SLA 
SLAG, is a term used by the persons 
employed in working minerals, to express 
any hard vitrescent generally coloured 
opaque mass, produced by the fusion of 
any stony or metallic mixture. It gene- 
rally consists of the matrix of the ore, and 
any saline or earthy flux that may have 
been used. The slag of iroK founderies is 
for the most part composed of the earthy 
part of the ore, of the lime used as a flux, 
and the whole coloured with a part of the 
oxide of iron. Slag differs from scoria, in 
being more dense, and more completely 
vitrified ; whereas the scoria, or dross is 
lighter and porous. When the slag is very 
opaque and heavy, it contains a considerable 
quantity of metal, and in that case it may 
be worth while to work it over again. 
Slag in many parts of the country is used 
for mending roads, for which it makes a 
capital material when a little worn down, 
being hard, and almost impenetrable by 
water. 
SLATE, a well known neat, convenient, 
and dnralrle material, for the covering of 
tlie roofs of buildings. Tliere are great 
varieties of tliis substance, and it likewise 
differs very greatly in its qualities and co- 
lours. In some places it is found in tliick 
lamina or flakes, while in others it is thin 
and light. The colours are white, brown, 
and blue. It is so durable in some cases, as 
to have been known to continue sound and 
good for centuries. However, unless it should 
be brought from a quarry of well reputed 
goodness, it is necessaiy to try its proper- 
ties, which may he done by striking the 
slate shaiply against a large stone, and if it 
produce a complete sound, it is a mark of 
goodness ; but if in hewing, it does not 
shatter before the edge of the sect, or 
instrument commonly used for that pur- 
pose, the criterion is decisive. The good- 
ness of the slate may be further estimated 
by its colour : the deep black blue is apt 
to imbibe moisture, but the lighter blue is 
always the least penetrable : the touch also 
may be in some degree a guide, for a good 
firm stone feels somewhat hard and rough, 
wliereas an open slate feels very smooth, 
and as it were greasy. And another me- 
thod of trying the goodness of slate, is to 
place tlie slate stone lengthwise, and per- 
pendicular in a tub of water, about half a 
foot deep, care being taken that the upper 
or uniramersed part of the slate be not 
accidentally wetted by the hand, or other- 
wise : let it remain in this state twenty- 
fpur hours j if good and firm stone, it will 
SLA 
not draw water more than half an inch 
above the surface of the water, and that 
perhaps at the edges only, those parts hav- 
ing been a little loosened in the hewing ; 
but a spongy defective stone will draw wa- 
ter to the very top. There is still another 
mode, held to be infallible. First weigh 
two or three of the most suspected slates, 
noting the weight ; then immerge them in 
a vessel of water twelve hours, take them 
out, and wipe them as clean as possible 
with a linen cloth, and if they weigh more 
than at first, it denotes that quality of slate 
which imbibes water ; a drachm is allow- 
abie in a dozen pounds, and no more. In 
the laying of this material, a bushel and a 
half of lime, and three bushels of fresh water 
sand, will be sufficient for a square of work, 
but if it be pin plastered, it will take above, 
as much more : but good slate well laid 
and plastered to the pin will lie an hundred 
years, and on good timber a much longer 
time. It has been common to lay the slates 
dry, or on moss only. When they are to 
be plastered to the pin, then about the first 
quantity of lime and sand will be sufficient 
for the purpose. Se,e Thonschiefer. 
SLAVES and SLAVERY. Pure and 
proper slaves do not, nay cannot, subsist 
in England; such, that is, whereby an ab- 
solute and unlimited power is given to the 
niaster, over the life and fortune of the 
slave. And indeed, it is repugnant to 
reason, that such a state should subsist any 
where ; and the law of England abhors, and 
will not endure, tlie existence of slavery 
witliin this nation, so that when an attempt 
was made to introduce it, by statute 1 
Edward VI. cap. 3, wliich ordained, that'" 
all idle vagabonds sliould be made slaves, 
and fed upon bread and water, .on small 
drink, and refuse meat, should wear a ring 
of iron round tlieir necks, arms, or legs ; 
and should be compelled by beating, chain- 
ing, or otherwise, to perform the work 
assigned them, were it never so vile; the 
spirit of the nation could not brook this 
condition, even in the most abandoned 
rogues ; and therefore tliis statute was re- 
pealed in two years afterwards, by statute 
3 and 4 Edward VI. cap. 16. And now 
it is laid down, that a slave, or negro, the 
instant he lands in England, becomes a free 
man, that is, the law will protect him in the 
enjoyment of his person and his property. 
Vet, witli regard to any right which the 
master may have lawfully acquired to the 
perpetual' service of John, or Thomas, this, 
says Blackstone, will remain exactly in the 
