May 28, 1891] 



NATURE 



83 



daily change in the variation, somewhat analogous to the daily 

 range of the barometer, although the daily minimum of variation 

 at Washington occurs at about 8 a.m., and the maximum between 

 I and 2 p.m. It is proposed to continue the publication of 

 these curves on this Chart for at least three months, and any 

 questions regarding them will receive immediate consideration 

 and reply. The attention of masters of vessels is called to the 

 form issued by this Office for the record of observations of varia- 

 tion at sea, and to the general importance of the subject in con- 

 nection with vessels' compasses and the variation curves plotted 

 on our charts. "] 



The Alpine Flora. 



In connection with this subject (see Nature, vol. xliii. p. 

 581) it may be well to draw the attention of botanists to the 

 fact that a young vigorous strawberry plant, in an exposed 

 garden, will, during the winter season, place all its leaves in a 

 perfectly horizontal position, some even close to and resting on 

 the ground, in striking contrast to its summer habit of erect 

 growth, whereby it is often damaged by strong winds. 



Whether direct climatal conditions be the sole cause of this 

 peculiarity, or whether inherited, I cannot determine ; presumably, 

 in its natural surroundings, the continual crowding and con- 

 sequent struggle would not necessitate the adoption of dwarfing 

 as a means of survival. J. Lovel. 



May 13. 



Magnetic Anomalies in Russia. 



The magnetic disturbances in England and Wales as com- 

 municated to Nature, vol. xliii. p. 617, by M. Mascart and A. 

 W, Rucker, are of great interest, but the size of the disturbances 

 between Charkov and Kursk in Russia is of much higher value. 

 More than 150 stations with magnetic elements have proved that 

 in the above region there are points where the declination differs 

 by 86°, the inclination by 29°, and the magnetic total force by 

 o*39 el. un. The principal centres are distant from each other 

 not more than 12 kilometres. The m. elements are : — 



The normal values are - 1° Decl. ; + 64° Incl. ; 0*48 total 

 force. The districts are covered by sedimentary rocks. 



St. Petersburg, April 30. A. de Tillo. 



THE REJUVENESCENCE OF CRYSTALS^ 



V 



'ERY soon after the invention of the microscope, the 

 value of that instrument in investigating the pheno- 

 mena of crystallization began to be recognized. 



The study of crystal-morphology and crystallogenesis 

 was initiated in this country by the observations of Robert 

 Boyle ; and since his day a host of investigators — among 

 whom may be especially mentioned Leeuwenhoek and 

 Vogelsang in Holland, Link and Frankenheim in Ger- 

 many, and Pasteur and Senarmont in France — have added 

 largely to our knowledge of the origin and development 

 of crystalline structures. Nor can it be said with justice 

 that this field of investigation, opened up by English 

 pioneers, has been ignobly abandoned to others ; for the 

 credit of British science has been fully maintained by 

 the numerous and brilliant discoveries in this department 

 of knowledge by Brewster and Sprby. 



There is no branch of science which is more dependent 

 for its progress on a knowledge of the phenomena of 

 crystallization than geology. In seeking to explain the 

 complicated phenomena exhibited by the crystalline 

 masses composing the earth's crust, the geologist is 



• The Friday Evening Discourse, delivered at the Royal Institution on 

 January 30, jSgi, by Prof. John W. Judd, F.R.S. 



NO. II 26, VOL. 44] 



constantly compelled to appeal to the physicist and 

 chemist ; from them alone can he hope to obtain the 

 light of experiment and the leading of analogy, whereby 

 he may hope to solve the problems which confront him. 



But if geology owes much to the researches of those 

 physicists and chemists who have devoted their studies to 

 the phenomena of crystallization, the debt has been more 

 than repaid through the new light which has been thrown 

 on these questions by the investigation of naturally-formed 

 crystals by mineralogists and geologists. 



In no class of physical operations is time such an im- 

 portant factor as in crystallization ; and Nature, in pro- 

 ducing her inimitable examples of crystalline bodies, has 

 been unsparing in her expenditure of time. Hence it is 

 not surprising to find that some of the most wonderful 

 phenomena of crystallization can best be studied — some, 

 indeed, can only be studied—in those exquisite specimens 

 of Nature's handiwork which have been slowly elaborated 

 by her during periods which must be measured in millions 

 of years. 



~ I propose to-night to direct your attention to a very 

 curious case in which a strikingly complicated group 

 of phenomena is presented in a crystalline mass : and 

 these phenomena, which have been revealed to the 

 student of natural crystals, are of such a kind that we 

 can scarcely hope to reproduce them in our test-tubes 

 and crucibles. 



But if we cannot expect to imitate all the effects which 

 have in this case been slowly wrought out in Nature's 

 laboratory, we can, at least, investigate and analyze them ; 

 and, in this way, it may be possible to show that pheno- 

 mena like those in question must result from the pos- 

 session by crystals of certain definite properties. Each 

 of these properties, we shall see, may be severally illus- 

 trated and experimentally investigated, not only in natural 

 products, but in the artificially-formed crystals of our 

 laboratories. 



In order to lead up to the explanation of the curious 

 phenomena exhibited by the rock-mass in question, the 

 first property of crystals to which I have to refer may be 

 enunciated as follows :- - 



Crystals possess the power of resuming their growth 

 after interruption; and there appears to be no limit to 

 the time after which this resumption of growth may take 

 place. 



It is a familiar observation that if a crystal be taken from 

 a solution and put aside, it will, if restored after a longer or 

 shorter interval to the same or a similar solution, continue 

 to increase as before. But geology affords innumerable 

 instances in which this renewal of growth in crystals has 

 taken place after millions of years must have elapsed. 

 Still more curious is the fact, of which abundant proof 

 can be given, that a crystal formed by one method may, 

 after a prolonged interval, continue its growth under 

 totally different conditions and by a very different method. 

 Thus, crystals of quartz, which have clearly been formed 

 in a molten magma, and certain inclosures of glass, may 

 continue their growth when brought in contact with solu- 

 tions of silica at ordinary temperatures. In the same 

 way, crystals of felspar, which have been formed in a 

 mass of incandescent lava, may increase in size, when 

 solvent agents bring to them the necessary materials 

 from an enveloping mass of glass, even after the whole 

 mass has become cold and solid. 



It is this power of resuming growth after interruption, 

 which leads to the formation of zoned crystals, like the 

 fine specimen of amethyst enclosed in colourless quartz, 

 which was presented to the Royal Institution seventy 

 years ago by Mr. Snodgrass. 



The growth of crystals, like that of plants and animals, 

 is determined by their environment ; the chief conditions 

 affecting their development being temperature, rate of 

 growth, the supply of materials (which may vary in 



