The Chemical and Physical Sfructure of Protoplasm - 69 



can be distinguished from all other similar and 

 dissimilar substances, on the basis of exact meas- 

 urements of its melting point, crystalline struc- 

 ture, degree of sweetness, solubility, and other of 

 its specific physical and chemical attributes. 



A mixture, in contrast to a substance, displays 

 variable properties, depending upon the propor- 

 tions of the mixed components. The properties 

 of pond water, for example, vary widely accord- 

 ing to the locality from which it is collected. It 

 may contain different mineral salts from the soil 

 and various amounts of the atmospheric gases. It 

 may display a wide variety of tastes and colors 

 and considerable variation in the boiling and 

 freezing points. It may even be toxic when drunk. 

 Or crude sugar may be light or dark, strongly or 

 weakly sweet, easy or difficult to dissolve, and so 

 forth, depending upon the refinement of the 

 methods used to extract it from the cane. The 

 variations of a mixture depend upon the varying 

 proportions of the mixed substances, and perhaps 

 the most complex and variable of all mixtures is 

 protoplasm itself. In protoplasm the number of 

 components is very great, and the problem of un- 

 derstanding their interactions is very difficult. 



Chemical Combination vs. Mixing. The fixity of 

 the properties of a substance, as compared to the 

 variability of the properties of a mixture, is clue 

 to the fact that the atoms that constitute the 

 molecules of a substance are chemically united in 

 fixed proportion; whereas the molecules that com- 

 pose a mixture may be present in any proportion. 

 This may be illustrated by a specific case. Take, 

 for example, a strong gas tank containing hydro- 

 gen gas (H 2 ) and oxygen gas (0 2 ) mixed together 

 (Fig. 4-3). In the tank the hydrogen and oxygen 



may be present in any proportion. With little hy- 

 drogen and much oxygen, the mixture as a whole 

 will be relatively heavy and dense; or conversely, 

 with little oxygen and much hydrogen, the mix- 

 ture will be lighter and less dense. In other words, 

 the properties of the mixture in the tank are al- 

 together variable. But suppose that gradually the 

 tank is heated. At a critical temperature, com- 

 bustion, which represents a chemical union be- 

 tween the atoms of the hydrogen (H) and the 

 oxygen (O). will start and then proceed with ex- 

 plosive speed. A fraction of a second later, all that 

 can be recovered from within the tank is a small 

 amount of water (H 2 0) and a residual quantity 

 of either oxygen or hydrogen— depending upon 

 whether the exploded mixture contained an ex- 

 cess of the one or the other (Fig. 4-3). But the 

 point to be made is that when two or more atoms 

 chemically unite to form a molecule, they do so 

 in fixed proportion. The fixed proportion that 

 determines the specific combining power of each 

 different kind of atom is given by its valence 

 number. Consequently this number must be 

 learned, at least for all atoms present in the pro- 

 toplasmic system (Table 4-1). 



Atomic Structure. The fact that the molecules of 

 all substances are constituted by specific combina- 

 tions of the atoms of a limited number of differ- 

 ent elements, has been recognized by scien- 

 tists for many years now. But it is only in the 

 recent years of "atom smashing" that conclusive 

 proof has been advanced that each atom is consti- 

 tuted of electrically charged smaller particles that 

 are of just two sorts. The center of each atom is 

 constituted mainly of protons, each bearing a 

 single positive charge, although there are some 



Fig. 4-3. The mixing of two substances is not the same as chemical 

 union between them. A, reaction tank contains a mixture of free 

 oxygen (O.,) and hydrogen (H 2 ) before ignition has occurred. B, 

 same tank after the hydrogen has burned (united with oxygen). Two 

 hydrogen atoms unite with each oxygen (i.e., the atoms unite in a 

 fixed proportion). Therefore the resulting compound, water (H 2 0), is 

 a uniform product. Large dots represent oxygen atoms; small dots, 

 hydrogen atoms. 



VALVE 

 — INLET TUBE — ! 



REACTION 

 CHAMBER 



