Mortar . 
267 
burning is deficient, in fuel or in time. The burning 
ought to continue in a strong red, approaching to a white 
heat for at least ten hours. 
Lime exposed to the air for about twenty days will be 
reconverted into limestone, and become perfectly useless 
as a cement. Dr. Watson converted into lime 540 grains 
of statuary marble : the lime weighed 304 grains : from 
the 10 th of February to the 4th of March, it had gained 
so much in weight, that it weighed 515 grains. Dr. Hig- 
gins exposed 2 lb. of well burnt, non -effervescent chalk 
lime, to the air for 22 days ; at which time it weighed 
3 lb. 2 ounces and 1 drachm and a half. The greatest 
proportion of this accession of weight, is gained in the 
first 4 or 5 days. 
This encrease of weight is owing to 1st. carbonic acid 
gas imbibed from the atmosphere : 2d. in smaller pro- 
portion, to water imbibed from the atmosphere. These 
substances thus imbibed, reconvert the lime into lime- 
stone. So that mortar made from such lime, might as 
well be made from powdered limestone. It is no cement z 
it produces no cohesion. 
The deeper and more circular, is the heap of lime, the 
better is the inner part protected from the bad effects of 
the atmosphere. The external surface is spoiled first* 
If the lime be slacked, and made into a paste with lime- 
water, (water wherein some lime has been previously 
slacked) and protected by boards, on all sides and on the 
top, it may be kept for a long time tolerably well. Or if 
slacked into powder immediately on being brought from 
the kiln, and made up into a compact heap. 
If lime from the kiln effervesces, or gives out air (car- 
bonic acid gas) on being dissolved in muriatic or nitric 
acid, it is ill-burnt in proportion exactly to the quantity of 
this air which it yields. 
