways 

 to reveal whether such variations exist. The most 

 effective of these is to determine the 'distribution of organic 

 solutes and of inorganic salts in the cell. The demonstra- 

 tion of the former is at present difficult, or even in some 

 cases impossible. The occurrence of soaps, which are 

 amongst the most effective agents in lowering surface 

 tension, may be revealed without difficulty micro- 

 chemically, as may also neutral fats, but we have as yet 

 no delicate microchemical tests for sugars, urea, and other 

 nitrogenous metabolites, and in consequence the part they 

 play, if any, in altering the surface tension in different 

 kinds of cells, is unknown. Further research mav, how- 

 ever, result in discovering methods of revealing their 

 occurrence microchemically in the cell. We are in a lilce 

 difficulty with regard to sodium, the distribution of which 

 we can determine microchemically in its chief compounds, 

 the chloride and phosphate, only after the exclusion of 

 potassium, calcium, and magnesium. We have, on the 

 other hand, very sensitive reactions for potassium, iron, 

 calcium, haloid chlorine, and phosphoric acid, and with 

 methods based on these reactions it is possible to localise 

 the majority of the inorganic elements which occur in the 

 living cell. 



By the use of these methods we can indirectly determine 

 the occurrence of differences in surface tension in a cell. 

 This determination is based on the deduction from the 



NO. 2136, VOL. 84] 



Gibbs-Thomson principle that, where in a cell an inorganic 

 element or compound is concentrated, the surface tension 

 at the point is lower than it is elsewhere in the cell. lf> 

 for example, it is concentrated on one wall of a cell, the 

 surface tension there is less than on the remaining sur- 

 faces or walls of the cell. The thickness of this layer 

 must vary with the osmotic concentration in the cell, with 

 the specific composition of the colloid material of the cyto- 

 plasm and with the activity of the cell, but it should not 

 e.xceed a few hundredths of a milimetre (o-02-o-04 mm.),, 

 v/hile it might be very much less in an animal cell the 

 greatest diameter of which does not exceed 20 /i. 



Numerous examples of such localisation may be observed 

 in the confervoid protophyta. In Ulothrix, ordinarily, 

 there is usually a remarkable condensation of the- 

 potassium at the ends of the cell on each transverse wall. 

 The surface tension, on the basis of the deduction from, 

 the Gibbs-Thomson principle, should be, in all these cases, 

 high on the lateral walls and low on those surfaces adjoin- 

 ing the transverse septa. 



The use of this deduction may be extended. There are 

 in cells various inclusions the composition of which gives 

 them a different surface tension from that prevailing in 

 the external limiting area of the cell. Further, the limit- 

 ing portion of the cytoplasm in contact with these in- 

 clusions must have surface tension also. When, therefore, 

 we find by microchemical means that a condensation of 

 an inorganic element or compound obtains immediately 

 within or v/ithout an inclusion, we may conclude that 

 there, as compared with the external surface of the cell, 

 the surface tension is low. It may be urged that the 

 condensation is due to adsorption only ; but this objection 

 cannot hold, for in the Gibbs-Thomson phenomena the 

 localisation of the solute at a part of the surface as the 

 result of high tepsion elsewhere of the solution is, in all 

 probability, due to adsorption, and is indeed so regarded.' 



It is in this way that we can explain the remarkable 

 localisation of potassium in the cytoplasm at the margins 

 of the chromatophor in Spirogyra, and also the extra- 

 ordinary quantities of potassium held in or on the in- 

 clusions in the mesophyllic cells of leaves. In infusoria 

 (Vorticella, Paramcecium) the potassium present, apart 

 from that in the stalk or ectosarc, is confined to one or 

 more small granules or masses in the cytoplasm. 



How important a factor this is in clearing the active 

 portion of the cytoplasm of compounds which might 

 hamper its action, a little consideration will show. In 

 plants, very large quantities of salts are carried to the 

 leaves by the sap from the roots, and among these salts 

 those of potassium are the most abundant as a rule- 

 Reaching the leaves, these salts do not return, and m 

 consequence during the functional life of the leaves they 

 accumulate in the mesophyllic cells in very large quanti- 

 ties, which, if thev were not localised as described in the 

 cell, would affect the whole cytoplasm and alter its action. 



Enough has been advanced here to indicate that surface 

 tension is not a minor feature in cell life. I would go 

 even farther than this, and venture to say that^ the energy 

 evolved in muscular contraction, that also involved in 

 secretion and excretion, the force concerned in the pheno- 

 mena of nuclear and cell division, and that force also 

 engaged by the nerve cell in the production of ^ a nerve 

 impulse, are "but manifestations of surface tension. On 

 this view the living cell is but a machine, an engine, for 

 transforming potential into kinetic and other forms of 

 energy through or by changes in its surface energy. 



To present an ample defence of all the parts of the 

 thesis just advanced is more than I propose to do in this 

 address. That would take more time than is customarily 

 allowed on such an occasion, and I have, in consequence, 

 decided to confine my observations to outlines of the 

 points as specified. 



It is not a new view that surface tension is the source 

 of the muscular contraction. As already stated, the first 

 to appiv the explanation of this force as a factor in 

 cellular 'movement was Engelmann in 1869, who advanced 

 the view that those changes in shape of cells which are 

 classed as contractile are all due to that force which is 

 concerned in the rounding of a drop of fluid. The same 

 view v,-as expressed by Rindfleisch in 1880, and by Berthold 

 1 See Freundlich, " KapiUarchemie," p. 50, igoj. 



