86 



SCIENCK. 



[Vol. XXI. No. 524 



The writer's attention was first attracted to the place through 

 receiving several small specimens of the selenite from sheep- 

 herders, who had discovered the deposit while searching for feed- 

 ing-places, and who claimed to have found a mine of mica, which 

 they called "isinglass." Their disgust was great when assured, 

 by the conclusive experiment of holding a bit of the material in 

 the flame of a candle, that the stuff was not what it seemed. I 

 first visited the place in April last, and my rapture at the superb 

 display of crystal beauty was checked by the evidences of vandal- 

 ism on every hand. Some of the finest crystals had been hacked 

 and carved, and cow-boys' initials were scratched and cut on 

 almost every prismatic face which the light could reach. Visit- 

 ing the place again six months later, I found that still greater 

 destruction had been waged, and, becoming convinced that good 

 crystals would soon be difficult to obtain, 1 took steps to secure 

 legal claim to the land, and proceeded to remove the remaining 

 crystals of greatest value to a place of safety. Under the auspices 

 of the Deseret Museum of Salt Lake City, the work of removal is 

 still in progress. Already over twenty tons of most beautiful 

 crystals have been taken out and shipped to this city. 



Some of the finest specimens will probably be on exhibition in 

 Chicago next summer. 



THE FUTURE OHM, AMPERE, AND VOLT. 



BY HENBY S. CABHAET, ANN ABBOR MICH 



Since the International Congress of Electricians in Paris in 

 1881, the most eminent physicists have been agreed as to the 

 theoretical values to be assigned to the three fundamental units 

 of electrical measurement ; but it has been a matter of ten years' 

 labor on the part of many distinguished investigators to embody 

 these theoretical definitions in practical units for universal use. 



Up to the date mentioned the two units of resistance in use 

 were the British Association (B.A.) unit and the Siemens unit. 

 Only the former represented an attempt to construct an ohm 

 corresponding to the theoretical definition. The B.A. unit has 

 served a useful purpose, but it is now known to be 1.34 per cent 

 too small. 



The "legal ohm" was provisionally adopted in 1883 by an in- 



Prisms of perfect form and varying in length from one to five 

 feet, and in weight from ten to one hundred pounds, are of fre- 

 quent occurrence. One of the most regular yet taken out is four 

 feet long, and the widest faces are six inches acrofs. Cleaved 

 slabs are obtainable six feet in length, and two and a half feet in 

 breadth. One of the longest perfect prisms yet obtained extends 

 fifty-one inches, and from one of its faces nineteen smaller crys- 

 tals sprout. Twins are common, as are also compound termina- 

 tions of very complicated structure. A magnificent group, 

 weighing over six hundred pounds, was removed from the floor 

 of the cavern; it was set up on the outside and photi graphed (see 

 Fig. 4). 



As to the habit of the crystals, in the midst of such variety it is 

 difficult to specify. Prisms short and stout, also long and com- 

 paratively slender, are numerous; and of twins, the "swallow- 

 tail'' vie with the cruciform and penetration varieties in points 

 of abundance and perfection. Some of the crystals are of perfect 

 transparency, and cleaved slabs of this quality are common. 

 Sometimes the prisms inclose sand and clay, which is so distributed 

 as really to add to the beauty ol the crystals in the eyes of all save 

 the mineralogist. When fracture planes are made visible by 

 striking a crystal containing such impurities, the particles appear 

 on the internal planes as on shelves of glass. 



ternational committee to which the Congress of 1881 bad com- 

 mitted the subject. It was in the nature of a compromise, and 

 fixed the practical ohm as the resistance at 0° C. of a column of 

 mercury one .square millimeter in cross-section and 106 centimeters 

 long. Competent investigators, like Lord Rayleigh and Professor 

 Mascart, contended that a column 106 3 centimeters in length 

 was nearer the true value; but a few smaller values obtained by 

 some well-known physicists decided the adoption of the mean 

 value 106 centimeters. This conclusion satisfied no one, and the 

 " legal ohm" was never legally oroificialty adopted by any Euro- 

 pean or American government. 



Subsequently, Professor Rowland came forward with his de- 

 termination of 106.82, and errors were found in the data of some 

 who had contended for the lower values. Hence the number 

 106.3 has been tacitly accepted for two or three years already, 

 and it is now believed that this does not differ from the true value 

 by more that two units in the fifth figure; that is, the length of 

 the mercurial column representing the true ohm is not less than 

 106.28 and not more than 106.32 centimeters. 



Somewhat over two years ago a commission was appointed by 

 the British Board of Trade to draft an "Order in Council" as a 

 legal settlement of the units to be employed by the Board of Trade 

 Electrical Bureau, and hence as the legal electrical units for Great 



