HARD WICKE 'S SCIENCE - G OS SIP. 



171 



had been conclusively proved to be vegetable, i.e. 

 -a lichen. 



I found by extended inquiry that the Portland, 

 the Bath, and the Barnacle (also lower oolite) stone 

 were all liable to the blackening in the course of 

 time. But the churches and buildings of sandstone, 

 in that most smoky of towns, Wolverhampton, were 

 not blackened at all. Evidently it was an effect 

 peculiar to limestone. 



I observed further, that in fluted columns, window- 

 jambs, arches, &c, the blackness was always in pro- 

 portion to the absence of sun-rays, and that the stone 

 remained quite white where the sun shone full upon 

 it. If any one will walk round St. Paul's, and com- 

 pare the south with the north side, he will see the 

 difference. Let him also notice the lines left white 

 by the oblique rays of the summer sun on parts of 

 the north wall. 



In Bath, you may see whole ranges of buildings, 

 like the Circus, so black on the sunless side, that in 

 many cases the walls have been painted with black 

 paint, as giving at least a more shiny and respectable 

 black than my mischievous little lichen, which has a 

 dingy, sooty, uncanny appearance. 



In other parts you may see a wall on the north side 

 perfectly black, while the east wall of the same 

 building is perfectly white. 



On the smoke-theory, this is inexplicable ; on 

 the light-theory, it is precisely what we might 

 expect. 



For if this blackness is really, as I now fully 

 believe, due to the gradual growth of a lichen, we 

 may conclude that it dislikes surfaces warmed and 

 dried by the sun ; and it is also to be inferred that 

 the lime is a necessary part of its food. I examined a 

 curious lichen, that grew in circular patches on the 

 Barnack stone in Peterborough Cathedral, and I found 

 that it had the property of extracting quantities of 

 lime from the texture of the stone. 



But its extremely slow growth, requiring a long 

 series of years before complete blackness results, its 

 very hard and stony texture, its amorphous form, and 

 its extremely low organization, — the lowest, perhaps, 

 that vegetable life can possibly possess, — render its 

 history a very interesting one. 



The practical result of the inquiry would be, to 

 ascertain if Portland and Bath stone can be treated with 

 some chemical solution, such as sulphate of copper, 

 which would prevent the growth of the lichen, sup- 

 posing it really to be such. It would be a valuable 

 scientific discovery that a brush and a pail would 

 restore surfaces that no water-washing will keep 

 dean, and not only restore, but prevent from 

 future discoloration. 



If it be true that the beautiful Caen stone used in 

 Paris does not become thus black, it must be due to 

 causes well deserving of investigation. One cannot 

 help hoping that some process of "pickling" build- 

 ing-stone may be discovered, which will tend to 



make churches and mansions less like an undertaker's 

 hearse. 



I have not given, in this brief paper, the substance 

 of nearly all the observations, experiments, and 

 reasonings, which led me to the result I have de- 

 scribed. But I think some grounds of probability 

 have been shown, enough to encourage those who are 

 competent to prosecute the inquiry. 



It is still open to conjecture, that some chemical 

 change in the texture of the stone, — some oxidizing 

 process gradually effected by the air,— may be the 

 cause of the blackness. But the singular effect of 

 sun-light in preventing it is a fact beyond all question, 

 and one that must be borne in mind in forming any 

 conclusion on the subject. 



A CHAPTER ON MICROSCOPIC FUNGI. 



(Perisporicurei. ) 



By Greenwood Pim, M.A., F.L.S. 



THIS is a small but very interesting group of leaf 

 parasitic fungi, and includes most of the forms 

 popularly known as Mildews. In their immature 

 condition three species of this order form the mildew 

 of the rose, vine, and hop, respectively. That the 

 vine mildew is a member of this group is to a certain 

 extent an assumption, as its perfect fruit has never 

 yet been discovered ; and it is only by its analogy 

 to, almost its identity with, the rose, pea, and hop 

 mildews that it is believed to be the conidiophorous 

 condition of an allied species. 



All Perisporiacei consist at first of a woolly growth, 

 consisting of delicate threads of concatenate cells, 

 arising from a mycelium, which makes its way through 

 the parenchyma of the stems and leaves of the plant 

 on which it occurs. Under a low power of the micro- 

 scope a mildewed rose-leaf looks like a delicate forest 

 of crystalline vegetation. The threads break up very 

 easily into their component cells, each of which, on 

 meeting with a suitable nidus, immediately com- 

 mences a separate existence, and, as is well known to 

 rose-growers and others, spreads with a rapidity 

 almost marvellous. In this condition the various 

 species obtained the name of Oidium, and it is only 

 comparatively recently that the identity of the Oidium 

 with the fully-developed conceptacles of the mature 

 form has been demonstrated. 



In the case of the vine mildew, only the oidium 

 or conidiophorous condition is known ; the cells of 

 these threads being known as conidia. In the other 

 species, towards autumn a kind of spherical capsules 

 are formed, each containing one or more sacs or asci, 

 which include 2, 4, 8 or more spores. These concept- 

 acles are usually furnished with curiously-formed ap- 

 pendages, threadlike, curved, hooked, horned, needle- 

 shaped, forked, &c. By these characters, as well as 

 he number of asci and spores, the species which were 



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