May lo, 1877] 



NATURE 



23 



Third do., faint, grey , 



The wave-lengths of the bands, and other positions in the 

 spectrum, roughly obtained, by which it may be possible to iden- 

 tify some of them in photographic spectra, although open to 

 some uncertainty from the inconstant length and strength of the 

 arc of llame in the electric lamp, which confused and shifted 

 some of the comparison lines, were as follows : — 



Electric Arc with Carbon Poles. Wave-lengths. 



End of the violet field (-4, K|3, and last 

 violet line in arc-spectrum of strontium, 

 4,080-4,100) About 4,100. 



First light-band ; faint violet-grey (IIj, 

 3,968 ; H„, 3,933) About4,ooo~3,95o. 



Second do., strong grey band | "^"^o"' 800™ ^'^°° 



(.Strong grey line of mercury About 3,700). 



\ Between 3,600 and 

 { 3,500 (?). 



Other metallic arc-spectra probably present lines in this por- 

 tion of the spectrum, of which it would be interesting to examine 

 the apparent brightness and the colours. At present the most 

 conspicuous that I have met with is the grey line of mercury, 

 which is brighter and more refrangible than the grey band of the 

 electric light between carbon points. Its very advanced position 

 in the spectrum, and the absence, or negative appearance of 

 colour in its pretty bright light, both taken together seem to 

 indicate very clearly that the grey or "lavender-grey " division 

 of the spectrum fully equals in extent, when seen prismatically, 

 the violet, the indigo, the blue, or any of its other better known, 

 and much more ordinarily visible companion regions, the seven 

 Newtonian colour-spaces of the spectrum. A. S. Herschel 



College of Science, Newcastle-on-Tyne, April 26 



Pele's Hair 



I HAVE read with great interest Mr. Moseley's description of 

 Pele's Hair in NATURE (vul. xv., p. 547), since it furnishes 

 information which I was most anxious to obtain. It seemed to 

 me extremely probable that the analogy between Tele's Hair 

 and the aitificial furnace products would not be confined to the 

 long fibres, and I did my best to ascertain whether irregular 

 glassy spherules occurred along with the natural products. I 

 was unable to obtain sjecimens for examination, but paid a visit 

 to my friend Mr. J. G. Sawkins, F.G.S., who had explored the 

 crater and collected the hair, in order to ask him whether he 

 had ever noticed the pear-shaped spherules. lie told me that 

 he had never seen arything but the glassy fibres. I must say 

 that I lelt very much inclined to believe that the specimens 

 usually collected ate the material which has been blown 

 some distance by the wind, consisting of the fibres from 

 which most of the spherules have been broken. Mr. 

 Moseley's letter in Nati;re, and another which he has 

 kindly addressed to me, make me believe that the analogy be- 

 tween the artificial and natural products is more complete than 

 I was able to ascertain before Mr. Moseley's observations were 

 published. In conclusion I would say that these facts in no 

 way invalidate my arguments in respect to meteorites. They 

 merely show that in certain cases the glassy volcanic spray, like 

 melted furnace-slag, can to some extent collect into more or less 

 imperfect spherules, so far analogous to those in meteorites as to 

 indicate how those remarkable bodies were formed, but these 

 spherules are accompanied by many fibres, which I have never 

 yet seen in meleorites. This difference appears manifestly to 

 depend on ihe difference in the temperature of the space into 

 which the glassy spray was thrown. If the temperature of the 

 air in the crater of Kilauea were equal to that of the melting 

 point of the lava, we should almost certainly find, as in 

 meteorites, many spherules and no hairs. ' H. C. Sorby 



The Critical Point of Carbonic Anhydride 



As the writer is not aware that any attempts have hitherto 

 been made by others to exhibit to a large class the phenomena 

 attendiirg the passage through the critical point of a liquid in 

 the presence of its gas, he is of opinion that the following ac- 

 count of a method which be has found very successful may be of 

 interest : — 



Dr. Andrews's apparatus for the study of gases was employed 

 in the experiments, and the image of the tube containing the 

 carbontc anhydride was projected on a screen by means of the 

 oxy-hydrogen lime-light and a solar microscope which magnified 



it about 120 diameters. Dr. Andrews's apparatus consists of a 

 thermometer tube filled with carbonic anhydride and a second 

 tube filled with dry air, which serves to measure the pressure 

 applied. The lower ends of these tubes dip beneath the surfaces 

 of mercury contained in test-tubes, which are suspended in strong 

 copper cylinders communicating with each other, and filled with 

 water, which presses on the mercury in the test-tubes. The 

 pressure is applied by means of long steel screws which pass 

 through the bottoms of the cylinders. For the filling and mount- 

 ing of these tubes the University of Cambridge is indebted to 

 the kindness of Dr. Andrews. The lantern was supported on 

 three screws, which allowed it to be raised or lowered so as to 

 bring any required portion of the thermometer tube into the field 

 of view of the microscope. The best height for the lantern was 

 found to be such that the top of the tube was rather less than 

 half an inch above the axis of the microscope. When the 

 oxygen was turned on, the radiation from the lime cylinder 

 raised the temperature of the portion of the tube within the field 

 of view above the critical point in little more than a minute, so 

 that no other source of heat was required ; but when the oxygen 

 was turned off the tube cooled through several degrees. 



The best method of performing the experiment is .as follows : — 

 The lantern having been properly adjusted, the gas should be 

 lighted, the Oxygen turned on, pressure applied until the sur- 

 face of the mercury conies into the field of view and the micro- 

 scope focussed so as to give a distinct image of this surface. The 

 pressure should then be relieved and a blast of cold atr from a 

 bellows or gas bag directed against the tube. This will cool it 

 considerably below the critical point. The pressure should then 

 be increased, the cold blast being continued until the inverted 

 image of the concave surface of the liquid reaches the middle of 

 the field of view appearing as a broad dark line possessing con- 

 siderable curvature, and, of course, concave downwards. The 

 focussing screw should now be finally adjusted so as to give the 

 best image of this surface, and the blast then stopped. Imme- 

 diately after cutting oft' the blast the operator must obtain com- 

 mand over one of the screws and carefully increase the pressure 

 as the temperature rises so as to keep the image of the liquid 

 surface just above the centre of the picture on the screen. As 

 the temperature and pressure increase the broad iniage of the 

 surface becomes narrower and less concave until, as the tem- 

 perature approaches the crttical point, the line becomts very 

 tliin and faint and loses its curvature altogether ; it then seems 

 to explode into mist and vanish as the critical point is reached. 

 Another half turn of the screw then produces the well-known 

 clouds or flickerings, which are best seen on the screetr some- 

 v.bat below the middle of the field, and in a few more seconds 

 all is steady. More pressure should then be applied until the 

 mercury rf^aches the axis of the microscope, but no change of 

 state will be manifested by the carbonic anhydride. 



It is important that the image of the surface of the liquid 

 should not be below the centre of the field of view on the screen, 

 for if the liquid stand in the tube above the axis of the micro- 

 scope, since the greatest heat is there concentrated, bubbles of 

 gas are liable to be formed within the liquid and to damage the 

 continuity of the surface. Perhaps the flickerings may be due to 

 unequal temperatures at different parts of the tube, so that some 

 are just above and others just below the critical point. The 

 mode of propagation of a sound wave through a substance just 

 at the critical point may be an interesting subject for inquiry. 



After passing the critical point the blast of air should be 

 directed against the tube for about a minute. This will, of 

 course, cause the image of the mercury to descend upon the 

 screen, but no change of state will appear to take place in the 

 carbonic anhydride. The pressure should then be rapidly dimi- 

 nished by turning the screws, when a violent ebullition wiU be 

 seen, showing that the whol^ of the contents of the tube had 

 assumed the liquid state during the cooling, the gas having 

 passed at the critical point into the liquid without breach of con- 

 tinuity, so that no indication of a change of state was apparent 

 on the screen. On increasing the pressure and continuing the 

 blast the liquid surface will again appear, and the experiment 

 can be at once repeated. \Vm. Garnett 



Cavendish Laboratory, Cambridge 



Floating Cast Iron 



Having read the interesting letter on this subject which ap- 

 peared in Nature (vol. xv., p. 529), I send the following copy 

 of notes of experiments which I made about three years ago. 



