PHYSICS, 



589 



although the brass bar showed a difference of 3.8 microtis and the glass 

 ones 3.G and 2.8 microns, respectively. The authors conclude, first, 

 that zinc is not a reliable metal for one of the components of a metal- 

 lic thermometer, much less for a standard of length ; and, second, that 

 bars of steel, copper, and brass are not likely to vary in length appre- 

 ciably at any temperature within the range to which standards are 

 ordinarily subjected. (Am. J. ScL, June, 1883, III, xxv, 448.) 



Wiedemann has measured the changes of volume which hydrated 

 salts undergo by the action of heat, using for this purpose a dilatom- 

 eter consisting of a bulb, in which is placed a known weight of the salt, 

 terminated by a capillary tube. The bulb is exhausted of air and is 

 then filled either with oil or with mercury, by the oscillations of which 

 in the capillary tube the changes of volume are measured. The alums 

 melt about 90°, remain surfused down to 00°, then undergo a modifica- 

 tion, accompanied, in the case of potassium-aluminum and ammonium- 

 iron alums, with a change of volume. Magnesium sulphate deposits 

 at 93° crystals of Mg S0 4 (H 2 0) 3 ; zinc sulphate at G9° deposits Zn S0 4 

 (H 2 0) 3 ; iron sulphate gives at 65° Fe S<_> 4 (H 2 0) 3 — all accompanied by 

 a change in volume. ( Wied. Ann., xvn, 561 ; J. Phys., August, 1883, 

 II, II, 374.) 



Mallard has made an elaborate study on the action of heat on crys- 

 tals of boracite and of potassium sulphate. He has observed that the 

 former, which are doubly refractive at ordinary temperatures and even 

 when heated to 260°, becomes suddenly singly refractive at or about 

 201°, remaining so at higher temperatures. Crystals of potassium sul- 

 phate which are biaxial below 380° pass through intermediate stages 

 and become uniaxial negative at 600° and above. Boracite crystallizes 

 in rhombic dodecahedrons rigorous^ cubic; its double refraction is 

 therefore anomalous. The author distinguishes between the crystalline 

 form which is due to the symmetry of the molecules themselves and 

 that which results from their arrangement in space, the reticular sys- 

 tem. Hence he maintains that, while the reticular system of boracite 

 is cubic, the molecular symmetry is orthorhombic. The results given 

 above he regards as proof of his position. The reticular system of 

 boracite is unaltered by the heat, being cubic at all temperatures. But 

 the molecular symmetry changes from the orthorhombic at ordinary 

 temperatures to the cubic at 261° ; a change accompanied by an ab- 

 sorption of 5.74 calories for each unit of weight. So potassium sulphate, 

 which is orthorhombic at the ordinary temperature, is hexagonal at 

 G00° and above. The dimorphism here noted has a parallel in the well- 

 known cases of niter and of sulphur. The former is orthorhombic and 

 hexagonal, biaxial in the former, uniaxial in the latter, the former be- 

 ing the stable form below 339°, the latter above this point. Sulphur is 

 orthorhombic and monoclinic. Toward 110° the former variety passes 

 into the latter. The author concludes as follows: 1st, a perfectly cubic 

 reticular system may be accompanied by an energetic double refraction; 



