Feb. 24, i&Si] 



NATURE 



597 



spectrum. They think that the blue colour of the sky may 

 probably be partly due to the ptesence of ozone. 



Brame (in Comptes reiidiis) veconimends the use of baryta in 

 place of soJium carbonate and charcoal, in the ordinary dry test 

 for arsenic. If arsenious oxide is heated with baryta a mirror is 

 obtained consisting partly of metallic arsenic, and partly of 

 barium arsenate : the test does not succeed so w ell %\ith arsenious 

 sulphide. 



A CONSIDERABLE deposit of crystallised (octahedral) sulphur 

 has been found under the soil of Paris, where organic refuse 

 matter has long accumulated. The sulphur appears to be a 

 product of the deoxidising action of the carbon compounds 

 present in the refuse on the calcium sulphate of the soil. 



M. I.OUGHININ continues, in the Journal of the Russian 

 Chemical Society, his interesting researches on the quantities of 

 heat produced by burning alcohols of the allyl series ; he pub- 

 lishes in the /otirnal the figures corresponding to two new 

 bodies of this series (CbHioO and CjoH^oO), which figures, 

 together with those he has already published in the Comptes 

 rendus (vol. xci.), allow him to draw a complete table of the 

 calories disengaged by the whole of the alcohols of this series. 



The first number of the Gazetla Chimica Jtaliana for the 

 present year is devoted, with the exception of a paper by M. Fileti 

 on gas analysis, to papers on organic chemistry : these include 

 work on Camphor Derivatives by Schiff; on Picrotoxin by 

 Paterno and Oglialoro ; and on Synthesis of Aromatic Aldehydes 

 by the use of Chromyl Bichloride, by Paterno and Scichiloni. 



In the course of a paper on the Photochemistry of Silver 

 Chloride, Eder states (in IVieu. Akad. Bcr.) that this substance is 

 more sensitive to li^ht when substances which absorb chlorine 

 are present, than when in the pure state. To develop the latent 

 image he recommends especially ammonium ferrocitrate, and 

 hydroquinone along with ammonium carbonate. 



By the action of potassium dichromate and sulphuric acid on 

 caffeine, Hinteregger has obtained as much as 40 per cent, of 

 dimethyl parabanic acid, and 39 per cent, of the monomethyl 

 acid from theobromine. 



Ik continuation of his investigations into the action of hydro- 

 chloric acid on metallic chlorides, Ditte describes (Coiiipt. rend.) 

 several new hydrated salts which crystallise from aqueous solu- 

 tions when these are saturated with hydrochloric acid. In the 

 absence of hydrochloric acid hydrated salts with more water of 

 crystallisation are always produced. The following table contains 

 the principal results obtained by Ditte : — 



Aqueous solution. 1 Solution saturated with HCl at 12". . 



Gramsofsalt iGramsofsalt 



dissolved Crystals which form I dissolved Crystals which form 

 per litre. i per litre. 



700 ... CaCl2.6H„0 ' 270 ... CaCl„.2H.,0 



500 ... SrCl^.eHa'O I 20 ... SrCl„'.2H;0 



720 ... MgCU.eHjO ; 65 ... MgCl,.2H.,0 

 415 ... CoClj'.eHjO 1 , ^ S 2CoCL.3H.,0 



600 ... Nia„.6H20 I ^°5 ■■• I and C5C1.H.,0 



870 ... MnCL.4H,0 ' 40 .. NiClo.H^O ' 

 630 ... CuCCaHjO 190 ... MnCL.HsO 



290 .. CuClj'.HjO 

 M. PoucHET describes in Compt. rend, a method for destroying 

 organic matter before testing for mineral poi-ons in contents of 

 a stomach, &c. ; the method is based on the oxidising action of 

 potassium-hydrogen sulphate followed by addition of sulphuric 

 acid. 



PHYSICAL NOTES 



In a little mathematical note in the Comptes rendus M. ThoUon 

 investigates the general equation for the passage of light through 

 a prism, and thence deduces the proposition that for every prism 

 there is an angle of minimum resolving power. Differentiating 

 the general equation with respect to the index of refraction, he 

 obtains, first, a differential equrtion expressing the dependence 

 of the angular distance between two rays upon the dispersive 

 index. A separate differentiation with respect to the angle of 

 incidence yields a second differential equation expressing the 

 dependence of the apparent width of the slit as seen through the 

 prism upon the angular aperttire of the slit, as viewed from the 

 prism through the collimator. Hence a relation can be obtained 

 between the angular distance between two rays and then* apparent 



breadth. Further examination of the equations .shows that for a 

 certain incidence there will be a minimu-u of resolution (i.e. an 

 incidence at which the rays are least well defined), and that for 

 anothei incidence there will be a minimum of dispersion ; these 

 two incidences being symmetrically related to the angle of 

 incidence corresponding to minimum deviation. M. ThoUon 

 states that these deductions may be readily verified by the 

 following experiment : — A dense flint glass prism is adjusted in 

 the position of miniumin deviation for the rays D upon its sup- 

 porting table in the spectroscope, lit by a sodium flame. The 

 slit is then narrowed or widened until the two yellow rays are 

 just in mutual contact. On then turning the prism around its 

 axis so as to increase the angle of incidence the two rays are 

 seen to separate and to become perfectly dis'inct, the angular 

 distance between them dinnniihing all the while. But if the prism 

 be turned in the o]5posite direction, so as to decrease the angle 

 of incidence, the yellow band is seen to become w ider, but without 

 being lesolved into two rays. Perhaps this research may explain 

 why the so-called "half prism" spectroscope failed to realise all 

 the hopes of its inventor. 



Recent observations by Hrn. Wulluer and Grotrian (WieJ. 

 Ann. No. 12) seem to prove that the s^iecific volume of 

 vapours is independent of the size of the space in which it is 

 determined. They also confirm Ilerr Herwig's result, that 

 vapour^ always undergo precipitation before reaching the so- 

 called maxiouim tension. Further, the tension at which conden- 

 sation begins is found to have a relation to the maximum tension, 

 which depends on the nature of the liquid, but is nearly inde- 

 pendent of the temperature. Experiments made in order to find 

 in W'hat measure vapour must be compressed so as to pre.sent 

 maximam tension, gave the unexpected result, that there is in 

 general no maximum tension in the sense hitherto accepted ; but 

 that the tension of saturated vapours, even when they are in 

 contact w ith a large and excessive quantity of liquid, is perceptibly 

 increased by compression. 



The varieties of the electric discharge in gases are fully 

 investigated by Herr Lehmann in a recent paper (ff7rt/. Ann 

 No. 12). The chief conclusion is that there are four well- 

 characterised modes of discharge to be distin:;uished, viz. glow, 

 brush, band, and spark discharge ; and these may all be 

 obtained in air of ordinary (a; well as of less) density, and also 

 in other gases, with inserted resistances and breaks, and 

 with sharp and rounded form of electrodes, at great or small 

 distances. The principal characteristics are these: — i. Glow- 

 discharge ; positive glow, negative light pencil, consisting of two 

 parts separated by a dark space. 2. Brush-discharge ; positive 

 brush, consisting of stem and branches ; negative light-pencil. 

 3. Band-discharge ; positive light with two places of intermit- 

 tence, sometimes stratified, and separated from the negative glow 

 by a dark space. 4. Spark-discharge: band of light connecting 

 both electrodes ; with two places of intermittence, brushes of 

 metallic vapour at both ends, the positive longer, the negative 

 thicker ; sometimes oblique dark spaces. 



The influence of traction and vibrations of a metallic wire on 

 its electric conductivity is the subject of a paper by Dr. De 

 Marchi in \.h(t Keale 1st . Lomb. Rend. (vol. xiii. fasc. xix.). The 

 results he arrives at are summed up thus : I. Any traction of a 

 metallic wire increases in general its resistance ; when the traction 

 is very slight however there is diminution instead of increase ; 

 with increase of traction the case comes under the general law. 

 2. In general the increments are proportional to the increments 

 of traction, up to a certain limit, beyond which the variations of 

 re-istance are manifested in sudden bounds, indicating an instan- 

 taneous and profound perturbation of the molecular state of 

 the wire. 3. The law of increments of resistance is apparently 

 independent of that of the elongations. 4. Any vibration of a 

 wire is accompanied by a variation of resistance generally very 

 perceptible. In most cases there is decrease of resistance if the 

 vibration be sonorous, and more so if harmonic; increase, if 

 the vibration be silent. This last law however requires con- 

 firmation. 



It is known that M. Plateau distinguishes between an internal 

 and a surface viscosity of liquids, a distinction which Signor 

 Marangoni does not consider warranted. Herr Oberbeck (IVied. 

 Ann. No. 12) has approached the question experimentally thus : 

 A brass cross is hung bifilarly with two platinum wires by one 

 arm ; its horizontal arms carry weights whose positions can be 

 varied by screwing, s 5 as to vary the swing ; it carries a mirro 

 reflecting a scale, and to the lower arm is attached a thin plat 



