2o6 



NA TURE 



[June 29, 1893 



the London and North-Western Railway lighted Holyhead 

 Harbour by electricity. The effect of the five arc-lamps em- 

 ployed was to break down the block signals in the district 

 within a mile. But the difficulty was removed by supplying 

 metallic circuits to the signals. At Blackpool the disturbing 

 currents from the electric tramway had lowered a block-signal 

 on the railway and fired a time gun at the same moment, a 

 minute or so before the time when the gun ought to have been 

 discharged. 



In the recently published number of the Proceedings of the 

 Societe Fran9aise de Physique there is an account of a standard 

 condenser formed by two plates of silvered glass separated by 

 three blocks of quartz accurately worked to the same thickness. 

 The instrument almost exactly realises a theoretical condenser, as 

 the central part is only separated from the guard-ring by a 

 narrow line along which the silver has been removed. The 

 only disadvantage is that the insulation is rather bad, and when 

 the air is not perfectly dry there is a small current between the 

 central disc and the guard-ring. To get over this difficulty the 

 author (M. P. Curie) joins the electrometer to the continuous 

 plate of the condenser, charges the central disc of the other 

 plate with the battery, and connects the guard-ring with the 

 earth. Under these conditions the field of force between the 

 plates is no longer uniform, but the charge of the condenser is 

 the same as in the ordinary arrangement. With this airange- 

 ment the insulation is all that can be desired, as the quartz 

 blocks are very good insulators, and little affected by moisture 

 in the air. 



A SIMILAR condenser to that described above has been em- 

 ployed by M. Abraham in his determination of the ratio between 

 the electromagnetic and electrostatic units (see Proceedings of 

 Sociele de Physique, p. 332, iSy3). The method employed for 

 measuring the distance between the platei is as follows. In 

 front of the space between the plates a finely-divided glass scale 

 is placed with its plane perpendicular, and the lines of the 

 graduations parallel to the p ates. The silvered plates consti- 

 tute excellent mirrors, and give a series of images of the 

 divisions of the scale, the distances between which were 

 measured by means of a microscope. This method gives the 

 mean distance between the plates, which was found to vary 

 each time the instrument was set up, and to differ slightly from 

 the length of the quartz blocks employed to separate the 

 plates. 



The photographic study of sources of light by means of a 

 carefully graduated series of exposures was first applied with 

 great success by M. Janssen to the investigation of the minute 

 structure of the solar surface. M. Crova has applied a similar 

 method to the study of the carcel standard and the electric arc. 

 A contrast between the various parts of the magnified pholo 

 graphic image of the carcel flame does not appear until the ex- 

 posure is reduced to the minimum necessary to secure an 

 impression, and to bring out this contrast the negative must be 

 developed slowly and subsequently intensified. Four photo- 

 graphs thus obtained were exhibited at a recent meeting of the 

 French Academy. The axis of the flame appears dark, and 

 the zone of combustion exhibits two bright lines representing 

 the external and internal surfaces of combustion of the hydro- 

 carbons, with a dark line between them corresponding to the 

 space where combustion is incomplete. Photographs of the 

 flames of a candle, an amyl-acetate burner, and a bat's-wing gas 

 jet were also exhibited, showing analogous phenomena. The 

 same method applied to the arc light yielded some interesting 

 results. As the time of expo^ure was reduced the arc gradually 

 vanished, the negative carbon was reduced to a very small sur- 

 face, and the positive carbon exhibited a surface riddled with 

 NO. 1235. VOL. 48] 



dark spots, and granulated like the surface of the sun in M. 

 Janssen's photographs. These granulations could be seen in 

 violent motion on the ground glass screen of a camera with the 

 lens sufficiently stopped down.- It follows that it is not admis- 

 sible to screen off all but a very small portion of the luminous 

 source in order to reduce the amount of light in the same pro- 

 portion as the area of luminous surface. With very small jor- 

 face elements both the amount of light and the temperature, and 

 hence also the tint of the light, may be constantly changing. 



Hekr von Lupin, of Munich, has recently called attention 

 to two thermometer liquids as free from certain drawbacks of 

 the spirit thermometer. One of these is sulphuric acid diluted 

 with water. According to experiments by Sohncke, the quan- 

 tity of water removed by distillation in the thermometer-tube 

 was a minimum even when the free end was surrounded with 

 ice ; and (what is still more important) in a short time this very 

 small quantity was reabsorbed. The expansion of the liquid is 

 approximately constant. In a recent expedition by Herr Vogel 

 to Central Brazil these minimum thermometers were used, aod 

 found to act very well. The other liquid referred to is chloride of 

 calcium in spirit (10 to 15 per cent, of the anhydrous salt is 

 best). This is specially recommended for medical use, because 

 its pronounced colour enables it to be more easily read a; night 

 than the mercury thermometer. Here, too, there is no distil- 

 lation-error. A further advantage is that the thermometer takes 

 the body-temperature very quickly (in about three miautes). 

 The regularity of expansion between 0° and 50° C. is good, thoogb 

 not in the same degree as with sulphuric acid ; and the propor- 

 tion of calcium chloride is here influential. The solution, like 

 that of sulphuric acid, does not solidify even in the artificial 

 cold of evaporating carbonic acid snow ; and with the propor- 

 tion of salt given, no salt is separated out in the bulb. 



To the current number of the Zeilschrifc Jiir physikalisehe 

 Chemie Herr Altschul communicates 'rom Prof. Ost»ald's 

 laboratory a series of observations on the critical constants of 

 some fatty and aromatic hydrocarbons. Unlike most observen 

 in this field the author thus deals with chemically related sub- 

 stances which have a comparatively simple structure. Wiih 

 the ascent of a homologous series it appears that the critical 

 temperatures increase and the critical pressures decrease at rates 

 which gradually diminish. Chemical constitution also affects 

 the magnitudes of the critical values, the three metameric 

 xylenes, for example, have different constants. From his obser- 

 vations the author deduces the values of (a) and {b) in Van de 

 Waals's equation, Guye's critical coefficient, &c., and traces 

 relationships between their magnitudes. 



Is colour-blindness a product of civilisation ? An investiga- 

 tion described in Science by Messrs. Blake and Franklin, Phy- 

 sical Laboratory, Kansas University, favours an affirnialiTe 

 answer to the question. Of 159,732 persons tested in Europe 

 and America, nearly four per cent, were found to be colour- 

 blind. But when the ordinary Berlin worsteds were used to 

 test the colour perception of a number of Indians, representing 

 many tribes, only 3 in 418, or 07 per cent., were found to be 

 deficient. These were full-blooded Indians, and all males. It 

 appears, therefore, that, as with civilised peoples, the p<!r- 

 centage of cjlour- blind males is greater than that of females. 



The peculiar phenomenon sometimes observed near the 

 Wetter Lake in Sweden, and called by the natives Motalas- 

 troms stadnande, the standing still of the Motala river, has' been 

 the subject of speculation ever since the times when it used to 

 be regarded as a miracle and a portent. The Motala river 

 emerges from the Wetter Lake, and the phenomenon in ques- 

 tion consists in the cessation of the flow and the drying up />f 

 the. bed, accompanied by a letention of water within the lafce. 



