12 



BAKERIAN LECTURE— ON OSMOTIC FORCE. 



[1854. 



found considerable difficulty in its reproduction, and will, 

 therefore, describe in detail a method which is applicable in 

 most eases for obtaining in this manner an example of the 

 spheroidal state. We will employ a particular instance. If 

 we take cymole, a hydrocarbon belonging to the benzole series, 

 and half fill a bottle two or more inches in diameter with it, 

 placing in the neck of the bottle a perforated cork through 

 which passes a funnel-tube filled with cymole, and having a 

 piece of sheet India-rubber stretched over its mouth, we shall 

 find that on adjusting the funnel-tube till its lower extremity is 

 rather less than half an inch from the surface of the liquid in 

 the bottle, and on letting fall a drop of cymole from it, beauti- 

 ful spheroidal globules will be formed and roll about for some 

 time, scarcely diminishing in size. This expeiiment may be 

 performed with great advantage if the cymole be warmed first. 

 An ordinary funnel with a "filter may be substituted for the 

 funnel-tube, and will answer well if the lower aperture of the 

 funnel has a diameter of about -13 of an inch. The experi- 

 ment may be repeated with other materials with similar efTects. 

 That the spheroids are not in contact with the surfaces on 

 which they roll, may be proved by saturating the liquid in the 

 tube with something that shall have a visible effect upon a sub- 

 stance dissolved in the liquid in the bottle. A beautiful illus- 

 tration of this occurs when, under conditions similar to those 

 before mentioned, we employ in the funnel-tube a solution of 

 sugar containing sulphooj'anide of potassium, and in the bottle 

 a solution of sugar, containing sesquichloride of iron ; no red 

 colouration takes place until the coalesceijce of the spheroids 

 with the liquid beneath them. Many other chemical reactions 

 may be made use of with similar results. It is very curious to 

 see a solution of ferrocyanide of potassium floating upon a 

 solution of sesquichloride of iron, while not a trace of Prussian 

 blue is formed. These experiments must, of course, be recom- 

 menced whenever any union of the liquids employed has taken 

 place. The cork spoken of above should have two perforations, 

 one to admit the funnel tube, and the other to allow the escape 

 of air. 



Is the employment of a volatile substance essential to the 

 production of these phenomena ? I imagined that this ques- 

 tion might be answered by the following experiment. A dish 

 of platinum might be heated strongly, and a drop of melted 

 lead then placed upon it; now if the production of vapour from 

 the substances employed were essential to the formation of the 

 spheroidal state, the lead should at once dissolve and perforate 

 the platinum ; if, however, the spheroidal state occurs when 

 when two non-volatile substances are employed, the platinum 

 vessel should not be perforated until its temperature has been 

 considerably reduced. The experiment was tried with every 

 precaution to prevent the oxidation of the lead and insure an 

 accurate result; a perforation of the platinum ensued the mo- 

 ment of placing the melted lead upon it. This proves that the 

 production of vapour is essential to the occurrence of the 

 spheroidal state ; for it cannot be urged that that condition is 

 never manifested when metals only are employed ; for a drop 

 of a volatile metal, mercury (melted mercury, we may say, in 

 order to render its relationship to melted lead the more appa- 

 rent,) placed on an intensely heated surface of platinum in- 

 stantly assumes the spheroidal form, and evaporating slowly, 

 dauces about in the vessel with peculiar movements'. Upon 

 thin, sonorous vessels of copper, &o., this movement takes 

 place with such rapidity as to produce a musical tone of high 

 pitch. I have sometimes seen globules of mercury and water 

 rise to the height of six inches from the capsules in which they 

 had been formed. 



A word or two in conclusion, as an attempt at an explana- 

 tion of the phenomena observed may not be out of place. 



Since a space always exists between the lower body and that 

 in the spheroidal state, and since that state is not manifested 

 by non-vol!itile substances, it seems reasonable to conclude that 

 the vapour proceeding from those parts of the liquids nearest to 

 the containing vessel or subjacent fluid tends to assist the 

 internal molecular cohesive force of the drops in assuming and 

 maintaining their spheroidal form. Other forces originated by 

 the temperature may also be in operation. 



I should premise that the solution of sulphide of sodium 

 employed in my first experiment made a dark stain upon silver 

 even in the cold. This renders the nullity of ^its action at a 

 high temperature the more remarkable. It will be scarcely 

 necessary to mention, that, for the successful performance of 

 most of these experiments, considerable manipulative care is 

 required.- — Royal College of Chemistry, March 18, 1854. 



Bakeriau Liecttu-c — On Osmotic Force* 



By Pkopessok Graham. 



This name was applied to the power by which liquids are impelled 

 through moist membrane and other porous, septa in experiments of 

 endosmose and exosmose. It was shown that with a solution of salt 

 on one side of the porous septum and pure water on the other side, 

 (the condition of tlie osmometer of Dutrochet when filled with a saline 

 solution and immersed in water,) the passage of the salt outward is 

 entia'ely by difl'usiou, and that a thin membrane does not sensibly 

 impede that molecul.ar process. The movement is confined 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 ciu'rent It is osmose and the woik of the osmotic 

 force to be discussed. As difiiision is always a double movement, — 

 while salt diffuses out, a certain quantity of water necessarily diffuses 

 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 : diff"usion therefore is 

 quite insufficient to account for the water cui-rent. The theoiy 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 correctly observed, and 

 many of the saline solutions which give rise to the highest osmose aire 

 indistinguishable 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 mem- 

 brane. The earthenware osmometer consisted of the porous cylinder 

 employed in voltaic batteries, about five inches in depth, sui'mounted 

 by an open glass tube 0'6 inch in diameter attached to the mouth of 

 the cylinder by means of a cup of gutta percha. In conducting an 

 experiment, the cylinder was filled with any saline solution to the base 

 of the glass tube, and immediately placed in a large jar of distilled 

 water ; and as the fluid within the instriiment rose in the tube during 

 the experiment, water was added to the jar so as to prevent inequality 

 of hydrostatic pressui-e. The rise (or fall) of liquid in the tube was 

 highly uniform, as observed from hour to hoiir, and the experiment 

 was generally terminated in five hours. From experiments made on 

 solutions of GY^ry variety of soluble substances, it ajjpeared that the 

 rise or osmose is quite insignificant with neutral organic substances in 

 general, such as sagar, alcohol, urea, tannin, &c. ; so also with neutral 

 salts of the earths and ordinary met.als, and with chloride of sodivun 

 and potassium, nitrates of potash and soda, and chloride of mercury. 

 A more sensible but still very moderate osmose is exliibited by hydro- 

 chloric, nitric, acetic, sulphirrous, citric and tartaric acids. These 



