LAD 
sentry, and serving to convey the chyle to 
its destined place. See Chyle. 
LACTESCENT, in botany, a term ap- 
plied to the juices of plants, of whatever 
colour, which flow out of plants when any 
injury is done them. The colour is either 
white, as in the campanula, maple, dande- 
lion, &c. ; or yellow, as in the celandine, &c. ; 
or red, as in the bloody dock. Mostjates- 
cent plants are poisonous, excepting those 
witli compound flowers, which are generally 
of an innocent quality. 
LACTIC acid, in chemistry, is contained 
in milk, and was discovered by Scheele, to 
whom modern chemistry is indebted for 
much important knowledge. The forma- 
tion of this acid depends on the change of 
the saccharine mucous matter ; for after 
the acid is once well formed, when the 
serous part of the milk reddens vegetable 
blues, no more is obtained by evaporation 
and crystallization. Scheele obtained this 
acid by the following process : he evapo- 
rated sour whey to one-eighth of its bulk, 
and tlien filtered it to separate the coagu- 
lated cheesy matter. He then added lime 
water to precipitate the phosphate of lime, 
and diluted the liquid with pure water. He 
next precipitated the excess, of lime by 
means of the oxalic acid, and then evaporat- 
ed the solution to the consistence of honey, 
poured on a quantity of alcohol which sepa- 
rates the portion of sugar, of milk, and other 
extraneous matter, and dissolves the lactic 
acid, and distilled the clear filtered liquor 
till the whole of the alcohol employed be 
driven oft’ : what remains is the lactic acid. 
This acid is never crystallised, but always 
appears in the form of a viscid mucilaginous 
substance ; it has a sharp taste ; it reddens 
tincture of turnsole ; and gives a reddish 
shade to the syrup of violets. It combines 
with alkalies, earths, and metallic oxides; 
and forms with them lactates. 
LACTUCA, in botany, lettuce, a genus of 
the Syngenesia Polygamia jEqualis class 
and order. Natural order of Compositae 
Semiflosculos®. Cichorace®, Jussieu. Es- 
sential character : calyx imbricate, cylindri- 
cal, with a membranaceous margin ; recep- 
tacle naked ; seeds even, with a simple 
stipitate down. There are eleven species, 
of which L. sativa, the common garden let- 
tuce, with its several varieties are too well 
known to need a particular description. 
LACUNAR, in architecture,' an aiched 
roof or ceiling, more especially the planking 
or flooi ing above porticos and piazzas. 
LADDERS, scaling, in the military art. 
LAD 
are used in scaling when a place is to be 
taken by surprise. They are made several 
ways ; sometimes of flat staves, so as to 
move about their pins and shut jike a paral- 
lel ruler, for conveniently carrying them : 
the French make them of several pieces so 
as to be joined together, and to be capable 
of any necessary length : sometimes they 
are made of single ropes knotted at proper 
distances, with iron hooks at each end, one 
to fasten them upon the wall above, and 
the other in the ground ; and sometimes 
they are made with two ropes, and staves 
between them to keep the ropes at a proper 
distance, and to tread upon. When they 
are used in the action of scaling walls they 
ought to be rather too long than too short, 
and to be given in charge only to the 
stoutest of the detachment. 
The soldiers should carry these ladders 
with the left arm passed through the second 
step, taking care to hold them upright close 
to their sides, and very short below, to pre- 
vent any accident in leaping into the ditch. 
The first rank of each division, provided 
with ladders, should set out with the rest at 
the signal, marching resolutely with their 
firelocks slung, to jump into the ditch ; 
when they are arrived they should apply 
tlieir ladders against the parapet, observing 
to place them towards the salient angle 
rather than the middle of the curtain, be- 
cause the enemy has less force there. Care 
must be taken to place the ladders within a 
foot of each other, and not to give them too 
much nor too little slope, so tliat they may 
not be over-turned, or broken with tlie 
weight of the soldiers mounting upon them. 
The ladders being applied, they who have 
carried them, and they who come after 
should mount up and rush upon the enemy 
sword in hand ; if he who goes first happens 
to be overturned, the next should take care 
not to be thrown down by his comrade ; 
but on the contrary, immediately, mount 
himself so as not to give the enemy time to 
load his piece. The success of an attack 
by scaling is infallible, if they mount the 
four sides at once, and take care to shower 
a number of grenades among the enemy, 
especially when supported by some grena- 
diers and piquets, wlio divide the attention 
and share the fire of the enemy. 
LADEN ; the state of a ship when she is 
charged with a weight or quantity of mate- 
rials equal to her tonnage or burthen. If 
the goods with which she is laden be ex- 
tremely heavy, her burthen is determined 
by the weight thereof; but if light, she 
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