6i6 



NATURE 



[April 30, 1896 



Somewhat opaque: Mica, tourmaline, wulfenite, axinite, 

 spinel, calcite, aragonite, kaolin, NiS04(NH4)2S04.6H20, 

 NiS04.K,S04.6H20, &c. 



Opaque : Roll sulphur, crystal of rhombic sulphur, fluor- 

 spar, topaz, beryl, ruby, quartz, NaCl, chalco-pyrite, 

 Hi,(NH4)As04, H2KASO4, KsFeCyg, KgCraO^, orpiment, 

 anhydrite, celestine, barite. 



Sulphuric acid is as opaque as the same thickness of sulphur. 

 Water is more opaque than paraffin wax. 



A number of crystals of about the same thickness were photo- 

 graphed on the same plate, and an attempt made to judge of 

 their relative transparency with a scale from i to lo, with the 

 following results : — 



H2KASO4 .« ... I 



H5,(NH4)As04 2 



NiS04.{NH4)2S04.6H20 3 



MgSO4.K2SO4.6H2O 3 



NiSO4.K2SO4.6H2O 3-5 



MgS04.(NH4),S04.6H20 4 



ZnS04.(NH4)2S04.6H20 4 



C0SO4.K0SO4.6H2O " ... ... 4 



CoS04.(NH4).,S046H20 4 



H2(NH4)P04" 5 



Paraffin wax ... ,.. ... ... 10 



The different values of arsenic and phosphorus in the iso- 

 morphous acid arseniates and phosphates are to be remarked. 



Thin sections of a granite composed principally of quartz and 

 feldspar and of a hornblende-gabbro were photographed. In 

 both cases the feldspar was found to be distinctly more trans- 

 parent than the other constituents. 



Prof. E. Doelter, of Graz, has communicated to the Nattir- 

 wissenschaftliche Verein fiir Steierniark some observations 

 relating to the opacity of different rocks and minerals for the 

 Rontgen rays, and their use as providing a test of the genuine- 

 ness of precious stones. Dr. Doelter finds that (i) the opacity 

 does not always increase with the density, although minerals 

 having a specific gravity greater than 5 are relatively opaque ; 

 (2) the complexity of the chemical constitution of a mineral 

 affects its opacity, but no general law of relationship can be 

 enunciated ; (3) dimorphous minerals exhibit but slight differ- 

 ences in their behaviour with regard to the rays in their different 

 forms ; (4) in most crystals, the amount of absorption does not 

 depend sensibly on the direction of the incident rays ; (5) all 

 minerals naturally fall into about eight well-defined groups, 

 according to their opacity, the order being as follows : diamond, 

 corundum, talc, quartz, rock-.salt, Iceland spar, &c. The 

 diamond is ten times as transparent as corundum, and 200 times 

 as transparent as tinfoil. 



Mr. W. Ackroyd and Mr. H. B. Knowles have systematically 

 examined the opacity of a number of substances for Rontgen 

 rays, with a view to determining whether it bears any relation 

 to molecular weight {Journal of the Society of Dyers and 

 Cleaners, April). 



With this view they have compared the isomorphous sulphates, 

 RSO4, 7H2O of the eighth group of metals, iron, nickel, and 

 cobalt ; the oxides, RO, of some members of the second natural 

 group, viz. magnesium, zinc, and mercury ; the isomorphous 

 oxides, R2O3, of the metals aluminium, chromium, and iron. 

 In each of these series there are presumably similarly shaped 

 molecules for comparison, and the disturbing factor is the 

 difference of molecular weight. The result of an hour and a half's 

 exposure showed that the alumina was practically transparent, 

 the chromium sesquioxide semi-transparent, while the ferric oxide 

 was opaque. In other words, the opacity of the substance was 

 in some direct relation to the molecular weight. There are here 

 marked differences with big jumps in molecular weights. The 

 same observation applies to the oxides of magnesium, zinc, and 

 mercury. The isomorphous sulphates of iron, nickel, and 

 cobalt are extremely interesting, because of the nearness of the 

 specific gravity numbers, and also of the molecular weights. 

 The iron compound, with lower specific gravity and molecular 

 weight, appears to be the least opaque of the three, while the 

 nickel and cobalt compounds of nearly the same specific gravity 

 and molecular weight have approximately the same degree of 

 opacity. The following table correlates these facts with other 

 properties : — 



NO. 1383, VOL. 53] 



Compound. 



FeS04, 7H2O 



NiS04, 7H2O 

 C0SO4, 7H2O 



AljOa 



CrgOj 



FeaOs 



MgO 



ZnO 



HgO 



Sp. j 



Mole- 

 cular i Colour 

 weight. 



1-85 



4-00 

 4-99 

 5-13 



3*42 

 5-47 

 II-I3 



278 



280 

 281 



103 

 153 

 160 



Light green 



Deep green 

 Pink 



White 

 Green 

 Brown 



Kehaviour to 

 Rontgen rays. 



Slightlyless opaque 



than others in 



this group. 



\ About the same 



j relative opacity. 



Transparent. 

 Semi-opaque. 

 Opaque. 



40 White 



81 \ White 



216 I Red . 



... Transparent. 

 ... Semi-opaque. 

 ...Opaque. 



The foregoing figures, conclude the authors, demonstrate the 

 weakness of an unqualified law of density, as the denser oxide 

 of zinc is more transparent than the less dense ferric oxide. But 

 they point out that, adopting the legitimate method, which they 

 have initiated, of comparing only compounds with kinship, each 

 of the above bodies conforms to the law of density as well as 

 of molecular weight in relation to opacity. 



Dr. A. Sella and Dr. Q. Majorana {Rettd. R. Accad. dei 

 Lincei) describe certain experiments on the influence of Rontgen 

 rays on the sparks produced by the discharge of an induction 

 coil in air. The sparking distance is found to be shortened by 

 the Rontgen rays, this effect taking place whenever these rays 

 fall on the positive pole. In this respect the phenomenon is the 

 reverse of that obtained by Hertz with ultra-violet light, the 

 effect of which is to lengthen the sparking distance whenever it 

 falls on the negative pole. The authors found that the simul- 

 taneous actions of Rontgen rays and of ultra-violet light could be 

 made to neutralise each other by arranging the coil to give a 

 spark of suitable length (in their experiments about 30 mm.). 

 When the sparking distance was less, the Hertz effect pre- 

 dominated ; when the sparking distance exceeded 30 mm., the 

 Rontgen rays had the greater influence. 



Dr. Filippo Campanileand Dr. Emilio Stromei communicate to 

 XheRe?idtconfodeir Accademia delle Scienze Jisiche e maietnatiche 

 (Naples) a note on the phosphorescence and the Rontgen rays in 

 Crookes' and Geissler's tubes. The conclusions arrived at are as 

 follows : (i) When in the circuit of an induction coil, containing 

 a Crookes' tube, a spark is thrown off from the positive pole, the 

 phosphorescence .of the tube and the efficacy of the Rontgen radia- 

 tions are augmented. (2) As the length of the spark increases 

 the phosphorescence at first increases to a maximum, and then 

 decreases. (3) If, on the other hand, the spark is thrown off 

 from the negative pole, the phosphorescence and the Rontgen 

 rays are thereby diminished. The same experimenters have 

 also succeeded in obtaining Rontgen rays with an ordinary 

 Geissler's tube. These radiations possessed all the characteristics 

 of those which emanate from a Crookes' tube. 



Signer E. Villari, writing in the same journal, considers that 

 the phenomena of discharges in tubes seem to indicate the 

 existence not only of kathodic but also of anodic rays. While 

 the kathodic rays travel in straight lines and produce a negative 

 charge wherever they strike the tube, the suggested anodic rays 

 diffuse them.selves all round the anode, and communicate a 

 positive charge to the whole surface of the tube over which they 

 are diffused. 



The fundamental character of the new rays has led specu- 

 lators to make various surmises as to a possible connection 

 between these radiations and the phenomenon of gravitation, 

 and two lengthy memoirs have been written on the subject by 

 Rudolf Mewes.^ In the second of these the author claims to 

 have proved experimentally that gravitation is propagated 

 through the ether with the velocity of light. 



Finally, attention may profitably be called in this summary to 

 the April number of the Proceedings of the Physical Society. In 

 the admirable collection of abstracts of physical papers there 

 published, will be found concise descriptions of the scope and 

 results of no less than forty papers concerned with Rontgen 

 rays. 



1 " Licht- Electricitats und X-Strahlen "-(pp. 54) ; " Die Fortpflanzungs- 

 Cleschwindigkeit der Schwerkraftstrahlen " (pp. 93). (Berlin : M. Krayn. 

 1896.) 



