404 LIFE : OUTLINES OF GENERAL BIOLOGY 



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 word was invented, we believe, 

 by Purkinje six years before, but perhaps von Mohl was the first to 

 use it as a general term for the kind of material that goes to the 

 making of all kinds of living creatures. 



But, as already said, the modem student would change some of 

 von Mohls words. Instead of opaque, he would say clear; instead 

 of white, he would say colourless; instead of viscid, he would 

 probably say jelly-like or colloid. The fact is that protoplasm shows 

 itself as a liquid in which there are suspended multitudinous minute 

 particles, occasionally, especially in plant cells, in a state of con- 

 stant ("Brownian") movement. When the protoplasm dies the liquid 

 "sets" as a jelly, and the Brownian movement of the particles 

 ceases. A similar change from "sol" to "gel" often takes place 

 temporarily during life. Everyone is familiar with the coagulation 

 of white of egg in boiling water, or with the liquefaction of gelatine 

 in similar conditions; but the changes in protoplasm are naturally 

 somewhat subtler. Raw white of egg can be partly solidified at a low 

 temperature and entirely solidified by drying; and it is possible to 

 work the change the other way by raising the temperature again or 

 by adding water. But protoplasm can pass from the solid to the 

 liquid, from the gel to the sol, or vice versa, without any change 

 in the temperature or in the water-content. 



Before saying more about the physical characters of protoplasm, 

 let us think of it chemically. Living matter cannot be analysed as 

 such, for the methods of analysis must kill it, and death may mean 

 that large molecules rapidly tumble down into smaller ones. But 

 the chemist tells us that there are no elements in the protoplasm 

 that are not common enough in the adjacent non-living world; it 

 is a question, not of the presence of rare elements, but of the com- 

 binations of common ones. The chemist also tells us that the dead 

 I)rotopIasm shows a mixture of proteins, carbohydrates, and fats 

 besides minute representation of some other materials. It is well 

 known that proteins are never absent, and it follows that they must 

 play an essential part in the chemistry of life. Proteins contain 

 carbon, oxygen, nitrogen, and hydrogen (in that order of percentage), 

 and usually a minute representation of sulphur. Examples, already 

 stated, arc the albumin of white of egg, the vitellin of yolk of 

 egg, the casein of cheese, and the gluten of wheat. The protein 

 molecules are very large and complex, sometimes consisting of 

 thousands of atoms, and there are so many different kinds that 

 it is safe to say that every distinctive type of animal or plant 

 has some peculiar protein, characteristic of itself. The red colouring 

 matter of the blood — the haemoglobin — is a protein, and we know 

 that this pigment in a dog is appreciably different from that which 



