474 LOWER OOLITIC ROCKS OF ENGLAND: 
protecting cover. Thus although the blocks obtained, were as a 
rule comparatively small in size, compared with those that may- 
be sawn out at a depth below ground, yet Nature had to some 
extent sorted the good from the bad stone, and the materials used 
have in most cases proved their durability. More stone, of course, 
can be obtained for the labour, when the strata are protected by 
clay, and if the material be properly seasoned the results will be 
found satisfactory. 
Good and sound stone is usually sonorou?, and the blocks will 
ring when struck with a hammer. If unsound or " venty " the 
note will be discordant ; and Mr. A. M. Wallis, a practical 
worker at Portland, states that. " by placing the hand on the stone 
when striking it, the vent may be detected by a slight undulating 
motion." 
Sandstones with calcareous cement are not very durable as a 
rule, although the Chilmark Stone may be taken as an exception. 
The purely siliceous varieties of sandstone are however too hard 
to admit of being readily dressed, and are more usually employed 
for road-metal. 
Stones that absorb a large amount of water may be acted upon 
more readily by frost ; but the durability of a rock does not 
necessarily decrease according to the absorbent powers.* 
The Portland Whit Bed is said, when dry, to be capable of 
absorbing from 6 to 7 per cent, of its weight of water in 24 hours. 
In the applications of stones it is necessary to consider the aspect of 
the building in reference to prevalent rains, &c. ; and also the situation 
in large towns and cities, where the atmosphere may be impregnated 
with an excess of carbonic acid, and even with sulphuric, nitric, and 
hydrochloric acids from various manufactories. To some extent, as 
remarked by Mr. F. W. Rudler, the smoke of London gives a protective 
coating to the stone of buildings. 
Most of the beds, and especially those with any tendency to a laminated 
or flaggy structure, should be placed in their natural position in buildings. 
In some cases however where the rock is much false-bedded, the stone 
may be better placed on end or "surbedded," and this is said to be the 
case with the Taynton Stone. Moreover in the case of mouldings and 
fine carvings attention must be paid to the position the stone will 
occupy. 
For the artificial protection of stone, various solutions and other pre- 
parations (silicates of soda and potash, &c.) have been suggested. The 
building-blocks in some cases may be immersed in the solution before 
use ; in other cases the stone in the buildings may be painted or coated 
with the material. At present the expense has been the chief drawback 
to the adoption of these processes.! The " Bath Stone Firms," however, 
hare lately introduced a "fluate " for the preservation of limestones, and 
rocks containing over 25 per cent, of carbonate of lime. The process, 
which was originated by M. Kessler, is described as " fluosilicatization.'* 
The liquid is applied with a brush, and it acts chemically on the stone, 
leaving it as calcium fluoride, and silica; and thus rendering it insoluble 
by rain-water. The liquid naturally extends further into porous and 
loose-textured stone ; and in the oolite rocks it acts more readily on the 
matrix. It does not, however, entirely close the pores. The cost of the 
* See C. Lloyd Morgan, on Bristol Building Stones, Proc. Bristol Nat. Soc., ser. 2, 
vol. v. p. 95 ; and remarks on Stone in " Notes on Building Construction," Part 3, 
ed. 2, 1889, pp. 11, 7fi ; Wallis, Proc. Dorset Nat. Hist. Club, vol. xii. p. 191. 
f G. P. Merrill, Smithsonian Keport for 1886, Part 2. 
