190 



KNOWLEDGE. 



[August, 1902. 



is now ready for further treatment, viz. : etching. The object 

 of tliis is to further Jevelop tlie structure. 



Etchiug is done by means of various reagents, the choice of 

 wliich is mainly a matter of ])ersonal opinion, but perhaps the 

 most generally used, an<i the best for beginners, are infusion of 

 liijuorice root and tincture of iodine. Very dilute nitric 

 acid and sulphuric acid are also used, but until the student 

 has become thoroughly accjuainted with the effects of the above 

 lie is not advised to use them. 



Before proceeding further it is advisable to give an outline 

 respecting the effects of the reagents, also the construction of 

 the metal. 



Steel is viewed as if it were a rock with various constituents 

 in it. There are three principal ones, viz. : Ferrite, Cementite, 

 and Pearlite (or Sorbite). « 



Ferrite: — This is free from carbon; it retains a very dull 

 polish, and is not stained by iodine or liquorice. To develop 

 the crystalline structure of Ferrite, a very dilute solution of 

 nitric acid in alcohol should be used. 



Cementite ; — This is a very hard substance, and stands in relief 

 after polishing as above. It is very rarely found in low-carbon 

 steels, and is left bright after the polished surface is attacked by 

 iodine. 



Pearlite .-—This is a very intimate mixture of Ferrite and 

 Cementite. If the steel has been allowed to cool slowly from 

 a very high temperature, Pearlite assumes a well-defined lamellar 

 structure ; on the contrary, if the metal has been forged, or re- 

 heated at a very low temperature, Pearlite assumes a granular 

 appearance. It is readily acted upon by iodine or liquorice. 



From this it will be seen that steel is made up of : (1) Ferrite 

 and Pearlite, (2) Pearlite, (,3) of Pearlite and Cementite. Other 

 constituents are found in steel after it has undergone certain 

 treatment, but enough has been said to guide the student to 

 make a commencement. 



The method of applying the reagent is as follows ; — The 

 S])ecimen is either coated with some i)rotective varnish — leaving 

 the surface free that is to be acted upon — and immersing the 

 whole in a bath ; or a few drops may be applied to the surface, 

 and then carefully spread by means of a glass dipping rod. The 

 solution should be allowed to act, say, for twenty seconds, then 

 carefully washed in alcohol or methylated spirits, gently rubbing 

 the surface with the little finger, finally washing in water, and 

 drying with a very soft piece of linen. 



The metal is now examined under the microscope, and it will 

 then be seen if the etching has been sufficient ; if not, it should 

 be repeated as above for another twenty seconds. The student 

 should do this several times, noting the effect of the reagent 

 each time until he becomes thoroughly acquainted with its 

 properties. 



So far we have only dealt with steel, but alloys of tin, copper, 

 &c., are treated in exactly the same way, with the exception that 

 liquorice and iodine are not used. The various acids, am- 

 monia and caustic potash, &c., are used in weak solutions as 

 etching reagents. 



With respect to the mounting of the specimens, from what has 

 already been s;iid regarding the new holders, it will be seen that 

 the affixing to glass slides is no longer necessary. Where a glass 

 slide is used a great deal of extra work is involved, because it 

 sometimes happens that the under side of the metal is left 

 jagged, or may be broken off at a very sharp angle, and before 

 this could be mounted there would be a long delay in filing and 

 grinding, also the thickness of the block of metal would have 

 to be kept within certain limits. No consideration has to be 

 paid to these matters when a holder is used. 



Some workers bed the metal on a glass slide in modeller's wax, 

 and then to render it quite plane attach to the microscope nose- 

 piece an uprighting tool and bring it down on the face of the 

 metal under examin.ation. It will be at once seen that all this 

 is really not needful with a champing holder. 



NOTES AND QUERIES. 

 Siiinia. — A glycerine immersion objective cannot he so efficient 

 as the homogeneous one, or that which is used with cedar wood 

 oil. Glycerine has not a refractive index the same as crown 

 glass, being only 1"47, and its dispersion is too high ; moreover, 

 its hygroscopic properties are against it. It has been pr.actioally 

 abandoned since the superiority of cedar wood oil has been 

 established. An air-angle of 170° would correspond to a 

 N.A. of -'jyu, which is very low for an immersion lens, 



J. IlilU. — Presumably you wish to know the best way to keep 

 insects that you may collect for future mounting as micro 

 objects. If so, you cannot do better than place those with hard 

 chitonous bodies in methyhited spirit, the more delicate ones 

 might bo more satisfactorily preserved in spirit and water, half 

 of each. 



Note.-— The Centipedes from Portugal which were offered 

 for distribution in the June number have been identified as 

 Seuliffera cohoptoata, and are stated to be very fine specimens. 

 A few of these are yet available on receipt of a stamped label 

 and a glass collecting tube. 



Communications and enquiries on Microscopical matters are 

 cordially invited, and should he addressed to M. I. CROSS, 

 Knowledge Office, 326, High Holbom, W.C. 



NOTES ON COMETS AND METEORS. 



By W. F. Denning, f.r.a.s. 



Jean Louis Pons, who began his comet-seeking about a century ago, 

 must still be regarded as the most successful worker who lias ever 

 ent.red this field. He was born in 1761, and died in 1831. When 

 doorkeeper at the Observatory of Marseilies he was instructed and 

 encouraged by the Director, lliulis (tlie discoverer of Eneke's comet 

 at its return in 1805;, to take up tlie searcli for new comets, and Pops 

 followed the work so successfidly durin;; the 26 years from 1801, 

 July 12, to 1827, August 26, that he independently discovered 37 

 comets. In several instances he was, ho»ever. anticipated by other 

 observers, tliough the fact remains that during the first quarter of the 

 19th century he discovered nearly all the comets tliat were seen. 

 During the 14 years he remained at Marseilles he picked up IS new 

 comets. His initial success was in 1801, July 12, but several other 

 astronomers, including Messier, Mechain, and Bouvard, shared this 

 discovery. It was just a century ago, namely, on 1802, August 26, 

 that Pons could claim to liave been absolutely the first discoverer of a 

 comet. Among tlie mure important objects he foimd may be 

 mentioned the periodical comets of Biela (1S05), and of Encke and 

 Winnecke (1819). In 1812 he picked \i\> a comet which proved to 

 have a jieriod of about 71 years, and belonged to the Xeptuiie family. 

 Pons supplied a pi'omineut example of an individual entirely devoting 

 himself, with an abiliiy only matched by bis diligence, to one field of 

 observation and achieving success beyond any historical parallel, either 

 before or after his time. 



Comet 1898 I. (Pekhine). — A definitive deteiTnination of the 

 orbit of this comet has been made by Heber D. Curtis, of Virginia. 

 In all the comet was observed 666 times lor position, and 34 different 

 observatories participated in the work. The comet remained visible 

 during about eight months (March 20th to November 15th), and an 

 especially fine series of observations were secured by tlie discoverer, 

 Mr. C. l3. Perrine, witli the 12 and 36-incli equatorials of the Lick 

 Observatory on Mount Hamilton. From the visual estimates of the 

 diameter of the coma, ranging from 2 minutes of arc on the date of 

 discovery to 10 seconds of arc at the middle of Ncvember, there 

 seems to have occurred a considerable shrinkage, the dimensions at 

 the two periods being 84,000 miles and 12,000 miles, while the distances 

 of the comet from the earth were 147 and 247 milhons of miles 

 respectively. The following are the definitive elements ■ — 

 Kquinox of 1898-0. 

 P.P. = 1898 March 17-130777 G M.T. 

 o) = 47° 19' ll"-85. 

 a. = 262° 26' 19 "06. 

 i = 72° 31' 47"01. 

 log. q = 00395112. 

 e = 0-9803852. 

 Period = 417-2 ± 2-2 years. 

 AlTGTTST Pebseids.— During the first half of August, moonlight 

 will offer little impediment to the visible progress of the display, and, 

 with clear weather, it will be possible to obtain an unusually success- 

 ful series of observations. There are several points in connection 

 with this rich annual shower which require further investigation, viz. : 

 the horary number of meteors visible, the exact position of the 

 radiant on each night of observation, and the character of the 

 radiation (i.e., whetlier limited to a contracted area or diffused over 

 a comparatively large space). Observers will also do well to record 

 the times and apparent courses of the brighter meteors seen (es)iecially 

 such as are uon-Pei->eids) so that data may be avadablc for computing 

 their real paths in the air. As 1900 was not leap-year the maximum 

 number of meteors will appear on the morning of Auaust 12 or 13. 

 An ephemeris of the radiant, based on a large number of observations 

 obtained in previous years, will be found in the Monthly Notices for 

 December, 1901, p. 109. 



