170 



HARD W1CKE 'S SCIENCE ■ G OSSIP. 



plaster of the mould tinted with washing-blue and 

 the perfectly white plaster of a better description 

 used in the casting, insert the edge of a broad chisel, 

 and tap it gently with the mallet ; make several very 

 cautious attempts along the edge of the two plasters, 

 until there is an evident sign of a parting having taken 

 place ; now plunge the whole under water, or pour 

 some over the edge, and the shrinkage will give the 

 desired cleavage. If, in endeavouring to break open 

 the plaster, a portion of the cast is knocked away, it 

 can be replaced, the cords drawn up through the half 

 set plaster have served to cut it up, but if the fish is 

 round, and the plaster has much "under-cutting," it 

 will require much skill to break away the mould 

 without injury to the cast beneath. It will be well, 

 therefore, to commence operations with a flat fish, 

 whose head, mouth, and eyes are the only highly 

 developed portions ; the fins can be pinned out, after 

 the skin has been washed with great care, to remove 

 all the slime with which most fish are covered. The 

 body of the mould taken from a flat fish of course 

 will come away like a seal, the impression beneath 

 being devoid of undercutting, but the head of the fish 

 cast requires great care, as about it are portions of 

 great delicacy of structure which may break away, 

 Casts of fishes in plaster are only of value to the 

 student ; they are heavy, and liable to injury ; to be 

 worth having, fish-casts should be made in paper. 

 From the plaster moulds we have described these 

 paper casts are made by successive layers of clean 

 white paper, and paste made from rice flour, backed by 

 coarse paper, chips, and thick wood shavings, inter- 

 spersed in the work, or a bit of common wire netting 

 cut to fit the mould, if the fish to be represented is 

 large in size, as a salmon or pollack. The first layers 

 of white paper need being spread with extreme care, 

 and if edges do occur in these first layers, the paper 

 must be torn and not cut, as the joining cannot other- 

 wise be concealed. The plaster cast cannot be painted, 

 all the attempts to make plaster and colour agree having 

 hitherto failed, andwhere gilding is resorted to, the sharp- 

 ness of the cast is utterly ruined. The paper cast can 

 be painted in fine washes of water-colour, or gold and 

 silver. The varnish over water-colour does not.injure 

 the sharpness of the mould, and it is possible so to 

 imitate a fish, in this way, that the veiy keenest 

 angler may be deceived as to the material. l There 

 are some fishes which can be cast showing both sides, 

 as a gurnard, or a cat-fish. Suspended by a fine 

 line, these casts are capable of accurately representing 

 the originals, but there is one difficulty about plaster 

 casting, — it is, without exception, the dirtiest occupa- 

 tion upon which an amateur can engage himself. 

 The plaster (if not properly dealt with) will follow 

 the operator over the house, adhering to his boots 

 and clothes, and hands, to the great discontent of 

 servants and others, who object to the filthy traces 

 which are so difficult to remove from floors and 

 carpets. 



IS THE BLACKNESS ON ST. PAUL'S 



MERELY THE EFFECT OF SMOKE ? 



By Professor Paley, M.A. 



MANY years ago I took a great deal of pains 

 to investigate a question which to many, 

 perhaps, will appear both trifling and useless, but 

 Avhich really has an important bearing on the aspect 

 of our great public buildings. It is often said, that 

 Paris is not such a smoky city as London, because 

 the stone buildings are much whiter. The north side 

 of St. Paul's Cathedral must, from its extreme black- 

 ness (curiously relieved as it is by lines and patches 

 of light), have attracted the attention and excited 

 the regret of most observers. There are other 

 buildings, of course, built with the same kind of stone, 

 which are equally black ; there are even towns, such 

 as Bath, built entirely of a similar (oolite) stone, 

 where all the new houses are of a rich creamy colour, 

 but most of those built a hundred years ago are as 

 black as a piece of black cloth. 



From investigations I made, and which I think 

 worth being recorded, in order that further inquiries 

 may be conducted with patient and scientific care, I 

 was led to believe that this blackness is due to a 

 hitherto unknown and undescribed species of lichen. 



Two of its peculiarities are, that it only grows 

 upon some kinds of limestone, and it will not grow 

 where the rays of the sun fall directly upon the sur- 

 face. 



I first noticed the latter fact in a wall of rusticated 

 Italian work at Cambridge. It faced due west, and 

 it overlooked the country for many miles, so that 

 smoke was not likely to have caused the blackness. 

 The whole wall in this part reminded me somewhat 

 of the lights and shades of a photograph. Those 

 surfaces were quite black on which the sun could not 

 fall, and those remained quite white on which the 

 rays were directly incident. I concluded that, at 

 least, sun-light was in some way concerned in the 

 appearance produced. But what reason can possibly 

 be alleged why a stone should contract less soot in 

 the light than in the dark ? 



I proceeded to scrape off some of the black sur- 

 face, which I collected, in the form of black dust, 

 exactly like gunpowder. If, I argued, the blackness 

 is really soot, surely a washing in hot water with 

 soap or soda will bring me white lime-dust, or lime- 

 sediment. But no ! I might as well have tried to 

 "wash a blackamoor white." The gunpowder was 

 gunpowder still, as far as the look of it went. 



Then I tried the microscope. The washed granules 

 were intensely black, somewhat amorphous in aj)- 

 pearance, and more or less angular. My power was 

 not very high, and my knowledge of such very 

 minute cellular structure was too small : I could not 

 say whether the object was organic or inorganic. I 

 was afterwards told that under a good microscope it 



