absorption 2 ;i 



Substances so capable of dissociation are termed electrolytes, and 

 their interest to us at the present time is in connection with osmosis ; 

 the act of dissociation liberates a number of ions, and by so doing 

 increases the number of particles moving in the fluid. It can be 

 shown that osmotic pressure — a term yet to be explained — is pro- 

 portional to the number of molecules of dissolved substances present, 

 and in the above example the ion behaves as a molecule in osmotic 

 pressure. Generally speaking, the greater the dilution, the larger 

 the number of ions dissociated. 



Though not directly connected with the matter under considera- 

 tion, it is convenient in this place to look at the part played in the 

 body by ionic action. Thus it is possible that the contractile tissues 

 which work in a saline artificial circulation (p. 49) are capable of 

 so doing in consequence of a due adjustment of ions in their surround- 

 ings ; and the same may be said of cilia, amoeboid action, and such . 

 like, each requiring its own definite proportion of ions. Loeb has 

 classified the ions responsible for rhythmic contractions, though 

 such a classification can at present only be regarded as provisional. 

 So much importance does this observer attach to ionic action that 

 he has brought forward evidence to show that fertilisation of the 

 ovum may be ionic in its nature,- the spermatozoa merely regulating 

 the proportion of ions. Even a nerve impulse he regards as depend- 

 ing on an electrolytic action. 



Osmotic Pressure. — If a solution of an electrolyte such as common 

 salt be enclosed in a specially prepared semi-permeable cell, water 

 will pass in, but salt will not pass out, either by filtration or diffusion. 

 This can be ascertained by placing the cell in distilled water, and 

 if a manometer be connected with the cell it will be found that as 

 the water passes in the pressure increases. This pressure is known 

 as osmotic pressure, and the term ' osmosis ' at the present day is 

 confined to the stream of water passing through a membrane, 

 while dialysis is restricted to the passage of the molecules dissolved 

 in water. If in the above experiment the solution of salt was 1 per 

 cent., it will be found that, if the strength be doubled, the mano- 

 meter will indicate twice the pressure, so that the amount of osmotic 

 pressure is always proportional to the number of molecules of the 

 dissolved substance in a given volume of the solution. 



The nature of osmotic pressure is unknown ; it can be shown to 

 be independent of the nature of the substance in solution, and pro- 

 portional to the number of molecules of the dissolved substance. 

 It is governed by laws closely analogous to those governing gaseous 

 pressure, and, as in the case of gases, it is affected by variations of 

 temperature and by the law of partial pressure. The latter, in the case 

 of osmotic pressure, is expressed by saying that the osmotic pressure 

 of a solution of different substances is equal to the sum of the pressure 

 which the individual substances would exert if they were alone in the 

 solution. Osmotic pressure is conveniently estimated by ascertaining 

 the freezing-point of a substance soluble in water, which is always 

 lower than that of the water itself ; the lowering of the freezing-point 

 is proportional to the molecular concentration of the dissolved sub- 

 stance, and this molecular concentration is proportional to osmotic 

 pressure. Sugar, for instance, not being an electrolyte, has a 

 smaller number of particles moving in the solution than sodium 

 chloride, which is capable of dissociation, so that the osmotic 

 pressure of salt is higher than that of sugar. A 1 per cent, solution 

 of sugar has an osmotic pressure of 473 mm. mercury, while 0-9 per 



