SI 



KN<)\\I.i:i)C.I 



rilHKfAKV. I'll. 



is exceptional, since lines separated by «,',o in. cannot l)i' 

 resolved by the luLiidcd eye. bnt tin- (cndcMicy to close up 

 is always (ircsont .iiul selecting soniclliinj; with vi-ry small 

 distances apart tends to show more clearly the phi-noincnon. 

 It is also found that the spreading varies with iliU'iTi'iil plates; 

 in one instance the vari.ition ln'tween a collndiim plate and 

 the same imaue taken on a (jdatirie one, ainonnled to .i ditlcr- 

 ence of ,',, millimetre in favour of the collodion even althonK'h 

 this h.ad been intensified. In I'l^ure 83 is shown the result 

 of doubling the exposure in taking the photo-mierof;raph. from 

 which it will be seen that tlie lines are tendiuK to become 

 thinner, due to the extra len^'th of exposure (jiven in the taking' 

 of the pliolo-micro.irraphs. These examples should be of 

 interest as illustrating the Rreat care necessary in photo- 

 sraphinj; subjects of this nature. 



rRlNTINC WITH S.M.TS Ol' CIIKOMUM. riH' 

 fdUowini; printing process, due tn the inveslif^ation of Robert 

 Hunt, and announced by him in 184J. imder the name of 

 ■■ Chromatype." deserves attention owinj; to its simplicity 

 together with tlie good results obtainable by its means. 

 Well-sized paper is washed over with a solution of cojiper 

 sulphate, and when dry coated with a fairly strong solution of 

 potassium bichromate. The paper when dry may lie kept for 

 a short time ready for use. In order to employ this paper, it 

 is pLaced with the object or objects upon it, in a printing 

 frame and the whole exposed to light for a sufficient length of 

 time to render the paper almost white, when a yellow positive 

 on a white ground results. On removal from the printing 

 frame the paper is floated upon a solution of silver nitrate 

 when the parts that have been unacted upon by light at once 

 assnme a dark purple-red colour from the formation of silver 

 bichromate. To fix the images the prints are well washed in 

 distilled water free from chlorides, as these latter have a 

 destructive action, even when present in small ([uantity. 

 Prints produced by this method appear to be stable, not 

 undergoing any change when exposed to strong light for a 

 considerable time. 



EXPOSURE TABLE FOR FEHRLWKY.— The calcula- 

 tions are made on the actino^raph for plates of speed 200 H 

 and D, the subject a near one and lens aperture F16. 



PRINTING WITH COB.-\LT O.XALATE.— This process, 

 originally devised by Messrs. Lnmiere, is capable of giving 

 very fine results. Unfortunately, perhaps, a certain amount 

 of skill in chemical manipulation is necessary to carry it out 

 successfully. It is based upon the action of light in reducing 

 cobaltic oxalate to cobaltous. and this on suitable treatment is 

 made to yield an image finally in sulphide of cobalt. In our 

 first experiments cobaltous hydrate was suspended iu water, 

 through which a current of chlorine was passed, until a black 

 precipitate of cobaltic hydrate was formed, and to ensme the 

 action being complete caustic soda was also present in the 

 water as well. The black precipitate was then carefully washed 

 and dissolved in a calculated quantity of o.xalic acid, it being 

 of importance not to have an excess of acid. The formation 

 of the oxalate should be gradual, and the operation allowed to 

 go on in darkness. The dark green solution of cobaltic oxalate 

 was then employed to sensitize gelatine-coated paper, which 

 must be used as soon as dry. The printing under a negative 

 is fairly rapid, the colour of the paper changing over the parts 

 exposed to light to a pale rose tint, due to the formation of 



cobaltous oxalate. The prints on removal from the printing 

 frame are treated with a 5 % solution of potassium ferricyanide. 

 which deepens their colour; they are then washed in running 

 water for about thirty minutes, after which they are placed iu 

 a dilute solution of annnonium sulphide for times varying with 

 the coloin' desired. A short immersion gives a sepia, .'i 

 prolonged one black. .\ fin.d washing completes the operations. 

 .\ simplified method of preparation of the oxalate consists in 

 treating cobalt sulphate with soda peroxide (Naj Oi), adding 

 the latter gradually and keeping the solution well cooled to 

 prevent decomposition. The precipitate is then well washed 

 to free it from soda sulphate, and dissolved in a calculated 

 ((uantity of nxalic acid .-is before. 



PHYSICS. 



By Al.iKi :l> C. G. Egerton, B.Sc. 



SIL1C.\. — Fused silica is now used largely in place of glass 

 and porcelain for vessels which have to stand heat without 

 risk of cracking. It possesses an exceedingly small coefficient 

 of expansion. Conse(|ucntly. when suddenly heated the 

 mechanical forces set up in the material owing to the expansion 

 are not great enough to cause cracking, as in the case of 

 similar bad-conducting substances with large coefficient of 

 expansion — such as glass or porcelain. It is eminently 

 suitable as the material for a primary standard of length. It 

 is cheaper than platinum or iridio-platinum, and is not so 

 greatly affected by change of temperature as silica. It has a 

 coefficient of expansion seventeen times less than that of 

 platinum. Invar, the nickel iron alloy, which possesses a 

 smaller coefficient of expansion than any other metal, is not 

 so suitable a material as silica for a standard of length. Dr. 

 (;. Kaye has prepared a standard metre from silica. It cnn- 

 sists of a tube of silica with slabs of the same material fu.sed 

 into its ends. The slabs are optically worked planes, and 

 platinised on the underside ; the platinum film is then ruled 

 with a diamond so as to give the defining lines between 

 which the standard length is measured. 



It will be remembered th.at the standard metre is the 

 distance between two marks on a bar of platinum which is 

 preserved at the International Bureau of Metric Standards, 

 Saint Cloud, near Paris. It was originally made to be as 

 nearly as possible equal to one ten-millionth of a quadrant of 

 the earth from equator to pole. However, it is now known 

 that the length of the standard metre is not exactly the 

 ten-millionth of the earth's quadrant, but this latter is equal to 

 10,001.472 metres. 



.Mthough the standard metre, and, in fact, any such arbitrary 

 standard that is based on the length of a certain piece of 

 metal, might alter in the course of ages owing to certain cosmic 

 changes, yet they can always be checked against the lengths 

 of certain waves of light. Different elements give out light of 

 different wave lengths ; each element has its own characteristic 

 spectrum. One can choose some special element and then 

 choose some special wave-length of light, given out by that 

 metal and find how many such wave-lengths make the 

 standard metre ; thus the standard of length can be expressed 

 in terms of the wave-length of a certain line in the spectrum 

 of a certain metal. This has been done by Professor 

 .Michelson. for the red line given by the metal cadmium. 



The manuf.-icturers of vessels of fused silica have so perfected 

 their processes that the most complicated apparatus can now 

 be obtained, and it is an economy to buy apparatus which is 

 to be subjected to sudden changes of temperature made of 

 silica rather than glass or porcelain. Silica is not attacked by 

 acids, with the exception of hydrofluoric acid. It melts at 

 about 1,600" C. It is harder than ordinary glass, .\bove 

 1 ,000" it is permeable to hydrogen. It is also readily permeable 

 to helium, and recently Professor O. W. Richardson has shown 

 that neon can penetrate through the walls of a tube made of 

 silica, though less easily than helium. It expands regularly 

 up to nearly 1 ,000' C, and is exceedingly well suited for making 

 high temperature mercury thermometers, which register tem- 

 peratures up to 700° C, the mercury being under pressure. 

 Owing to its small coefficient of expansion, silica is a convenient 



