18i9.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



343! 



that this bed, which may be seen over a preat part of Denmark 

 always in the same position, the same fossiliological character, and 

 the same thickness in the hill of Faxtie, is enlarged to a thickness 

 which cannot be much less than 150 feet. Here the Faxoe lime- 

 stone is covered by a bed of dolomite, ivhich again is covered by a 

 bed of limestone, consisting almost entirely of fragments of 

 Bryozoa, and belonging likewise to the chalk formation. The 

 limestone of Faxoe contains about 1 per cent, of carbonate of 

 magnesia, arising from the shells and corals which always contain 

 it in a small quantity, but which in some instances, as in the Isis 

 and some Serpulre, amount to 6 or 7 per cent. The bryozoan lime- 

 stone which covers the dolomite does not contain more than 1 per 

 cent, of carbonate of magnesia, while the dolomite contains 16 or 

 17 per cent, of carbonate of magnesia. The dolomite occurs 

 generally in round globular masses; very similar to those of Hum- 

 bledon Hill, and are evidently (like most of the globular masses of 

 limestone, such as confetti di Tivoli, and the peastone from Carls- 

 bad) the produce of springs, — an opinion which is still more con- 

 firmed by a number of large vertical tube-like cavities, which pass 

 through the compact limestone, and are completely similar to those 

 described by several English geologists as passing through the 

 chalk, which have been recognised as the natural pipes of springs. 

 Thus the Faxoe dolomite is the produce of springs; but then these 

 springs have deposited stalagmitical limestone wherever they 

 have passed through the crevices of tlie limestone rock, which as a 

 more or less thick coating covers all the fossils. Now, this produce 

 of tbe springs contains only a very small quantity of magnesia, 

 but besides lime, a great quantity of oxide of iron. It appears 

 thus, that the springs do not deposit carbonate of magnesia, if no 

 other reaction takes place than the escape of carbonic acid; but 

 that the dolomite is formed where the carbonic acid springs come 

 in contact with sea-water. The author has made a great number 

 of experiments on the decomposition which takes place when water 

 containing carbonates dissolved by carbonic acid acts upon sea- 

 water, and found that always a more or less great quantity of car- 

 bonate of magnesia was precipitated with the carbonate of lime. 

 AVhen using water containing only carbonate of lime, the quantity 

 of carbonate of magnesia thrown down at a boiling heat amounted 

 to 12,^ per cent., the rest being carbonate of lime. The results of 

 this decomposition vary, however, very much, and according to 

 conditions not yet well known. So much, however, may be stated, 

 that the quantity of carbonate of magnesia precipitated increases 

 with the increasing temperature. AVater which, besides carbonate 

 of lime, contains carbonate of soda, throws down a much larger 

 quantity of carbonate of magnesia, amounting in one experiment 

 to 27 '93 per cent, of the precipitate. At last, the author tried 

 what kind of precipitate some of the most famous mineral springs 

 of Germany would form, if they at the boiling point aeted upon 

 sea-water. Thus, he obtained : — 



From the water of Sellers. 



Carbonate of lime 86'55 



„ maguesia 13'45 



loo-uo 

 From the water of Pyrmont, 



Carbonate of lime 84-33 



„ magnesia b-\2 



Protoxide of iroa 10 50 



lillf 01) 



The oxide of iron in the experiment was of course precipitated 



as peroxide of iron, and from tliat the carbonate was calculated — 



From tlie water of Wildengen, 



Carbonate of lime 92'12 



„ magnesia 7'8ej 



lOo-iiu 

 Remarks. — Professor Ansted agreed with the view of Professor Forch- 

 hammer relative to the formation of dolomite, at the same time observing 

 that there were undoubtedly several other ways in which it may be pro- 

 duced, but that suggested by M. Forchhammer was undoubtedly one. — Dr. 

 Daubeny was glad to see that such subjects were enjoying the attention of 

 chemists. Had Von Bucks had a small amount of chemical knowledge, he 

 would have avoided the elaborate but untenable theory relative to the forma- 

 tion of dolomite which he had advanced. 



6. On the Colouring of Glass by Metallic Oxides. By M. 



BoNTEJJPS. 



Oxide of iron gives usually a green colour, but by various 

 methods of treatment the author obtained by its use all the colours 

 of the spectrum. In the manufacture of earthenware a red was 

 obtained by iron, and at some degrees of heat it gave a yellow 

 colour. Manganese gives a purple or pink colour. The light 

 pink colour given by manganese is liable to change by exposure 

 to a low heat; it passes first to brownish red, then to yeUow, 



and lastly to green. Flint-glass, in which small quantities of 

 manganese are used, is liable to become of a light yellow by 

 exposure to light. Copper produces a fine red colour, and also 

 a green; the former being produced by the lowest degree of 

 oxidation, and the latter by the highest. Silver produces a 

 yellow colour, which may vary fi-om lemon yellow to deep orange. 

 Gold, in the form of the purple precipitate of cassius, gives a 

 ruby colour to glass, but it requires careful treatment; the mixture 

 when first melted is colourless, but becomes red on re-heating. 

 The various colours whicli at different temperatures the same 

 oxide produces, are attributable to some molecular change in the 

 glass. 



M. Bontemps has found that similar changes take place in the 

 annealing oven. He has determined, by experiments made by 

 him on polyzonal lenses for M. Fresnel, that light is the agent 

 producing the change mentioned; and the author expresses a 

 doubt whether any change in the oxidization of the metal will 

 explain the photogenic effect. A series of chromatic changes of a 

 similar character were observed with the oxides of co])per; the 

 colours being in like manner regulated by the heat to which the 

 glass was exposed. It was found that silver, although with less 

 intensity, exhibited the same phenomena; and gold, although 

 usually employed for the purpose of imparting varieties of red, 

 was found by various degrees of heating at a high temperature 

 and re-casting several times to gi^'e a great many tints, varying 

 from blue to pink, red, opaque yellow, and green. Charcoal in 

 excess in a mixture of silica alkaline glass gives a yellow colour, 

 which is not so bright as the yellow from silver, and this yellow- 

 colour may be turned to a dark red by a second fire. The author is 

 disposed to refer these chromatic changes to some modifications of 

 the composing particles rather than to any chemical changes in 

 the materials employed. 



Remarks. — Dr. Faraday said, in the beautiful facts brought forward by 

 M. Bontemps, it appeared that many of the changes of colour mentioned 

 are purely physical. The phenomena of the change of manganese fiom 

 white to pink in glass appeared to him inexplicable as a chemical effect. 



Mr. DiLKE inquired upon what peculiarity depended the .iiBerences dis- 

 covered to exist in the coloured glass of the windows of old churches and 

 that of modern manufacture. 



M. Bontemps stated that the observed differences were entirely due to 

 age and imperfections in manufacture. 



Dr. Faraday remarked that any irregularities tended to produce the 

 diffusion of the rays which permeate the glass; and that the opacity of 

 ancient church windows was probably due to a superficial change of the 

 external surface. 



M. Bontemps stated that old glass was by repolisbing rendered as trans- 

 parent as any modern glass. 



Dr. Faraday concurred with M. Bontemps in regarding the phenomena 

 of coloured glasses not as purely chemical nor purely physical, and bL'lieved 

 that it is only by considering them conjointly as physical and chemical that 

 they can be successfully studied. 



7. On the cause of the Colouration of Porcelain by Oxide of Iron, 

 and the general theory of Kilns. By M. Leon Arnoux. 



This communication, which was read by the author in the French 

 language, entered very extensively into the general detail of the 

 process of forming porcelain, and particularly adverted to many 

 defects to which it was liable in the process of manufacture. Par- 

 ticular attention had, however, been directed to discover the cause 

 giving rise to a disagreeable yellow tint which frequently presented 

 itself. The author comes to the conclusion that this yellow colour 

 is due to the presence of oxide of iron. It had been thought by 

 jM. Ebelman that infinitely small quantities of carbon produced 

 this defect, but all the experiments of M. Arnoux went to prove 

 the presence of iron whenever the porcelain presented this peculiar 

 tint. 



A short discussion followed between the author and M. Bontemps, the 

 latter contending that carbon was far more likely to produce this yellow 

 than iron, and instanced the effects of carbon on glass. He was disposed to 

 think that at the high temperature of firing, the presence of iron would give 

 rise to a blue rather than a yellow colour. 



8. On an improvement in the Preparation of Photographic Paper, 

 for the purpose of Automatic Registration, in which a long-continued 

 action was necessary. By Mr. Charles Brooke. 



The preparation of the paper described may be thus briefly 

 stated: — The paper is washed over by a brush with a solution of 

 12 grains of bromide of potassium, 8 grains of iodide of potassium, 

 and i gi-ains of isinglass in 1 fluid ounce of distilled water, and 

 dried quicklj'. When about to be used it is washed over by a 

 brush with a solution of 50 grains of nitrate of silver to 1 fluid 

 ounce of water, and placed on the cylinder of the registering 

 apparatus, on which it remains in action for twenty-four hours. 



