MATTER 



13 



blood there is about 0.7 per cent of salt in solution. If the blood is diluted 

 with water, the red blood cells will immediately absorb additional water, 

 swell, and may even burst. If, however, salt is added to the blood the red 

 cells will shrink and their surrounding membranes will become very 

 wrinkled or crenated. This swelling and shrinking of the blood cells 

 caused by water entering or leaving them rapidly are phenomena of 

 osmosis and osmotic pressure (Fig. 4). Osmosis is a veiy important 

 phenomenon in all living things. The intake into Hving tissues of oxygen, 

 digested foods, and water and the elimination from living tissues of carbon 

 dioxide gas and other waste products are brought about by the passage of 

 molecules through a semipermeable membrane from a place of higher con- 

 centration of these molecules to a place of lower concentration. In this 

 physical manner chemical substances necessaiy for life activities come 

 into living tissues, while the harmful chemical waste substances that are 

 produced in living tissues are eUminated from them. 



.7 % so// /n 

 '/as ma 



ph 



Wafer- 

 More wa/-&T~ 

 Less sa// 



Less ivofer' 

 More sa/f 



Fig. 4. — Diagrams illustrating osmosis and osmotic pressure in red blood cells by- 

 increasing or decreasing the water in the blood plasma. A, water and salt molecules in 

 the same ratio on the inside and outside of the semipermeable cell membrane. B, by 

 increasing the ratio of water molecules over the salt molecules on the outside, the water 

 molecules migrate from a place of higher concentration through the cell membrane to the 

 inside of the cell, thereby causing the whole cell to swell. C, by decreasing the ratio of 

 water molecules on the outside, the water molecules on the inside again migrate from a 

 place of higher concentration of water molecules through the cell membrane to the outside 

 of the cell and cause the whole cell to shrink and become crenated. 



20. Colloidal Emulsions. — In case the suspended droplets in an 

 emulsion are colloidal, the mixture is termed a colloidal emulsion. The 

 liquid in suspension is dispersed and is called the disperse phase, while 

 the other Uquid is called the dispersion medium, or the continuous phase. 



A colloidal emulsion is more or less jelly-like. It may at one time 

 become thinner and assume the condition of a sol or at another time 

 become thicker and assume the condition of a gel. This may be due to 

 the transfer of liquid from the disperse phase into the dispersion medium, 

 or vice versa, without the addition of more liquid from without. The 

 two phases, in other words, tend to change places, one being at one time 

 dispersed in the other, at another time the other in the one. When the 



