February 17, 1893.] 



SCIENCE. 



87 



Britain. After this comniiltee had made its report, but liefore 

 tbe "Order in rouncil" had been signed by the Queen, an inti- 

 mation was received frona Professor von Helmholtz that something 

 might be done toward international agreement if the order were 

 delayed till he could communicate in person the results of the 

 most recent determinations in Berlin. Acrordingly von Helmholtz 

 and some others were invited to be present at the British Associa- 

 tion last August, and to sit with the famous B. A. " committee ap- 

 pointed for the purpose of constructing and issuing practical 

 standards for use in electrical measurements," The report of the 

 committee, recently published, says: "During the Edinburgh 

 meeting the committee were honored with the presfrceof Dr. von- 

 Helmholtz, M. Guillaume of Paris, Professor Carliart of the 

 United Slates Dr. Lindeok and Dr. Kahle of the Berlin Relchs- 

 anstalt. These gentlemen came by invitation to consider the 

 question of establishing identical electrical standards in various 

 countries." The committee had two long sessions, and there 

 were present Professor Carey Foster, chairman, Lord Kelvin, Pro- 

 fessors Ayrton, Perry, and Sylvanus Thompson, Dr. Oliver Lodge, 

 Mr. Glazebrook, secretary, Mr. Preece of the PostOffice, Major 

 Cardew of tbe Board of Trade Bureau, and others. 



The most important results of these conferences were the aban- 

 donment of the time-honored B.A. unit, the disregard of the 

 "legal" ohm, and the adoption of the mercury standard of 106.3 

 centimeters. The reports from Berlin and Paris showed most 

 conclusively that mercurial standards, set up with the precau- 

 tions recently adopted, agree with surprising accuracy. The un- 

 certainty of tbe relation between the centimeter and the gramme 

 was avoided by defining the mass of the mercury column of 106.3 

 centimeters in length, which has a resistance of one ohm. It is 

 14.4531 grammes. This corresponds to a specific gravity for 

 mercury of 13.5956. "In reality tbe square-millimeter cross-sec- 

 tion remains the elementary definition, but with the specification 

 that this is arrived at by mercurial weighmgs." 



Standards of resistance for industrial purposes in solid metal 

 will still be made as heretofore. But it must be borne in mind 

 that such resistances, especially when made of alloys, should be 

 kept at a temperature near the one at which they have been 

 standardized ; otherwise small changes take place in the resis- 

 tance, due perhaps to a kind of annealing and recrystallizing 

 process. 



It was further agreed with regard to the unit of current that 

 the number 0.001 1 18 should be adopted as the number of grammes 

 of silver deposited per second from a neutral solution of nitrate of 

 silver by a current of one ampere. This corresponds to 4.025 

 grammes per hour. The silver voltameter, with tbe proper 

 manipulation, becomes, theri-fore, a secondary standard for the 

 determination of the unit current 



The electromotive force of the Clark standard cell has been rede- 

 termined both in Berlin and Cambridge, England, within a year 

 or two; and the results are in rather surprising agreement. A 

 comparison of these determinations led the B.A committee to de- 

 cide upon 1.434 as the number of volts representing tbe electro- 

 motive force of the old form of Clark cell at 15° C. containing a 

 saturated solution of zinc sulphate and crystals in excess. This 

 is .001 volt lower than the value heretofore assigned to this cell. 

 It was not determined to adopt this form of cell as the standard, 

 but only to decide upon its voltage when set up by competent 

 persons in accordance with certain specific directions. My own 

 form of Clark cell is perfectly portable, has an electromotive 

 force of 1 44 volts at 15° C, and its change of electi-omotive force 

 with temperature is only half as great as that of the old t lark 

 cell containing crystals. 



We have as yet in this country no buieau where concrete stan- 

 dards of resistance and standaid instruments for other electrical 

 units are preserved. Such a bureau, under federal control, is 

 greatly to be desired. Germany has its Reichsanstalt, under the 

 direction of von Helmholtz, in Berlin ; England has not only the 

 standards of the British Association in the keeping of Mr. Glaze- 

 brook at Cambridge, but also the Board of Trade Bureau in Lon- 

 don, under the directorship of Major Cardew. Mr. Glazebrook 

 undertakes the comparison and certification of standard coils for 

 the English-speaking world while the bureau in London issues 



certificates of instruments for commercial purposes in Great 

 Britain. 



Government bureaus mean certified standards and legally 

 adopted units. The decisions of the B.A. committee last August 

 were with the fuH concurrence of Professor von Helmholtz. and 

 it is understood thai the German government will adopt the B.A. 

 proposals. The committee appointed by the Board of Trade in 

 London has already made its supplementary report in accordance 

 with the conclusions of the B.A. committee, and these units will 

 doubtless very soon accjuire a legal character in England. The 

 coming electrical congress in ( hicago will afford an opportunity 

 for official delegates to adopt the>e same units for their respective 

 countries, and official ratification will then naturally follow. 

 Lord Kelvin (Sir Wra. Thomson) predicted at the close of tbe 

 Edinburgh meeting that the system r{ units adopted by tbe B.A. 

 committee will become thoroughly international. It should be 

 tbe duty and pleasure of all electricians to contribute toward this 

 result. 



THE CLASSIFICATION AND NAMING OF IGNEOUS ROCKS. 



BY W. S. BAILEY, WATERVILLE, ME. 



The discussions of Mr. Iddings' relating to the crystallization 

 of lava have led him to conclusions that will undoubtedly prove 

 of vast significance in the attempt to ground the study of rocks 

 in a firm and sure foundation. Heretofore, most petrographera 

 have busied themselves with descriptions of rock-types, confining 

 their discussions principally to the mineralogical composition and 

 the structure of the specimens studied, and to their similarity to 

 other specimens assumed as types. Such work as this is of course 

 absolutely necessary to the right treatment of rock classification. 

 It is evident that we must first know the characteristics of bodies 

 to be classified before we can hope to separate them into genetic 

 groups. But tbe time has now come when students of rocks 

 must seek for a generalization that will do for their science what 

 the atomic theory has done for chemistry or the theory of evolu- 

 tion for the biological sciences, viz.. elevate petrography from the 

 position of a descriptive science to that of a philosophical one. 

 Mr. Iddings's recent studies in tbe causes producing the differ- 

 ences noted in different lavas emanating from the same volcanic 

 centre, and tbe generalizations drawn from them, will go far 

 toward affording a philosophical basis for rock classification, and, 

 consequently, toward the inception of a broader study of rocks in 

 their relationships to each other than has heretofore been possi- 

 ble. 



The rocks on the surface of the earth all tend toward the pro- 

 duction of a few simple types, in which tendency may be traced 

 the action of chemical laws, under the definite conditions existing 

 at the surface, producing from unstable compounds those that are 

 most stable under these conditions. 



Mr. Iddings believes that the relation existing between chemi- 

 cal action and tbe conditions under which it occurs is discovera- 

 ble not only in the breaking down (degradation) of rocks, but also 

 in their construction. He believes that the intimate gradations 

 in composition and structure that are known to exist between the 

 types of eruptive rocks are due to the action of chemical laws 

 under changing but definite conditions — conditions that are de- 

 termined largely by the position of the magmas from which the 

 rocks are derived. If this be true, pelrographers have at last a 

 thread to which they can tie the results of their investigations : 

 they have offered them a conception as to the caiise of the ex- 

 istence of eruptive rock-types, whose discussion jiro and con will 

 compel them to study not simply rock-specimens, but rather 

 rock-masses, in the attempt to learn just what relations exist 

 between their various parts, with respect to composition and 

 structure, and to discover the conditions under which these parts 

 were formed. In other words, petrography, as the result of this 

 discussion, will become petrology, just as " natural history " has 

 become "biology." 



' J. P. Iddlrgs, The Origin of Igneous Eorts, Bull. Phllcs. Soc., WashlDg- 

 ton, vol xil, 1892, p. 89. 



