348°

11.82

373°

2:06

12.24 12:36

FIG.I.

3y

y trihydrol molecules pass into

(H2O)2

2

(H2O)2

2

& monhydrol molecules pass into The net increase in volume due to these actions is — x (2−1·231 −0·462)+y(2—1·846)+(0·615—0*462)

=-0.307x+0154y+0*1538.

The volume at 277° is 1817 and as the average coefficient of expansion of water is o00045 the volume is increased between 277-373° A. by 7.85. −0·307x+0*154y+0*1538=7·85 i.e., -2x+y+8=513

No likely results are obtained by giving any value to x, i.e., the action (H2O),⇒(H2O)2+H2O ends at 277° (under atmospheric pressure); unless it follows the course.

2(H2O),→2(H2O)2+2H2O
→2(H2O),+(H2O),
→3(H2O)2

Suitable values for y and a can be found so that at 373° will be 2:04. This condition is satisfied when x=0, y=13, and 2=38.3. Water at 373° consists, then, of

..

518 0385 0.187 713 0509 0*335 550 0538 0325

2:06 2.00 1.52 1'92 1.66

339 309

2.01 307 2.80

243

08315 03447 241 08524 0.3585 2.38

2.48 273

1.81

127

H2SO, (solid) 243 H2SO, (solid) 273 H2SO, (liquid) 293 H2SO, (liquid) 323 (SO, (solid) ... 127 SO, (liquid) 1.65 253 0516 0.313 (SO2 (liquid) 273 0532 0317 167 Na2S2O,.5H,O 329 1064 0.569 1.86 Na,HPO,. 12H2O

343

CaCl2.6H2O 340

1492 0.758 1.96 1159 0552 2.10

(NH, (solid) 85 1858 0.5 NH, (liquid) 273 2.678 0.876 3.05

3.72 85

The results in Table VI. are not only interesting but perplexing. We should expect that p would be less in the liquid than in the solid state. Taking ice at 273° as trihydrol, p for AgCl., KNO,, AgNO,, &c., is practically the same at the M.P. in both the liquid and solid states; so is it for H2SO, and SO2. The polymerisation curve for H2SO, &c., is practically continuous on change of state (see Fig. II.). There is a fall in

the value of on melting for K,Cr2O,, KCIO,, H.O, &c. ; but a rise in the cases of P, Bi, Ga, Hg, LINO,(?), AgBr, &c. To make always greater in the solid state than in the liquid state we should have to assume that ice at 273° was (H2O),, or more complex still, for all the values of (in the solid state) are based on (H2O), at 273° (ice).

When is greater in the liquid than in the solid state I should imagine it is for the same reason that for water is greater at 373° than at 273° A. So, solid phosphorus (P, and P1) on melting becomes more associated still, i.e., P2, P1, P ̧, &c. In the liquid state p for NH, is probably 3, so that Nis pentavalent;

TEMP

(To be continued).

2 Sulphuric Acid Wafer Phosphorus

OSMOTIC PHENOMENA.*

By CHARLES A. SHULL, The University of Kentucky, U.S.A.

A THOUGHTFUL consideration of the various relationships which exist between plants and their environment, leads one to the conclusion that none is of gerater importance than the relationship established with the moisture of the soil through the roots and root hair mechanism. The fundamental character of this relation is seen clearly in the facts that soil moisture is the dominant edaphic factor in the geographic distribution of vegetation, and that some of the most striking modifications in the form and structure of plants are produced by physical or physiological dryness of the environment. The deserts of the earth have a sparse vegetation not because of infertility, but from low soil moisture content. And the succulent types of plants found in xerophytic and halophytic habitats have been developed through the physiological conditions set up inside the plant incident to low availability of water.

As has been shown recently by MacDougal and others, the course of carbohydrate metabolism is determined in certain succulents by the amount of water existing in the cells, and the probability is that most succulent plants have been modified by the osmotic conditions in their cells. Whenever the water content exceeds the critical amount, the carbohydrates form polysaccharides of low imbibitional capacity. But if the saturation deficit becomes great enough, the tendency is for the carbohydrates to go over into pentosans which are hydrophilous. In such cases,if in addition there is the production of much free acid during the carbohydrate transformations, succulence will probably result from the combination of circumstances. Thus low availability of soil moisture may fundamentally change the metabolism of the plant, and modify its whole form and structure.

Whether we consider the influence of the plant upon soil moisture, or vice versa, the influence of the quantity of soil moisture upon the plant, these influences must act through the same mechanism,

*A Contribution to a General Discussion on "Physico-Chemical Problems Relating to the Soil" held by the Faraday Society, on Tuesday, May 31, 1921.

the osmotically active membranes of the plant. Usually these are living membranes, but osmotic membranes of cellulose, and dead cells, as in seed coats, are now well known.

When dry seeds with osmotically active coats are placed in moist soil, they absorb moisture with great power, such forces an imbibition, capillarity, surface force, hydration power of colloids, and osmotic pressure from internal salts being involved. The total power of the seed to absorb water is the resultant of all these forces working together. Opposed to these internal forces are external forces which tend to prevent intake of water. The surface forces of soils, capillarity, surface tension, adhesion, cohesion of water thin films, osmotic action of solutes, &c., are pitted against these internal forces; and water movement into or out of the seed will take place in accordance with the lack of equilibrium between the two resultant forces. It is an interesting fact that seeds will actually germinate in a soil that has a moisture content below its wilting coefficient.

in

As soon as the seed begins to germinate, the characteristic osmotic mechanism for soil water intake is established. The view point is general that the intake of water, its transfer through the plant body, and its distribution to the living cells, are processes involving osmotic action at various places in the organism. Although some suggestions have been made assigning to the root a rather passive rôle in water intake, it seems to me a mistake to adopt the idea that the root is mainly an anchorage organ, and that water merely filters through the root under the negative pressure of atmospheric evaporation and the cohesion of water. The osmotic delivery of water by the root to the lower end of the cohesive water columns probably goes on at a rate determined by all of the physical and chemical conditions within the soilroot system, including the cohesive pull upon the root cells. And if this does not keep pace with evaporation, the deficit in the aerial portions becomes greater and greater, and the tension stronger, until breakage of the column and permanent wilting ensues.

There is reason for believing that the drying power of the atmosphere, which fluctuates greatly with changes in relative humidity, averages not far from 1000 atmospheres. And this is the tremendous force which is responsible for the disturbance of moisture equilibrium in the leaves, generating in the cell walls bounding intercellular spaces, in the protoplasm itself, and in the vacuoles of leaf cells, those imbibitional and osmotic forces which lift the water column of the transpiration stream to the tops of even the tallest plants.

The root hair itself is very well adapted to its function as an absorbing organ. As Miss Roberts has shown, the wall of the root hair is usually the outgrowth of the middle lamella of the epidermal cell. It is lined inside by a cellulose layer deposited upon it by the living protoplasm. The external layer, therefore, is largely calcium pectate, and a pectin mucilage brings it into almost intimate contact with the particles of soil minerals. As this pectic material is a hydrophilous colloid, it is highly adapted to imbibe surface moisture from the particles of soil with which it is in contact. It is worth while to note in passing

that the whole problem of water intake and water outgo from the plant begins and ends in imbibition, imbibition of water from the protoplasm and vacuole by the exposed cell walls in leaf interspaces, and imbibition of water from the surface of soil particles by the pectic walls of absorbing root hairs.

There are several main problems connected with the osmotic phenomena of plant life which are worthy of consideration. These are, the nature of osmotic pressure itself, the cause of semipermeability in membranes, and the cause of unilateral movement of water across the semipermeable septum. Before considering the nature of osmotic pressure, the power of the plant to adjust itself osmotically to its environment should be mentioned. It has been fully established by the work of Hill, Drabble, and Drabble, and Miss Roberts, that the plant increases or decreases the osmotic concentration of its cell sap pari passu with changes in the environment which determine the scarcity or availability of water for the plant. Miss Roberts' work is especially significant as related to soil moisture, since she made her determinations on root-hair producing cells. She found that the osmotic pressure of the root hairs is maintained a few atmospheres in excess of that of the surrounding medium. Such changes in the root cells are accompanied by corresponding changes in the osmotic mechanism more or less throughout the entire plant, so that appropriate osmotic gradients are constantly maintained. With plants growing in the soil, a similar gradient for moisture intake exists. At the wilting coefficient the soil withholds water from the plant with a force of about four atmospheres, while the usual osmotic concentration of the sap of root cells of land plants is seven or eight atmospheres. In desert regions, of course, these pressures would run much higher. Under normal field conditions, therefore, the pressure gradient in ordinary mesophytic plants should run from four to eight atmospheres, as the fluctuation of the water withholding power is from zero to four atmospheres. At the wilting coefficient the effectiveness of the intake gradient is lost because the rate of transfer of moisture from soil particle to soil particle toward the plant becomes entirely inadequate. And now, even though the plant's osmotic pressure rises rapidly, it does not succeed in securing the needed moisture, because it is trying, as it were, to draw water from a dry well. Permanent wilting must quickly ensue in most cases after the soil reaches the wilting coefficient.

The main criticism to be made of the current discussions of the nature of osmotic pressure is that they attempt to simplify too much a process that must, like other life processes, be complex. We may attribute osmotic pressure to the difference in free energy of the solvent and solution, or to the kinetic energy of the solute acting on the semipermeable membrane. It seems to me more appropriate to consider osmotic pressure a complex force, the resultant of numerous factors. The free energy of the solvent, the kinetic energy of the solute, the chemical forces of hydration, especially in more concentrated solutions, and the attraction of ions in solution for particles of water charged oppositely while passing through the differential septum, are all factors in producing osmotic pressure. In the plant this complex force

is smallest in the root hairs, and progressively increases to the farthest limits of water transfer.

The most important problems in connection with osmosis centre in the nature and causes of semipermeability in membranes. The sieve theory, the various solubility theories, the surface tension theory, and the hydrone theory are all well known. Every one of them has some supporting evidence, but all of them have their limitations, and prove inadequate to account for the known facts.

Armstrong's hydrone theory has come particularly to my attention in connection with my own work. The assumptions which are made in this theory as to the character of semipermeable membranes and the salts which they eclude from passage are of such a nature that they seem to me to necessitate the fundamental likeness in behaviour of all semipermeable membranes. If only hydronated membranes can exhibit semipermeability, and if hydronated membranes exclude all hydronated or hydrolated salts, then all membranes of this kind should have identical behaviour. The variability in behaviour of the natural semipermeable membranes, then, is a challenge to the hydrone theory.

Again, it is not possible to attribute semipermeability to some particular chemical substance in the membrane, like tannin, or suberin, or cutin, &c. Many different kinds of membranes are semipermeable, and no single substance, or class of substances, is common to them al!. When copper ferrocyanide and other chemical precipitates, cellulose cell walls, or suberised cellulose, gelatin tannate, celloidin, cutinised tissue, parchment paper, and living protoplasm all exhibit semipermeability, it is certain that this peculiar property does not depend on chemical composition, but must be related rather to the structure of the membrane.

might enlarge until the gel is reversed, the discontinuous phase becoming a continuous medium, and the former continuous phase becoming discontinuous.

Changes in permeability and semipermeability would be related to these phase interchanges. Semipermeability would exist when the continuous phase existed in such thin films that molecular diffusion through it was impossible. Adequate provision is found here for natural differences in membranes, for they could hardly have identical behaviour unless the colloidal material were identical in kind. Two different kinds of membranes could exhibit differences in reaction toward the same solution, and one would not expect to find semipermeability behaviour identical in all mem

branes.

It seems clear to me that we must have a theory of semipermeability that recognises the nature of the membrane as Free's theory does. Living protoplasm is an amphoteric substance with an isoelectric point; and it may react as an acid or base depending on hydrogen ion concentration. As it is played upon by various solutes, or by temperature, or light, changes in the permeability or semipermeability relations would undoubtedly occur according to the nature of the solutes, and the effects of other factors on the aggregation in the colloidal system. In non-living membranes like seed coats, the gels are much firmer, more difficult to change, and the characters more stable than in living protoplasm. But even here one might expect changes from permeability toward semipermeability, or vice versa, with reagents having powerful effects on colloidal aggregation. In the problem of soil-moisture intake by roots, of course, we are dealing with a very delicate protoplasmic gel.

Finally, the movement of water through the membrane offers an interesting problem ̧ When the osmotic cell is completely surrounded by the solvent, like an enclosed sack, and if the membrane is elastic, one can think of the pressure

Water can undoubtedly penetrate both phases of the colloidal gel; but salt molecules attempting to penetrate the membrane would probably be prevented from travelling through the disperse phase by surface tension and other surface forces on the contact surfaces between the disperse and continuous phases. If so, they would have to travel in the contiuous phase.

Recently Free has proposed a theory of protoplasmic permeability based upon this conception. He assumes that protoplasm is made up as a twophase system in which the phases differ mainly in their percentage of water content, arranged as colloidal globules dispersed in a colloidal medium. These two phases are supposed to be so related that interchange of water between the two can occur accordingly as changes in the physics or chemistry of metabolism, or of the environment, necessitate. By such changes, the colloidal globules may increase in size while the continuous phase decreases; or, vice versa, the globules may become smaller while the continuous phase separating them increases in thickness at their expense. Conceivably the globules might reach such a size that the continuous phase would remain as the thinnest possible films between them, or they

to include more water, rather than water moving in through the membrane. It is the membrane that moves. But in the living plant osmotic and imbibition forces set up a unilateral movement which has not been satisfactorily explained. The suggestions of Pfeffer as to protoplasmic differences on opposite sides of the cell, and of differences in concentration on opposite sides of the vacuole are too well known to need discussion. Rhythmic pulsations of the osmotically active cells cannot be assumed.

A suggestion which merits attention has recently been made by Loeb. In amphoteric membranes like the protoplasm of root hairs, and of vacuolate cells generally, the opposite sides of the membrane may be oppositely charged. If they were, Loeb points out that positively charged water particles would be driven through the membrane from positive toward negative side, where it would lose its charge after completing passage. The loss of charge allows other positively charged particles to follow; and if a whole series of cells one after the other had such properties, the series would set up a unilateral current of water; in the case of roots let us assume toward the open trachea of the vascular system. At least this is one of the possible factors in addition to osmotic pressure

gradients, and might be responsible in part for the removal of water from the cortical cells into the open tracheæ. Electric exosmose has been suggested to account for the glandular secretion of water from cells of high osmotic concentration into ducts with lower concentrations. It can just as easily be a factor in the secretion of water into the trachea of roots.

The plant and its whole environment may be looked upon as a system in which atmospheric evaporation is the chief disturber of moisture equilibrium. In response to this disturbance, imbibitional and osmotic forces are set up which reach back from leaves to roots through cohesive water columns. These water columns are fed from below by an active root system which delivers its water by osmotic action and possibly electric transfer of water. The water is imbibed by the hydrophile colloids of the root hairs, from the surface of contiguous soil particles, which in turn exert surface tension, surface, and capillary forces to draw moisture from particle to particle toward the plant. This disturbance finally reaches to the ground water surface, which slowly lowers as the result of this upward movement of water during periods of no precipitation, but is maintained by precipitation from the same atmosphere which, when dry, disturbs the water equilibrium

I am fully aware that there are many pointwhich need investigation before an entirely satisfactory picture of the osmotic phenomena of plant life can be drawn. They are very complex pheno mena, perhaps much more complex even than the considerations here brought forward indicate. But with a working hypothesis such as I have briefly outlined, we should be able by appropriate investigations to throw much light on those more of less obscure regions of the problem.

NEW BRITISH CHEMICAL STANDARD STEELS.

(ANALYTICALLY STANDARDISED TURNINGS)

Two new plain carbon steels standards are now read for issue, viz., "M", needed for some time mainly for colour carbon tests round about_023 per cent, and "o 1" which fills the vacancy for a colour carbon standard of about 0'33 per cent, in addition to being available for the other elements shown below.

The analyses have been undertaken as usual by a number of experienced chemists representing the following interests: British Government Department; U.S. Bureau of Standards; Referee Analysts, independent; Railway Analysts, representing users issuing specifications; Works Analysts, representing makers and users. The standard figures are as follows:

The standards may be obtained either direct from Organising Headquarters, 3, Wilson Street, Middlesbrough; or through any of the leading laboratory furnishers at a price just sufficient to cover the cost. A certificate, giving the names of the analysts co-operating, the types of methods used, and a detailed list of their figures, will be supplied with each bottle.

ILLUSTRATION OF MOLECULAR MOTION.*

By J. NORMAN TAYLOR,

Washington Preparatory School, Y.M.C.A, Washington, D.C.

A PROPER appreciation by secondary school students of the "habits of Nature," which are expressed in abstract statements called "laws" because of their unfailing truth, is more readily brought about if concrete examples are given to illustrate their application.

For instance, it is very difficult for an immature mind to appreciate the modern theory of the composition of matter. When it is said that matter is not continuous but is of its own nature discrete, i.e., composed of unit particles known as molecules, it is difficult for the student to grasp the full meaning of this abstract statement. His idea regarding the behaviour of these molecules is also a vague one. It is very hard for him to visualise the movements of particles infinitely small, contained in a transparent vessel, and themselves invisible. If he can perform an experiment which will illustrate to him how molecules move very rapidly in straight lines until they collide with each other or come into contact with the walls of the containing vessel, then he will be able to perceive that molecules are elastic. He will also be able to accept the postulate that the interstices between the molecules must of necessity be larger than the molecules themselves. Furthermore, if a student is enabled, through an appropriate illustration, to understand that when heat is applied to the system the molecules move much more rapidly, then he will be in a much better position to take up the study of the gas laws. He will also be enabled to understand more thoroughly the kinetic theory of gases which is based upon all of these considerations.

A device illustrating the assumed behaviour of molecules, and described by E. R. Stoekel in Science, xlviii., No. 1245, has been employed by the writer in assisting chemistry students to a better understanding of the molecular theory. It consists essentially of a hard glass tube about Ioin. in length and in. in diameter, containing a pool of mercury which supports a small quantity of finely crushed material. In using this form of apparatus in the Association School laboratory it has been found that particles of cobalt glass of twenty mesh are very satisfactory. After preparing the tube, as here indicated, it is evacuated so that a pressure of less than a millimeter obtains and is then sealed from the pump. During the subsequent demonstration the tube may be held in the hand without inconvenience, but may, if desired, be clamped to a standard.

Upon the application of heat to the mercury

*Reprinted from June, 1920, issue School Science and Mathematics' XX, No. 6.