
18 A TEXTBOOK OF GENERAL BOTANY 
supplied. Proteins are very complex chemical compounds with 
large molecules and always contain the elements carbon, hydro- 
gen, oxygen, and nitrogen, and frequently in addition other 
elements, such as sulphur and phosphorus. In protoplasm there 
are many different kinds of proteins. The white of an egg is 
a very good example of one kind of protein. 
Colloidal state of protoplasm. ‘The proteins are dispersed 
in the water of the protoplasm as particles of colloidal size. In 
order to understand what is meant by a colloidal dispersion in 
water (sometimes called a colloidal solution) we may consider 
the differences between a suspension, a colloidal dispersion, and 
a true solution. If we were to take small particles of soil that 
are still large enough to be visible, and stir them in a vessel 
of water, they would become suspended in the water. We 
should then have a suspension of soil particles in water. We 
frequently see such a condition in muddy rivers or in agitated 
pools. Each soil particle is composed of many molecules. These - 
particles would settle to the bottom if the water were to remain 
still for a sufficient length of time. Each soil particle could be 
divided into two, and we should have a suspension of smaller 
particles. Theoretically this process of division could be con- 
tinued until each particle would consist of only a few molecules 
and finally of only a single one. The last condition would be 
a true solution, in which the individual molecules of the soil 
would be dispersed in water. When we put, a little sugar in 
water, the sugar seems to disappear and to sweeten the water. 
In such a case we have a solution (or dispersion of molecules) 
of sugar in water. From the foregoing consideration it is evi- 
dent that there must be every gradation between a suspension 
and a true solution. The condition intermediate between a true 
solution (dispersion of molecules) and a suspension (dispersion 
of visible particles) is known as a colloidal dispersion. <A col- 
loidal dispersion in water is a dispersion of particles whose size 
may vary between 0.1 w and 0.001 u. (1 mw equals 0.001 milli- 
meter.) The lower limit is regarded as the size of a large mole- 
cule, while the upper limit is slightly smaller than the smallest 
