Page images
PDF
EPUB
[graphic]

and there are additional figures showing structural details. It is a very striking indication of the modern tendency towards small genera that the 47 species are placed in 39 genera; one genus (Solaster) has 3 species (but 1 of these is usually considered generically distinct!) and half a dozen genera have 2 representatives each-the remaining 32 genera have in the British area but 1 species each so far as at present known.

In dealing with the brittle-stars, Dr. Mortensen is again faced with the problem of an unsatisfactory classification, and, in the opinion of the present writer, treats it in an unsatisfactory way. In rejecting Matsumoto's classification, Dr. Mortensen returns to the old arrangement of the ophiurans in two orders, a distinctly backward step and quite unnecessary. There is no need of rejecting all of Matsumoto's work, much of it of very great value, merely because his first order, the Phrynophiurida1 seems to be an unnatural assemblage. Probably we shall have to recognize 5, and possibly 6, orders, when we more perfectly understand the problem and have the necessary data. Mortensen finds it more difficult to make a satisfactory key to the 11 families of British (or potentially British) ophiurans, which he puts in the old heterogeneous order Ophiurae, than he does in the case of any other group, but he succeeds admirably in spite of the inherent obstacles. No fewer than 141 species of ophiurans, grouped in 48 genera, are indicated as potentially British but only 50 species are actually known from the area, as yet, and of these only about a dozen are found in shallow water. The bulk of this section of the book therefore, deals with forms, the average zoologist, even though a frequenter of marine laboratories, is never likely to see. Particular attention is paid to the larval forms and a key to the known larvae of British species is interesting and of real value. The discussion of the ecology of brittle-stars and of their parasites is particularly good.

In the handling of the echini, Dr. Mortensen is dealing with the group of which he is preeminently the master and this section is therefore, of great interest. The account of the morphology is clearly written and on the whole satisfactory, but in discussing the "lantern," the perignathic girdle of the test, with which it is intimately associated, is slighted and the important distinction between auricles and apophyses is ignored. No reference is made to the absence of the "compasses" in the "lantern" of clypeastroids. There is a little confusion about the

1 Dr. Mortensen wrongly calls it Phrynophiurae and uses the same erroneous termination for the other three orders.

use of the term "irregular echini" for while in the key to orders, the "Irregularia" are made to include the clypeastroids, elsewhere statements are made which indicate that Dr. Mortensen had the spatangoids only in mind. Thus (p. 262) the posterior gonad is said to have disappeared in the "irregular echinoids" whereas it is present in a large number of clypeastroids. The paragraphs concerning the larval forms and the key to those known from British seas are particularly important and useful. The classification used is open to little criticism and has the great merit of being simple and yet adequate. There are 33 genera and 53 species included in the keys but only 21 genera and 33 species are actually known from Great Britain. The illustrations in this section of the book are worthy of special praise. An interesting side-light on Dr. Mortensen's attitude towards rules of nomenclature is shown by a footnote on p. 321, in which he objects to quoting the name of the first describer of Aëropsis rostrata as authority for the species because it will deprive another more eminent authority of "the honour"!

The introductory pages to the section on holothurians are particularly good reading and give a very clear account of the class. The classification used includes results from some very recent researches and the keys are as good as can be prepared for a group so difficult of satisfactory preservation. The assistance of Dr. Elizabeth Deichmann in the preparation of many of these keys is cordially acknowledged. Some 44 genera and 116 species are regarded as potentially British but only 14 genera and 30 species are actually known as yet from the area, so that here as among the ophiurans, the book deals with a preponderance of forms which the average zoologist, even though he live at a marine laboratory, will never see. This is of course not a defect; it simply emphasizes the extraordinary scope of the book. Naturally the illustrations of holothurians are not as numerous or attractive as those in the other classes, but they are well-chosen and satisfactory.

The book concludes with a brief appendix, 8 pages of bibliography, a list of abbreviations used for authors' names, and no fewer than 5 admirable and very useful indexes. It is difficult to conceive of more satisfactory indexing. From any point of view the volume is a credit to those responsible for it and the Oxford Press, Professor J. Stanley Gardiner, who induced Dr. Mortensen to undertake the work, and the author himself are to be heartily congratulated. It unquestionably adds new honors and prestige to the record of the eminent Danish zoologist. HUBERT LYMAN CLARK

MUSEUM OF COMPARATIVE ZOOLOGY,
HARVARD UNIVERSITY

[merged small][merged small][merged small][ocr errors][merged small]
[graphic]
[graphic]
[ocr errors]
[ocr errors]

IONIZATION BY POSITIVE IONS

3

THE question as to whether positive ions can ionize atoms has been the subject of much controversy. Experimentally no direct evidence exists which will enable one to decide between the two apparently opposing views held. If one regards the phenomenon from the point of view of the ionization of an atom by a moving charged particle in virtue of the action of the charge on the electron of the neutral atom, one must agree with J. J. Thomson1 that it is unlikely that on this mechanism ions with less than a few thousand volts equivalent of energy can ionize. On the other hand, as J. Franck2 points out, and as is indicated by certain phenomena in ionization by slow canal rays, in the discharge of electricity from positively charged points, and from the temperature ionization observed by King, and also by Noyes and "Wilson, we have definite evidence of the ionization of gases by impact between charged or uncharged atomic masses moving with velocities corresponding to only two or three times the ionizing energy. These two apparently contradictory5.1.2 views, together with the conflicting experimental evidence put forward in an attempt to decide the question, have led to a good * deal of confusion. In part this has been clarified by Joos and Kulenkampff30 but not completely. In a seminar course the writer recently had the opportunity of reviewing the literature on the subject and with the benefit of the criticisms of his two colleagues, Professor R. B. Brode and Dr. Arthur von Hippel, believes that he has been able to clarify the situation still more and that he has been able to show that there is no real contradiction in the two views.

[ocr errors]

4

It is the purpose of this article to briefly set forth these conclusions. To do this we may regard three distinct processes. They are:

1. The ionization by rapidly moving charged particles, e.g., electrons, protons, and doubly charged He

atoms.*

2. The ionization of molecules of a gas of lower or equal ionizing potential by ionized atoms or molecules which may be in motion or at rest (e.g., an exchange of charge).

1 Thomson, J. J., Phil. Mag. 48, 1, 1924; and also 23, 454, 1912.

2 Franck, J., Zeits. f. Phys. 25, 312, 1924; Handbuch der Physik. Vol. 23, p. 731.

3 King, A. S., Astrophys. J. 48, 13, 1918, and many other papers in this journal.

4 Noyes and Wilson, Astrophys. J. 57, 20, 1923.

5 Rüchardt, E., Handbuch der Physik, Vol. 24, p. 99. Singly charged He atoms and all rapidly moving #charged particles should be included here. As, however, these carriers also can act to ionize in other ways which confuse the issue, and in order to emphasize the mechanism of the process, these have been purposely left out as typical examples of this class.

[ocr errors]

3. The ionization resulting from the impact of atoms or molecules, charged or neutral, which possess electrons and which have an energy which is a small multiple of the ionizing energy of one of the atoms. 1. The first class of ionizing processes is characterized and governed by the classical laws of electrodynamics and the laws of momentum and energy®,7 with the limitation that energy transfer to an electron of an atom acted on must follow the quantum conditions, 8.9.10 (i.e., energy can only be absorbed if the electron in question receives a quantum of energy demanded by its change in status). The applications of these laws to the fast electron, the proton and the alpha particle, have been adequately proven by the agreement in order of magnitude between the predicted results and observations. That is, the ionization by electrons (Whiddington's7.11 law and possibly even the application to ionization for slower electrons, though the latter is more doubtful), by protons12 and by a particles as calculated by Henderson,8 Fowler,9 , Bohr and others, agrees within a factor of two or three with the observations. These laws, as Thomson1 points out, demand that ionization by such particles in virtue of the action of the charge ceases at velocities easily computed from laws of momentum and energy, corresponding to values of the order 5 x 10-1 × 108 cm/sec. Such velocities correspond to energies of the order of the ionizing potentials for atoms in the case of electrons, to energies of the order of 2,000-3,000 volts for protons, and to energies of the order of 10,000 volts for alpha particles. The efficiency of this type of ionizing action is very high and is more or less successfully predicted from classical theory, assuming the ionization potentials as observed to be correct. 7,8,9,10 The conclusions are substantiated by the sudden cessation of ionization by a particles and protons13 at the end of their range observed, and by the recent work of Dempster14,5 on the long free paths of protons of 900 volts velocity. It is the only process by which such single charges can produce ionization.

2. The second class of phenomena belong, properly speaking, in that class of phenomena called "inelastic

6 Thomson, J. J., Phil. Mag. 23, 449, 1912, “Conduction of Electricity through Gases," pp. 370-382. 7 Bohr, N., Phil. Mag. 25, 10, 1913, and also 30, 581, 1915.

8 Henderson, G. A., Phil. Mag. 44, 680, 1922.

9 Fowler, R. H., Proc. Camb. Phil. Soc. 21, 521, 1923. 10 Loeb and Condon, Jr. Frank. Inst. 200, 595, 1925. 11 Whiddington, R., Proc. Roy. Soc. 85A, 323, 1911; also 86A, 360, 1912.

12 Dempster, A. J., Phys. Rev. 8, 656, 1916.

13 Baerwald, H., Ann. der Phys. 65, 167, 1921.

14 Dempster, A. J., Proc. Nat. Acad. Sci. 11, 552, 1925; also 12, 96, 1926; Phil. Mag. 3, 115, 1927.

impacts of the second class," first discovered experimentally by Franck 15 and Cario, and later observed directly by Erikson16 and Harnwell.1?

They explain many of the phenomena observed by Dempster12 on canal rays of low velocity. They occur fairly readily, and are largely independent of the velocity of the carrier. It is, however, possible that through the third class of ionizing phenomena the energy of motion could be utilized to make this group include ionization by moving ions of appropriate velocity of molecules of higher ionizing potential. To date, however, no certain evidence exists for this extension, though from indirect observation it seems probable. This process obviously can not lead to the production of a very much larger number of charged carriers than the initial number of charged carriers. Thus in a great many problems of ionization by means of charged particles their importance is secondary.

3. The third class of processes are definitely established by the existence of temperature ionization observed by Kings and treated theoretically exhaustively by Eggert,18 by Saha,19 and Fowler.20 Even if some of the assumed quantities (e.g., the energies of the atoms necessary for ionization in such a process) in the equations turn out to be in error by a factor of two or three, the correctness of the deduction is unquestioned. As regards other evidence for ionization of gas molecules by positive ions, or moving neutral molecules of relatively low velocities, the evidence is less clear if one exclude occurrences of the type of Class 2 above.12 The evidence from direct measurement on positive rays has been seriously questioned by Horton and Davies21 and by Hooper,22 due to the fact that secondary emission of electrons from the walls through the positive ion bombardment and photoelectric phenomena were not rigorously excluded. The work of Baerwald and others23 on emission of secondary electrons from metals by positive ion bombardment upholds this. Hooper concludes that if ionization of a gas by positive ions below 1,000 volts occurs, the process is inefficient. He very believes that at high pressures (where many collisions can take place and the ionization could be observed for inefficient agencies) there is some evidence that it occurs in his experiments. The evidence from the experiments on ionization phenomena in gases and 15 Franck and Cario, Zeits. für Phys. 11, 3, 1922. 16 Erikson, H. A., Phys. Rev. 28, 372, 1926. 17 Harnwell, G. P., Phys. Rev. 29, 830, 1927. 18 Eggert, J., Phys. Zeits. 20, 570, 1919.

19 Saha, M. N., Phil. Mag. 40, 478, 1920.

20 Fowler, R. H., Phil. Mag. 45, 1, 1923.

21 Horton and Davies, Proc. Roy. Soc. 95A, 333, 1919. 22 Hooper, W. J., Jr. Frank Inst. 201, 311, 1926.

23 Rüchardt, E., Handbuch der Physik, Vol. 24, p. 105.

sparking potentials in fairly uniform fields, as interpreted by Townsend, 24 has recently been seriously called into question 25,26 on the basis of the probable actions of positive ions or radiation on the cathode. Townsend27 himself agrees that such processes would fit his equations as well as experimental uncertainty admits. He however points out that only by assuming ionization by positive ions of low velocity in a gas can we explain the discharge from positive points at high potentials. 28, 29 In this assertion he is undoubtedly correct if we add the possibility that such ionization may be in part indirect as later described. There is thus evidence that neutral atoms, molecules, canal rays, and slowly moving positive rays directly or indirectly can ionize gas molecules by impact, though the efficiency of the process is obviously very low. This type of activity is, however, essentially different from that under Class 1 in that it is independent of the charged state of the ionizing atom or molecule, so that the charge is but incidental to the mechanism. The process, however, depends on one additional feature. Every atom ionizing in this fashion must have at least one electron in an orbit about it, and possibly more.

It is in fact the presence of the electrons in these ionizing systems that enables them to produce ionization independently of charge, and thus give a mechanism which can be clearly differentiated from the first class of ionization. With electrons in each of the atoms or systems colliding, transfers of energy between the electrons of the two systems again become possible at low velocities. However, it is difficult to postulate the exact mechanism of such transfers, in which the relative energies of two atoms are transferred to one or two of their electrons in a molecular impact. To date the new quantum mechanics has been unable to cope with the problem. The earlier discussions of Franck, and Joos and Kulenkampff treated the atoms as elastic spheres. If one could conceive of the electrons being rigidly held in stationary positions by the binding forces of the nucleus, interactions of the observed sort might be expected. Such an assumption enables one to find a plausible explanation for inefficiency of the process; for it would be a relatively rare atomic encounter that brought two electrons of the colliding atoms into such 24 Townsend, J. S., "Electricity in Gases," Chap. IX, p. 322.

1

[merged small][merged small][merged small][ocr errors]

T

a relation that the energy of atomic motion was concentrated on one electron and thus made possible its escape. However, the electrons are more probably in orbits in the atoms and the flexibility of this type of binding, coupled with the experimentally observed fact that the orbital momentum of electrons in atoms is not manifested in ionization processes, 31 makes it difficult, if not impossible, to explain the facts in a simple mechanical fashion. We can only conclude that there exists a mechanism in atoms which in rare collisions by means of the interactions of the electrons in the atoms enables the relative energy of the atoms to be transferred to one of the electrons.

The presence of a positive charge on one of the two colliding atoms at low velocities should affect the ionization by such a mechanism but slightly. As Franck1 has stated it increases the energy necessary to cause ionization, as with the charged atom the electron must escape against an attractive charge of two units instead of one. Besides this minor influence the charge plays an important indirect rôle, in low velocity phenomena, in that it enables a molecule or atom to acquire its ionizing energy from an electrical field, an energy which it otherwise would practically never acquire at room temperatures as a result of the heat motions. Such an atom or molecule having acquired the energy through its charge is then able to ionize molecules itself, or perhaps is able by impact to impart its energy to a neutral molecule which can ionize slightly more effectively. In any case whatever its manner of producing ions, the function of the charge is but indirect enabling the ion to acquire energy. It has little to do with the subsequent mech"anism of removal of electrons by the ion, thus clearly differentiating its ionizing mechanism from that of swiftly moving charged particles.

[ocr errors]

It is also conceivable that one ion may ionize by any two or even all three mechanisms simultaneously, although at high speeds the preponderating mechanism for an ion with electrons will be processes of Class 1, while as it slows down the processes of Class 2 and 3 will entirely predominate. At intermediate speeds probably all mechanisms are active and thus lead to some of the apparently contradictory results obtained.

We thus see that in terms of the three different mechanisms, the outstanding conflicting observations can be simply explained and it is seen that there is no essential contradiction even between the extreme

views of Thomson and Franck; for we have seen that neither a proton nor a doubly charged helium atom can ionize below certain minimum velocities as classi

cal theory demands that they should not, while hydrogen atoms, singly charged helium atoms and neutral

81 Watson, E. C., Phys. Rev. 30, 479, 1927.

[blocks in formation]

IN studying the distribution and the dominance of pasture plants, it was observed that there is a definite correlation between the dominance of certain pasture plants and the natural or native vegetation. An attempt was made to correlate the growth and dominance of the various plants with the soil acidity, but it was soon found that there is no very close correlation between acidity of the soil and the dominance of certain types of pasture plants.

Since no very definite correlation was found between the acidity of the soil and the growth and dominance of certain plants, an attempt was made to correlate plant growth with plant residues, particularly the basic nitrogenous materials, including ammonia, amines, etc., and here again only a partial correlation was found between the availability of the basic nitrogenous organic residues and plant growth.

The nitrogen carbon ratio in the organic residues probably affects the mobility of the nitrogen in the soil. There is a difference in the nitrogen carbon ratio in various plant residues. The difference in the nitrogen carbon ratio in peat soils illustrates the points in question. It has been found that some peat soils have

a nitrogen carbon ratio as narrow as 1: 8, while others have a ratio as wide as 1: 70 or wider. This difference in nitrogen carbon ratio undoubtedly affects the availability of anionic nitrogen. It has been found that there is a close correlation between the calcium oxide content and the width of the nitrogen carbon ratio in peat soils. Where the nitrogen carbon ratio is narrow, it indicates that there is a relatively large amount of high oxidation potential mineral basic material present. And in such a situation it has been found that there is often an accumulation of toxic amounts of nitrates. But when the nitrogen carbon ratio is wide it indicates that there is a limited amount of high oxidation potential mineral basic material present. And where such a condition prevails it may result in a prolonged nitrogen starvation period, especially early in the growing season. Where the nitrogen carbon ratio is very wide such plants as some of the conifers, poverty grass (Danthonia spicata), certain species of Agrostis, etc., which may readily utilize cationic nitrogen, are apt to dominate in nature. Other plants, such as certain species of oak, hickory, poa, etc., seem

[graphic]

to grow best when supplied with anionic nitrogen. It is not possible at present to say whether anionic nitrogen determines the growth response of plants or whether it is the nutritional complex commonly associated with available anionic nitrogen. Wide nitrogen carbon ratios would very probably have much less effect upon the mobility of the cationic nitrogen from organic residues. The basic nitrogenous materials from such residues undoubtedly function similarly to mineral bases in the soil colloidal complex. These organic bases may partially satisfy the basic needs of the soil colloids, but the oxidation potential of such materials is apparently not sufficient to produce optimum growth of many plants. The low oxidation potential of basic organic materials may partially account for the lack of close correlation between the hydrogen-ion concentration of a soil solution and plant growth. The desirable crop sequence in rotations and the succession of native plants on abandoned crop land, as well as the succession of plants on virgin soil, is probably closely correlated with the ability of various plants to utilize cationic nitrogen or nonionized nitrogenous materials. Basic material is very often the limiting factor in many of our depleted soils. Nature has an abundant potential supply of basic material in the nitrogen of the atmosphere, but apparently many plants can not readily utilize low oxidation potential cationic nitrogenous materials.

After failing to find sufficient correlation between the acidity of the soil and the availability of the basic nitrogenous organic residues to account for the difference in plant growth and associations, an attempt was made to correlate plant nutrition with the electromotive series and oxidation potentials. As life is probably dependent upon a difference in electrical potential it was believed that the electromotive series and oxidation potentials, which are the best single expressions of the properties of ions, would correlate with plant growth. The entire chemical activity of the metals corresponds fairly closely with the above series. Here we found a very striking correlation between the electromotive series and the absorption of plant nutrients. Indeed the electromotive series may be the key to many of the perplexing problems in plant and animal physiology. The various ions differ very much in the voltage they produce. Such ions as K, Na and Ca produce high voltages, other ions, such as Mg, Al, Mn, NH, amines and other basic nitrogenous materials, produce medium voltages, while still other ions, such as Fe, H. As, Cu, Hg, etc., produce very low voltages. Various plants and animals apparently tolerate different potential levels. Many crop plants, such as alfalfa, sweet clover, celery, barley, millets, asparagus, beets, etc., seem to be tolerant of very high electrical potentials as, for example, the

potentials produced by high concentration of such ions as K, Na and Ca, often encountered in semi-arid to arid climates. Other plants, such as blackberry, blueberry, cranberry, raspberry, strawberry, oats, buckwheat, red top, cotton, sweet potatoes, watermelon, etc., grow well at relatively low potential levels as, for example, the potentials produced by high concentration of such ions as Mg, Al, Mn, NH,, amines, protein acid salt ions, Fe, H. etc. It is evident that a given H-ion or OH-ion concentration resulting from the presence of various acidic or basic materials may produce different oxidation potential levels or physiological gradients. The gradients produced by such high potential materials as K, Na, Ca, etc., would be different from the gradients produced by such low potential materials as NH,, amines, etc. Hence a close correlation between H-ion concentration and plant growth could not be expected.

Cropping may deplete various soil types until they reach approximately the same biological fertility level as, for example, the fertility level suited for the dominance of pine, etc. Since the accumulation of basic nitrogenous materials is one of the important factors in the natural restoration of the productivity of soils, it is evident that the climax vegetation on different soils would be different, depending upon the ability of the soil colloidal complex to retain organic bases. Mass action resulting from the accumulation of organic bases may make available mineral bases that have a higher oxidation potential. The capacity of the soil colloidal complex to retain the organic bases may partially determine the climax vegetation. The above condition is probably one of the important factors controlling the more or less definite plant successions in the depletion and the restoration processes of various soil types. Therefore, certain soil types can not be restored above the pine fertility level while others may be restored to levels suited to the various hard woods.

It has been possible to trace the influence of the oxidation potential levels from plutonic magmas to the various igneous rock, thence to the soil colloids, and finally through the plant to the animal. The ash of certain plants, such as alfalfa, sweet clover, foxtail millets, etc., grown in semi-arid to arid climates may contain very large amounts of potash. Ionic potassium may produce a very high voltage and it is, therefore, very readily absorbed by plants. Under certain conditions the rapid absorption of the potassium or similar ions may exclude or limit the absorption of other desirable nutrient cations. The feeding of large quantities of plants with high potash content may seriously affect animals. Another striking example of the effect of a high oxidation potential material is the probable correlation between high potash content in

[graphic]
« PreviousContinue »