122 



NATURE 



[April 6, 191 6 



1 have a iew copies of my paper, and will send one 

 to anybody who has not access to the Journal of the I 

 Institution of Electrical Engineers, and who is specially \ 

 interested in the subject. A. P. Trotter. i 



Athenaeum Club, Pall Mall, London. 



international Latin. 



I HAVE always shared the ret^ret of your correspond- 

 ents that Latin has now ceased to be employed as the j 

 international language of science, although for more 

 than a thousand years after it had ceased to be a 

 vernacular it had, among men of education, 

 maintained its position as a living language, adapt- 

 ing itself to the varying needs of die times. 1 have 

 devoted some attention to the development during the 

 Middle Ages and succeeding centuries of the branches 

 of science in which I am more especially interested, 

 and have been struck by the clear, fluent Latin in 

 which the majority of the scientific treatises were 

 written. That of Agricola, Encelius (Entzelt), Gesner, 

 Camden, and Caesalpinus in the sixteenth century; 

 Francisco Imperato and Aldrovandi in the seventeenth ; 

 and Isaac Lawson, Cramer, and Linnaeus in the 

 eighteenth, and most of their fellow- workers is, as 

 a rule, as easy to follow as French, in spite of the 

 handicap of the want of articles, the most serious 

 defect of Latin. 



It was the Latinists themselves who were priniarily 

 responsible for the modern disuse of the language. 

 They insisted that the diction of Cicero, rather than 

 that of Pliny, should be followed, and as they spoke 

 with authority, there was no one hardy enough to 

 contradict them, so that the unfortunate man of 

 science had to face a hail of ridicule if he failed to 

 reproduce the mannerisms which were held up for his 

 imitation, while if he succeeded his pages were almost 

 unreadable for most of those who would have been 

 interested in their contents. 



If Latin were ever again to come Into use for 

 scientific purposes, it would only be by assimilating 

 its style and idioms to those of its daughter languages 

 at the present time, by accommodating itself to the 

 changes of meaning which have overtaken so many of 

 its words, and by borrowing freely from their vocabu- 

 lary, especially in the case of terms which are prac- 

 tically international. At the same time the inflexions 

 and syntax of classical Latin would be generally, but 

 not slavishly, followed. 



I am afraid, however, that it is too late to undo 

 the work of those who have slain the object of their 

 affections by strenuous efforts to renew the golden 

 age of its early youth when by far the more impor- 

 tant portion of its long career of usefulness still lay 

 before it; and now that it is, as it would seem, 

 really dead, had we not better regretfully but reso- 

 lutelv bury it out of sight and turn our thoughts to 

 the flexible idiom handed down to us by our fore- 

 fathers, which is already understood throughout the 

 whole civilised world? John W. Evans. 



Imperial College of Science and Technology, 

 South Kensington, March 28. 



Osmotic Pressure or Osmotic Suction— Which? 



The interesting controversy between Profs, van 

 Laar and Ehrenfest, referred to in Nature of 

 March 16, again raises the question of the cause for 

 the approximate equality between osmotic pressure 

 and gas pressure. In this connection the following 

 simple proof, based on the kinetic theory, of van't 

 Hofl''s well-known relationship, may be of interest. 



The tendency of a liquid to diffuse is measured by 

 its diffusion pressure, which may be defined as 

 the bombardment pressure exerted by the liquid 

 molecules on either side of a plane of unit 



NO. 2423, VOL. 97] 



area placed anywhere within the liquid. If we regard 

 a perfect liquid as formed by the compression of a 

 perfect gas until the molecules almost touch one 

 another, it will be seen that the diffusion pressure In 

 proportional to the number of molecules in unit 

 volume, or the absolute concentration, and also to the 

 absolute temperature. The diffusion pressure for 

 ordinary liquids has a very large value. For instance, 

 if water were a perfect liquid, its diffusion pressure 

 would be about 1200 atmospheres. 



Since the absolute concentration of a solvent is- 

 reduced by the introduction of a solute, it is evident 

 that diffusion pressure Is reduced in the same way ; 

 so that the diffusion pressure of the solvent in a 

 solution is always less than that in the pure solvent 

 itself. Hence solvent travels across a membrane from 

 the pure solvent side to the solution side, unless a 

 hydrostatic pressure equal to the difference between 

 the two diffusion pressures is placed on the solution. 

 The osmotic pressure, which is defined as the afore- 

 said hydrostatic pressure, is therefore proportional to 

 the difference between the absolute concentrations of 

 the solvent on the two sides of the membrane. 

 Further, it is also approximately proportional to the 

 concentration of the solute because the latter is itself 

 approximately equal to the difference in solvent con- 

 centration on the two sides, since the process of solu- 

 tion consists essentially in the spatial replacement of 

 part of the solvent molecules by a more or less equal 

 number of solute molecules. 



Next as regards the gas relationship. Consider the 

 case of a pure solvent separated from its solution by 

 means of a semipermeable membrane (diagram I.). 

 Remove from the solution side all the solvent mole- 

 cules ; and also an equal number from the pure solvent 

 side. The sys- 

 tem then as- 

 sumes the ap- 

 pearance 

 shown in dia- 

 gram II. Now 

 this process 

 merely reduces 

 the absolute 

 c o ncentratlons 

 and diffusion 

 pressures o n 

 the two sides 

 of the mem- 

 brane to an 

 equal extent ; 

 but leaves un- 

 altered the 

 differences. The 

 residue of sol- 

 vent molecules [0j will diffuse across the membrane 

 just as before ; whilst the solute molecules will bom- 

 bard the membrane. Moreover, the pressure of the 

 solvent residue on the one side will be equal to the 

 pressure of the solute on the other ; and both will be 

 equal to the corresponding gas pressure, since the 

 molecules are at distances from one another com- 

 parable to gas distances. Hence to prevent the residue 

 of solvent from flowing across the membrane, a hydro- 

 static pressure equal to the gas pressure will have to 

 be applied. The usual way of doing this is to make 

 the solution Into a kind of piston. 



On the other hand, the solute bombards the mem- 

 brane with a pressure equal to the corresponding gas 

 pressure, whether a hydrostatic pressure is placed on 

 the solution or not. The solvent itself can exert no 

 pressure on the membrane, since It is supposed to be 

 able to travel across the membrane just as if the 

 latter were non-existent. 







© 







X 



DiAcKAM II. 



