82 



SCIENCE-GOSSIP. 



[April 1, 1866. 



ported to all parts of the world for polishing steel 

 and other metals. The clay from which the scouring 

 bricks are made is composed of an immense number 

 of silicious shells of Infusoria, only to be distin- 

 tinguished by the aid of a powerful microscope. 

 Now, silex or flint is much harder than steel, and, 

 it being an elementary rule in mineralogy that the 

 harder minerals are capable of cutting the softer, 

 the roughness of the steel is easily removed, or, in 

 other words, a fine polish is produced. 



If you place a piece of steel under a micro- 

 scope the polish will be found to consist of minute, 

 equal scratches, produced by the hardness of tbe 

 broken shell. These shells lie probably at the 

 mouth of the river, between high and low water- 

 mark, mixed with mud, sand, &c, forming a species 

 of "Delta," and are washed up and carried on by 

 the rising tide and deposited at the reflux. That 

 this mud, impregnated with Infusoria, exists near 

 the mouth of the river, and above low water-mark, is 

 obvious. Firstly, because the deposit, being speci- 

 fically heavier than water, could not have been 

 carried far without falling to the bottom unless a 

 much greater mechanical force were exerted on it 

 than the sea under ordinary circumstances would be 

 capable of exercising. Secondly, because it is a 

 well-known fact, that the sea is never disturbed by 

 any external force, such as wind, tide, &c, more 

 than twelve feet below its surface — the deep sea 

 being always perfectly calm except where under- 

 currents exist ; and yet at the lowest tide, with the 

 surface of the sea unruffled, the deposit is steadily 

 brought in. Still further, in support of this view of 

 the case, there are extensive mud flats at the 

 entrance of the river, many of which are laid bare at 

 low water; whereas a little further on each side the 

 coast consists of sand and rocks. 



The fact of the clay being unfit for scouring 

 purposes a mile or so below the town, aud the same 

 distance above it, is easily explained. The Parrot 

 is one of those rivers into which the tide rushes with 

 considerable force, and, passing over the mud flats 

 in question, takes up mechanically, not only the so- 

 called clay, but also coarse sand, small stones, aud a 

 variety of other things ; but, having a winding 

 course of sixteen miles to pass over, before arriving 

 at Bridgewater, these last, together with the heavier 

 refuse, are for the most part deposited lower down ; 

 the lighter shells, together with the mud and some 

 of the sand, are carried up further, and deposited in 

 the immediate neighbourhood of the town. Hence 

 it is evident that it can only be collected from a 

 limited space along the river's bank, as the coarser 

 substances being mixed with it, its use for giving a 

 high polish is destroyed. 



This scouring-brick clay is not, however, entirely 

 peculiar to Bridgewater. It is, for instance, also 

 found on the banks of the Vistula, near Dantzig, in 

 Prussia, where, when the river overflows, as it does 



about once in twelve months, it leaves a deposit 

 very similar to our own; but, though extending 

 over a space of several miles, the deposit (being 

 only two and a half inches in depth) is quite useless 

 for commercial purposes. The long intervals 

 between the periods at which the river overflows is 

 also an insurmountable difficulty. At Bridgewater 

 the clay is obtained in the following manner : the 

 bank is cut away perpendicularly, so that a hori- 

 zontal bed is left on which the clay is deposited at 

 the rate of six feet in twelve months. Much more 

 is deposited in dry weather than in wet, in conse- 

 quence of the large amount of fresh water which 

 finds its way into the river in a rainy season, and 

 which is, of course, specifically lighter than salt. 

 The clay is afterwards dug up, well mixed and 

 ground, the object of which is thoroughly to unite 

 all the constituent parts, and to make it a uniform 

 quality. It is then moulded into bricks, dried, and 

 burned. 



There is another substance essentially the same 

 as Bath-brick, namely, tripoli, in speaking of which 

 Sir Charles Lyell says: — "There are a variety of 

 stony deposits in the earth's crust now proved to 

 have been derived from plants and animals of 

 which the organic origin was not suspected until 

 of late years even by naturalists. Great surprise 

 was therefore created by the discovery of Professor 

 Ehrenberg, of Berlin, that a certain kind of silicious 

 stone, called 'Tripoli,' contained millions of remains 

 of organic beings, which the Prussian naturalist 

 refers to microscope infusoria." Ehrenberg esti- 

 mates that in the Bilin Tripoli there are 41,000 

 millions of individuals of the Gaillonella distans in 

 every cubic inch, which weighs 220 grains, or about 

 187 milUons in a single grain. It is but fair to 

 state that there are those who believe the fossil 

 remains of the tripoli to be of vegetable origin, and 

 by them they are called Biatomacece. Tripoli is 

 obtained in large quantities from Bilin, in Bohemia. 



In conclusion, it may be interesting to remark 

 that during a short stay at Madrid, curiosity having 

 prompted me to inquire the price of a "Bath-brick," 

 I found it no less than fifteen pence of our money. 



Martin Henry Payne. 



The specimen of clay furnished by our corre- 

 spondent was divided between two expert manipu- 

 lators, who report that they can detect no organisms 

 present therein, and although their observations 

 were conducted independently of each other, their 

 conclusions are identical. The question is still an 

 open one, and worthy of further investigation : — 

 Does the scouring-brick clay of the River Parrot 

 contain organisms, and if so, of what kind ? — Ed. 



" No scientific truth can possibly be too trifling 

 or unimportant to be worthy of preservation." — 

 Sir James Edward Smith. 



