May io, 19 17] 



NATURE 



215 



theory more than held its own. It was claimed by 

 Prof.' A. W. Porter that this theorv" is the only one 

 which gives directly the experimentally obtained values 

 for dilute solutions ; that it has now been placed on a 

 sound experimental basis as a result of Perrin's inves- 

 tigations, which show^ that particles suspended in a 

 liquid, and therefore also the molecules of the solute, 

 are in rapid motion to the precise amount 

 required by the theory; and that any other theory of 

 osmotic pressure must not only be competent to 

 account for the obser\-ed facts, but must explain the 

 absence of the effects thai we have a right to expect 

 from the molecular agitation of the solute. These 

 claims were not seriously ' shaken by the criticisms 

 of subsequent speakers, and towards the close of the 

 meeting the chairman expressed his general agree- 

 ment with the arguments put forward in favour of the 

 kinetic theor}-. 



Mr. W. R. Bousfield's contention that it is the sol- 

 vent and not the solute which is active in osmotic 

 pressure may be met, as Sir Oliver Lodge pointed 

 out, in a simple and therefore necessarily incomplete ' 

 way as follows. Imagine a closed vessel full (or prac- 

 tically full) of water, and divided into two compart- 

 ments by a semipermeable membrane. The pressures 

 on the two sides of the membrane compensate each 

 other, but if a little sugar is dissolved in one com- 

 partment an additional pressure, due to the presence 

 of the solute, is set up on that side. The contention that 

 it is necessary to look to the solvent, and the solvent 

 onlv, as the source of the pressure is therefore not 

 established, but Bousfield's view that osmotic pressure 

 is connected with the presence of solvent vapour (ap- 

 proximately obeying the gas laws) in the molecular 

 interspaces deserves consideration on its merits. 



It will not be denied that there are difficulties in 

 applying the kinetic theory to relatively concentrated 

 solutions (more particularly as regards the correction 

 for the volume of the solute), just as there are diffi- 

 culties in the application of the kinetic theory to com- 

 pressed gases. It is remarkable that the deviations 

 from the simple gas laws are smaller for solutions 

 than for gases, and in one case at least (compare 

 Sackur and Stern, Zeitsch. physikal. Chem., 1912, 

 vol. Ixxxi., p. 441) this has been shown to be in accord- 

 ance with the kinetic theor}- of osmotic pressure. 



Both Prof. Porter and Mr. Bousfield ascribe the 

 deviation of osmotic pressure from simple laws solely 

 to hydration of the solute, and proceed to calculate 

 the degree of hydration of the solute particles on this 

 assumption. As, however, such simple laws do not 

 hold for the gaseous state, in which hydration is 

 necessarily absent, these "hydration numbers" do not 

 inspire much confidence, more particularly as the 

 variation of some of them with concentration in rela- 

 tively dilute solution appears difficult to reconcile with 

 the law of mass action. Unfortunately they cannot 

 be independently tested, as no satisfactory method of 

 measuring hydration in solution has yet been dis- 

 covered. 



.Although the magnitude of the osmotic pressure, as 

 equilibrium pressure, is independent of the nature of 

 the membrane provided the latter is truly semi- 

 permeable, the mechanism of osmosis, including the 

 part played by the membrane, is of great interest and 

 importance. The very suggestive investigations of 

 Adrian. Brown and Tinker on the permeability and 

 other properties of membranes have already added 

 substantiallv to our knowledge of these questions. 

 As regards the bearing of theories of osmotic pressure 

 on osmosis, the suggestion of van Laar that the pres- 

 sure of the sugar molecules as postulated by the kinetic 

 theory would prevent water flowing inwards does not 

 appear well founded. The most satisfactory picture 

 of the process is probably obtained by analogy with 



NO. 2480, VOL. 99] 



Ramsay's well-known experiment with a cell provided 

 with a palladium membrane permeable for hydrogen, 

 but not for nitrogen. Although the cell contained 

 nitrogen at half an atmosphere pressure, when it was 

 surrounded by hydrogen the latter entered until its 

 partial pressure inside was practically equal to its 

 pressure outside. G, S. 



ECONOMICS OF LIFE INSURANCE. 



OIX paf>ers relating to problems of life insurance, 

 *^ read to the Ecgnomic Sectidn of the American 

 Association for the Advancement of Science last 

 December, are printed in the Scientific Monthly for 

 -April. Of these, the most important, in view of 

 what has happened since it was read, is that by Prof. 

 Huebner, of the University of Pennsylvania, on " Life 

 Insurance and the War." For all the belligerents 

 he finds that the financial effects of the war on the 

 companies by depreciation of investments have been 

 serious. For England and Canada the war claims 

 have been between 11 and 12 per cent, of the total 

 claims, which is a favourable experience ; for the 

 enemy countries little information is to be had. The 

 same observation applies to the terms upon which 

 companies undertake war risks. The after-effects Of 

 the war on the health of survivors must not be over- 

 looked. The author urges that the companies should 

 have latitude to charge such extra premiums as may 

 be necessary, but that the burden should to some 

 extent be borne by the community as a whole. 



Prof. Huebner submits for consideration by the 

 companies proposals for refunding any excess extra 

 premium at the close of the war, and for reinstate- 

 ment of the policy where the insured, owning to the 

 war, is unable to produce evidence of good health. 

 These proposals will no doubt be attractive, but they 

 are open to the, objection that contracts based on a 

 calculation of averages cannot be modified by after- 

 results in individual cases. 



A paper by Mr. E. E. Rittenhouse shows that the 

 life insurance companies of New York State have 

 increased their new insurances in the thirty years 1885- 

 1915 from 65,000,000/. to 385,000,000/., and that the 

 proportion of insurances lost by lapse and surrender 

 during^ that period is a little more than 40 per cent., 

 and tends to diminish. 



Three of the papers relate to a recent development 

 of the functions of life insurance companies in the 

 United States. Dr. Fisk, as medical director of the 

 Life Extension Institute, a body organised in 1914, 

 urges health conservation as a duty incumbent upon 

 these companies. Mr. Cox, who represents an in- 

 fluential association of life insurance presidents, asserts 

 that nearly every large company in the United 

 States is doing something intended either to prolong 

 the lives of its policy-holders or of the people gener- 

 ally. Mr. Haley Fiske gives particulars of the work 

 of a life extension bureau for medical examination 

 of insured persons and of other comprehensive 

 measures adopted by the companies for the preserva- 

 tion of the health of the insured. 



Dr. Hoffman, statistician to the Prudential Insur- 

 ance Company of America, improves the occasion 

 offered by these recent developments to expose some 

 fallacies of compulsory health insurance. He con- 

 siders that voluntan.' effort can be relied upon to 

 bring about all the benefits that could be expected 

 from compulsory' measures. He meets the argument 

 that compulsory' insurance has had good effect in 

 Germanv and in England in the fight against tuber- 

 culosis by statistics showing that Massachusetts has 

 reduced its mortality from that cause by 238 in 10,000, 

 while the corresponding reduction in Germany has 



