78 ROOFING SLATE. 
out it is to be wiped with a cloth, and again weighed. 
If it have not acquired any considerable increase of 
weight, it is a proof of its being sufficiently compact. 
If, on the contrary, it have absorbed much of the water, 
and have become considerably heavier by the immer- 
sion, it is shown to be of a porous texture. Slates that 
are brittle are bad. If they emit a tolerably clear 
sound, when struck with a hammer, it is considered a 
proof that they are not too brittle : if, on the contrary, 
the sound be dull, they are soft and shattery. A good 
slate ought also to resist the action of a considerable 
degree of heat. 
The slates that are principally used in London are 
brought from North Wales, from quarries that are 
worked near Bangor. There are also extensive slate 
quarries near Kendal, in Westmoreland; and the Ken- 
dal slates, which are of a bluish green colour, are more 
highly esteemed than those from Wales. They are not 
of large size, but they possess great durability, and 
give a peculiarly neat appearance to the roofs on which 
they are placed. The slate quarries near Easdale, in 
Scotland, are so extensive as to furnish annually more 
than 5,000,000 in number, and to give employment to 
upwards of 300 men. 
French slates were much used in London about 
seventy years ago ; but they have been found too small, 
thin, and light, to resist the winds and storms of this 
changeable climate. 
Dark-coloured, compact, and solid slates are ma- 
nufactured into writing slates, or table slates, as they 
are sometimes called. In the preparation of these, the 
slate, after it is split of proper thickness, is smoothed 
with an iron instrument. It is then ground with sand- 
stone, and slightly polished with tripoli (119), and, 
lastly, rubbed with charcoal powder. It is cut into the 
requisite shape, set in a wooden frame, and is then 
ready for use. 
For writing on these slates, pencils are used which are 
also made of slate. These, which are called slate pen- 
