Royal Society, 151 



of the mean of the whole year. The phsenomena of the solar annual 

 variation superimposed upon those of the solar diurnal variation, — 

 and those of the solar diurnal variation itself, — are in this respect 

 contradistinguished by important differences. 



'' To have completed the view of the solar variations of the Decli- 

 nation at St. Helena would have required a notice of the so-called 

 irregular disturbances of that element, which are now known to have 

 a periodical character dependent on solar hours ; and also of the re- 

 markable cycle which is found to pervade all the magnetic variations 

 depending upon the sun, corresponding in its period and epochs 

 with those of the phaenomena of the solar spots ; but as both these 

 subjects have been recently brought before the Society in separate 

 memoirs, the author does not think it necessary to do more than 

 merely advert to them on the present occasion." 



June 15. — The Earl of Rosse, President, in the Chair. 



The following paper was read : — " On Osmotic Force." By Prof. 

 Graham, V.P.R.S. (The Bakerian Lecture.) 



This name was applied to the power by which liquids are im- 

 pelled through moist membrane and other porous septa in experi- 

 ments of endosmose and exosmose. It was shown that with a solu- 

 tion of salt on one side of the porous septum and pure water on the 

 other side (the condition of the osmometer of Dutrochet when filled 

 with a saline solution and immersed in water), the passage of the 

 salt outward is entirely by diffusion, and that a thin membrane does 

 not sensibly impede that molecular process. The movement is con- 

 fined to the liquid salt particles, and does not influence the water 

 holding them in solution, which is entirely passive : it requires no 

 further explanation. The flow of water inwards, on the other hand, 

 affects sensible masses of fluid, and is the only one of the movements 

 which can be correctly described as a current. It is osmose, and the 

 work of the osmotic force to be discussed. 



As diffusion is always a double movement — while salt diffuses 

 out, a certain quantity of water necessarily diffusing in at the same 

 time in exchange — diffusibility might be imagined to be the osmotic 

 force. But the water introduced into the osmometer in this way 

 has always a definite relation to the quantity of salt which escapes, 

 and can scarcely rise in any case above four or six times the weight 

 of salt, while the water entering the osmometer often exceeds the 

 salt leaving it, at least one hundred times. Diffusion is therefore 

 quite insufficient to account for the water current. 



The theory which refers osmose to capillarity appears to have no 

 better foundation. The great inequality of ascension assumed among 

 aqueous fluids is found not to exist, when their capillarity is cor- 

 rectly observed : and many of the saline solutions which give rise 

 to the greatest osmose are undistinguishable in ascension from pure 

 water itself. 



Two series of experiments on osmose were described, the first 

 series made with the use of porous mineral septa, and the second 

 series with animal membrane. The earthenware osmometer con- 



