372 LIFE : OUTLINES OF GENERAL BIOLOGY 



the conclusion that active protoplasm is a fluid of very varjnng 

 viscosity; that it is often crowded with mobile granules which, 

 however, are unessential; that it is homogeneous except in so far 

 as there may be transient films di\'iding the cell-substance into 

 areas; and that it always forms a surface film and forms it afresh 

 when it is destroyed. 



In many cases the protoplasm changes from a fluid "sol" state to 

 a jelly-like "gel" state. Up to a limit this is reversible, and there 

 may even be an alternation between more solid and more liquid 

 conditions in one part of the cell, notably in amoeboid movement. 

 LeaWng further details for subsequent discussion, we may describe 

 protoplasm as consisting of diversified organic material partly in 

 colloidal and partly in true solution, and of a percentage of mineral 

 salts, all dissolved in water, which forms about 80 per cent, of the 

 total. 



Let us sum up. While there is "one kind of flesh of men, another 

 flesh of beasts, another of fishes, and another of birds", the term 

 protoplasm indicates that in all kinds of "flesh" there is a more or 

 less similar physical basis. The word was invented by Purkinje in 

 1S40, but von Mohl seems to have been the first to use it as a 

 general term for the kind of material that goes to the making of all 

 living creatures. Writing in 1846, he said: "The remainder of the 

 cell is more or less densely filled with an opaque, viscid fluid of a 

 white colour, having granules intermingled with it, which fluid I 

 call protoplasm." The modern student would be inclined to change 

 some of von Mohl's words, saying clear instead of opaque, colourless 

 instead of white, but in most ways von Mohl's description holds 

 good still. Perhaps the two main advances are that we think of 

 protoplasm as an intricate mixture of proteins, carbohydrates, fats, 

 ferments, and mineral matter, and that we think of it as in a colloid 

 state. The multitudinous particles of protein and other substances, 

 and the crowds of unmiscible droplets as well, present a very large 

 surface in proportion to the total mass. This allows of great intensity 

 of cliemical and physical changes, because the area of the surface 

 is proportionally so enormous. Thus there is usually an electric 

 charge on the contact surface between any two "phases", e.g. 

 between a complex solid particle and a complex liquid medium. As 

 the multitudinousness of the particles, often quivering under the 

 bomljardnient of the restless molecules of the fluid, means a very 

 large surface, there is consequently a copious spring of electrification. 

 Many of the properties of living matter arc wrapped up with this 

 colloidal state. 



Two difliculties, often stated by the thoughtful student, may be 

 met at this jx)int. If protoplasm is a liquid emulsion, mostly con- 

 sisting of water, how are animals as firm as most of them are? and 

 how do trees rise three hundred feet into the air? Part of the answer 



