Scientific Culture, and Other Essays Part 6

In concluding, we must not forget to mention that most genial trait of Graham's character, his sympathy with young men, which gave him great influence as a teacher in the college with which he was long a.s.sociated.

There are many now prominent in the scientific world who have found in his encouragement the strongest incentive to perseverance, and in his approval and friendship the best reward of success.

VI.

MEMOIR OF WILLIAM HALLOWES MILLER.

_Reprinted from the "Proceedings of the American Academy of Arts and Sciences," Vol. XVI, May 24, 1881._

William Hallowes Miller, who was elected Foreign Honorary Member of this Academy in the place of C. F. Naumann, May 26, 1874, died at his residence in Cambridge, England, on the 20th of May, 1880, at the age of seventy-nine, having been born at Velindre, in Wales, April 5, 1801. His life was singularly uneventful, even for a scholar. Graduating with mathematical honors at Cambridge in 1826, he became a fellow of his college (St. John's) in 1829, and was elected Professor of Mineralogy in the University in 1832. Under the influence of the calm and elegant a.s.sociations of this ancient English university, Miller pa.s.sed a long and tranquil life--crowded with useful labors, honored by the respect and love of his a.s.sociates, and blessed by congenial family ties. This quiet student-life was exactly suited to his nature, which shunned the bustle and unrest of our modern world. For relaxation, even, he loved to seek the retired valleys of the Eastern Alps; and the description which he once gave to the writer, of himself sitting at the side of his wife amid the grand scenery, intent on developing crystallographic formulae, while the accomplished artist traced the magnificent outlines of the Dolomite mountains, was a beautiful idyl of science.

Miller's activities, however, were not confined to the University. In 1838 he became a Fellow of the Royal Society, and in 1856 he was appointed its Foreign Secretary--a post for which he was eminently fitted, and which he filled for many years. In 1843 he was selected one of a committee to superintend the construction of the new Parliamentary standards of length and weight, to replace those which had been lost in the fire which consumed the Houses of Parliament in 1834, and to Professor Miller was confided the construction of the new standard of weight. His work on this important committee, described in an extended paper published in the "Philosophical Transactions" for 1856, was a model of conscientious investigation and scientific accuracy. Professor Miller was subsequently a member of a new Royal Commission for "examining into and reporting on the state of the secondary standards, and for considering every question which could affect the primary, secondary, and local standards"; and in 1870 he was appointed a member of the "Commission Internationale du Metre." His services on this commission were of great value, and it has been said that "there was no member whose opinions had greater weight in influencing a decision upon any intricate and delicate question."

Valuable, however, as were Professor Miller's public services on these various commissions, his chief work was at the University. His teacher, Dr. William Whewell--afterward the Master of Trinity College--was his immediate predecessor in the Professorship of Mineralogy at Cambridge.

This great scholar, whose encyclopaedic mind could not long be confined in so narrow a field, held the professorship only four years; but during this period he devoted himself with his usual enthusiasm to the study of crystallography, and he accomplished a most important work in attracting to the same study young Miller, who brought his mathematical training to its elucidation. It was the privilege of Professor Miller to accomplish a unique work, for the like of which a more advanced science, with its multiplicity of details, will offer few opportunities.

The foundations of crystallography had been laid long before Miller's time. Hauy is usually regarded as the founder of the science; for he first discovered the importance of cleavage, and cla.s.sed the known facts under a definite system. Taking cleavage as his guide, and a.s.suming that the forms of cleavage were not only the _primitive forms_ of crystals as a whole, but also the forms of their _integrant molecules_, he endeavored to show that all secondary forms might be derived from a few primary forms, regarded as elements of nature, by means of _decrements_ of molecules at their edges. In like manner he showed that all the forms of a given mineral, like fluor-spar or calcite, might be built up from the integrant molecules by skillfully placing together the primitive forms. Hauy's dissection of crystals, in a manner which appeared to lead to their ultimate crystalline elements, gained for his system great popular attention and applause. The system was developed with great perspicuity and completeness in a work remarkable for the vivacity of its style and the felicity of its ill.u.s.tration. Moreover, a simple mathematical expression was given to the system, and the notation which Hauy invented to express the relation of the secondary to the primary forms, as modified and improved by Levy, is still used by the French mineralogists.

The system of Hauy, however, was highly artificial, and only prepared the way for a simpler and more general expression of the facts. The German crystallographer, Weiss, seems to be the first to have recognized the truth that the decrements of Hauy were merely a mechanical mode of representing the fact that all the secondary faces of a crystal make intercepts on the edges of the primitive form which are simple multiples of each other; and, this general conception once gained, it was soon seen that these ratios could be as simply measured on the axes of symmetry of the crystal as on the edges of the fundamental forms; and, moreover, that, when crystal forms are viewed in their relation to these axes, a more general law becomes evident, and the artificial distinction between primary and secondary forms disappears.

Thus became slowly evolved the conception of a crystal as a group of similar planes symmetrically disposed around certain definite and obvious systems of axes, and so placed that the intercepts, or parameters, on these axes bore to each other a simple numerical ratio.

Representing by _a_:_b_:_c_ the ratio of the intercepts of a plane on the three axes of a crystal of a given substance, then the intercepts of every other plane of this, or of any other crystal of the same substance, conform to the general proportion _m__a_:_n__b_:_p__c_, in which _m_, _n_, _p_ are three simple whole numbers. This simple notation, devised by Weiss, expressed the fundamental law of crystallography; and the conception of a crystal as a system of planes, symmetrically distributed according to this law, was a great advance beyond the decrements of Hauy, an advance not unlike that of astronomy from the system of vortices to the law of gravitation. Yet, as the mechanism of vortices was a natural prelude to the law of Newton, so the decrements of Hauy prepared the way for the wider views of the German crystallographers.

Whether Weiss or Mohs contributed most to advance crystallography to its more philosophical stage, it is not important here to inquire. Each of these eminent scholars did an important work in developing and diffusing the larger ideas, and in showing by their investigations that the facts of nature corresponded to the new conceptions. But to Carl Friedrich Naumann, Professor at the time in the "Bergakademie zu Freiberg,"

belongs the merit of first developing a complete system of theoretical crystallography based on the laws of symmetry and axial ratios. His "Lehrbuch der reinen und angewandten Krystallographie," published in two volumes at Leipzig in 1830, was a remarkable production, and seemed to grasp the whole theory of the external forms of crystals. Naumann used the obvious and direct methods of a.n.a.lytical geometry to express the quant.i.tative relations between the parts of a crystal; and, although his methods are often unnecessarily prolix and his notation awkward, his formulae are well adapted to calculation, and easily intelligible to persons moderately disciplined in mathematics.

But, however comprehensive and perfect in its details, the system of Naumann was c.u.mbrous, and lacked elegance of mathematical form. This arose chiefly from the fact that the old methods of a.n.a.lytical geometry were unsuited to the problems of crystallography; but it resulted also from a habit of the German mind to dwell on details and give importance to systems of cla.s.sification. To Naumann the six crystalline systems were as much realities of nature as were the forms of the integrant molecules to Hauy, and he failed to grasp the larger thought which includes all partial systems in one comprehensive plan.

Our late colleague, Professor Miller, on the other hand, had that power of mathematical generalization which enabled him to properly subordinate the parts to the whole, and to develop a system of mathematical crystallography of such simplicity and beauty of form that it leaves little to be desired. This was the great work of his life, and a work worthy of the university which had produced the "Principia." It was published in 1839, under the t.i.tle, "A Treatise on Crystallography"; and in 1863 the substance of the work was reproduced in a more perfect form, still more condensed and generalized, in a thin volume of only eighty-six pages, which the author modestly called, "A Tract on Crystallography."

Miller began his study of crystallography with the same materials as Naumann; but, in addition, he adopted the beautiful method of Franz Ernst Neumann of referring the faces of a crystal to the surface of a circ.u.mscribed sphere by means of radii drawn perpendicular to the faces.

The points where the radii meet the spherical surface are the poles of the faces, and the arcs of great circles connecting these poles may obviously be used as a measure of the angles between the crystal faces.

This invention of Neumann's was the germ of Miller's system of crystallography, for it enabled the English mathematician to apply the elegant and compendious methods of spherical trigonometry to the solution of crystallographic problems; and Professor Miller always expressed his great indebtedness to Neumann, not only for this simple mode of defining the position of the faces of a crystal, but also for his method of representing the relative position of the poles of the faces on a plane surface by a beautiful application of the methods of stereo-graphic and gnomonic projection. This method of representing a crystal shows very clearly the relations of the parts, and was undoubtedly of great aid to Miller in a.s.sisting him to generalize his deductions.

From the outset, Professor Miller apprehended more clearly than any previous writer the all-embracing scope of the great law of crystallography. He opens his treatise with its enunciation, and, from this law as the fundamental principle of the subject, the whole of his system of crystallography is logically developed. Beyond this, all that is peculiar to Miller's system is involved in two or three general theorems. The rest of his treatise consists of deductions from these principles and their application to particular cases.

One of the most important of these principles, and one which in the treatise is involved in the enunciation of the fundamental law of crystallography, is in its essence nothing but an a.n.a.lytical device.

As we have already stated, Weiss had shown that, if _a_:_b_:_c_ represent the ratio of the intercepts of any plane of a crystal on the three axes _x_, _y_, and _z_, respectively, the intercepts of any other possible plane must satisfy the proportion--

_A_:_B_:_C_ = _m__a_:_n__b_:_p__c_,

in which _m_, _n_, and _p_ are simple whole numbers. The irrational values _a_, _b_, and _c_ are fundamental magnitudes for every crystalline substance;[G] and Miller called these relative magnitudes the parameters of the crystals, while he called the whole numbers, _m_, _n_, and _p_, the indices of the respective planes. But, instead of writing the proportion which expresses the law of crystallography as above, he gave to it a slightly different form, thus:

_A_:_B_:_C_ = (1/_h_)_a_:(1/_k_)_b_:(1/_l_)_c_,

and used in his system for the indices of a plane the values _h_:_k_:_l_, which are also in the ratio of whole numbers, and usually of simpler whole numbers than _m_:_n_:_p_. This seems a small difference; for _h_ _k_ _l_ in the last proportion are obviously the reciprocals of _m_ _n_ _p_ in the first; but the difference, small as it is, causes a wonderful simplification of the formulae which express the relations between the parts of a crystal. From the last proportion we derive at once

(1/_h_)(_a_/_A_) = (1/_k_)(_b_/_B_) = (1/_l_)(_c_/_C_),

which is the form in which Miller stated his fundamental law.

[G] For example, the native crystals of sulphur have _a_:_b_:_c_ = 1:2340:1233.

Crystals of gypsum have _a_:_b_:_c_ = 1:0413:0691.

Crystals of tin-stone have _a_:_b_:_c_ = 1:1:06724.

And crystals of common salt have _a_:_b_:_c_ = 1:1:1.

If _P_ represents the "pole" of a face whose "indices" are _h_ _k_ _l_, that is, represents the point where the radius drawn normal to the face meets the surface of the sphere circ.u.mscribed around the crystal (the sphere of projection, as it is called), and if _X_, _Y_, _Z_ represent the points where the axes of the crystal meet the same spherical surface,[H] then it is evident that _X Y_, _X Z_, and _Y Z_ are the arcs of great circles, which measure the inclination of the axes to each other, and that _P X_, _P Y_, and _P Z_ are arcs of other great circles, which measure the inclination of the plane (_h_ _k_ _l_) on planes normal to the respective axes; and, also, that these several arcs form the sides of spherical triangles thus drawn on the sphere of projection.

Now, it is very easily shown that

(_a_/_h_)cos _P X_ = (_b_/_k_)cos _P Y_ = (_c_/_l_)cos _P Z_;

and by means of this theorem we are able to reduce a great many problems of crystallography to the solution of spherical triangles.

[H] The origin of the axes is always taken as the center of the sphere of projection.

Another very large cla.s.s of problems in crystallography is based on the relation of faces in a zone; that is, of faces which are all parallel to one line called the zone axis, and whose mutual intersections, therefore, are all parallel to each other. If, now, _h_ _k_ _l_ and _p_ _q_ _r_ are the indices of any two planes of a zone (not parallel to each other), any other plane in the same zone must fulfill the condition expressed by the simple equation

u_u_ + v_v_ + w_w_ = _o_,

where _u_ _v_ and _w_ are the indices of the third plane, and u v w have the values

u = _k__r_ - _l__q_ v = _l__p_ - _h__r_ w = _h__q_ - _k__p_.

Since _h_ _k_ _l_ and _p_ _q_ _r_ are whole numbers, it is evident that u v w must also be whole numbers, and these quant.i.ties are called the indices of the zone. The three whole numbers which are the indices of a plane when written in succession serve as a very convenient symbol of that plane, and represent to the crystallographer all its relations; and in like manner Miller used the indices of a zone inclosed in brackets as the symbol of that zone. Thus 123, 531, 010 are symbols of planes, and [111], [213], [001] symbols of zones.

An additional theorem enables us to calculate the symbols of a fourth plane in a zone when the angular distances between the four planes and the symbols of three of them are known, but this problem can not be made intelligible with a few words.

The few propositions to which we have referred involve all that is essential and peculiar to the system of Professor Miller. These given, and the rest could be at once developed by any scholar who was familiar with the facts of crystallography; and the circ.u.mstance that its essential features can be so briefly stated is sufficient to show how exceedingly simple the system is. At the same time, it is wonderfully comprehensive, and the student who has mastered it feels that it presents to him in one grand view the entire scheme of crystal forms, and that it greatly helps him to comprehend the scheme as a whole, and not simply as the sum of certain distinct parts. So felt Professor Miller himself; and, while he regarded the six systems of crystals of the German crystallographers as natural divisions of the field, he considered that they were bounded by artificial lines which have no deeper significance than the boundary lines on a map. How great the unfolding of the science from Hauy to Miller, and yet now we can see the great fundamental ideas shining through the obscurity from the first!

What we now call the parameters of a crystal were to Hauy the fundamental dimensions of his "integrant molecules," our indices were his "decrements," and our conceptions of symmetry his "fundamental forms." There has been nothing peculiar, however, in the growth of crystallography. This growth has followed the usual order of science, and here as elsewhere the early, gross, material conceptions have been the stepping-stones by which men rose to higher things. In sciences like chemistry, which are obviously still in the earlier stages of their development, it would be well if students would bear in mind this truth of history, and not attach undue importance to structural formulae and similar mechanical devices, which, although useful for aiding the memory, are simply hindrances to progress as soon as the necessity of such a.s.sistance is pa.s.sed. And, when the life of a great master of science has ended, it is well to look back over the road he has traveled, and, while we take courage in his success, consider well the lesson which his experience has to teach; and, as progress in this world's knowledge has ever been from the gross to the spiritual, may we not rejoice as those who have a great hope?

Although the exceeding merit of the "Treatise on Crystallography" casts into the shade all that was subordinate, we must not omit to mention that Professor Miller published an early work on hydrostatics, and numerous shorter papers on mineralogy and physics, which were all valuable, and constantly contained important additions to knowledge.

Moreover, the "New Edition of Phillips's Mineralogy," which he published in 1852 in connection with H. J. Brooke, owed its chief value to a ma.s.s of crystallographic observations which he had made with his usual accuracy and patience during many years, and there tabulated in his concise manner. As has been said by one of his a.s.sociates in the Royal Society, "it is a monument to Miller's name, although he almost expunged that name from it."[I] It is due to Professor Miller's memory that his works should be collated, and especially that by a suitable commentary his "Tract on Crystallography" should be made accessible to the great body of the students of physical science, who have not, as a rule, the ability or training which enables them to apprehend a generalization when solely expressed in mathematical terms. The very merits of Professor Miller's book as a scientific work render it very difficult to the average student, although it only involves the simplest forms of algebra and trigonometry.

[I] "Obituary Notices from the Proceedings of the Royal Society,"

No. 206, 1880, to which the writer has been indebted for several biographical details.

Independence, breadth, accuracy, simplicity, humility, courtesy, are luminous words which express the character of Professor Miller. In his genial presence the young student felt encouraged to express his immature thoughts, which were sure to be treated with consideration, while from a wealth of knowledge the great master made the error evident by making the truth resplendent. It was the greatest satisfaction to the inexperienced investigator when his observations had been confirmed by Professor Miller, and he was never made to feel discouraged when his mistakes were corrected. The writer of this notice regards it as one of the great privileges of his youth, and one of the most important elements of his education, to have been the recipient of the courtesies and counsel of three great English men of science, who have always been "his own ideal knights," and these n.o.ble knights were Faraday, Graham, and Miller.

VII.

WILLIAM BARTON ROGERS.