Capacity of the Gases for Caloric.—On Water-cements. 461 
drained off,—2 ewt. of the mass being put into the retort instead 
ef coal will produce more gas, and be less offensive than the 
same quantity of canal coal. This process will probably be found 
very convenient in. some cireumstanées for the consmnption of 
the tar prodneed by the distillation of coal in gas-works.— 
Journal of Science. 
CAPACITY OF THE GASES FOR CALORIC. 
J. H. Mallet, secretary to the Academy of Lyons, announces, as 
the result’ of soine well contrived experiments, which be has pub- 
lished, that At the same temperature the particles of different 
gases are at equal distances 5 that their molecules have different 
volumes; and that the quantity of calorie which a gas can ad- 
init deperids on the space which separates the molecules. 
ON WATER-CEMENTS, MORTAR, AND LIME, 
Tlie following particulars ate extracted from the work of M. 
L. J. Vicat on this subject. . 
Limestones vary greatly in quality. Those which approach 
to marble in purity, or consist almost entirely of carbonate of 
lime, are called rich ; those on the contrary are called meagre; 
which contain otable portions of sarid Or silex, alumine and iron, 
The former when burned, slacked, and miade into paste, will re- 
fain their softness for ages under water, or excluded from the air; 
but exposed to the air, they contract a remarkable hardness by 
the double effect of desiccation, atid union with the earbonic acid 
of the atmosphere. They even become susceptible of a beautiful 
polish. 
But the meagre limestones, it geiteral, tredted in the same 
manner, if kept under water, harden ina few days, and at length 
form a kind of freestone which could be acted upon or broken 
only by the pickaxe. Exposed to the air, it acquires a crumbly 
consistence, aud will never adinit of polish. From this circum 
stance, the linie which possesses the quality last mentioned is 
called hydraulic lime. But some of the meagre limestones are 
unfit for hydraulic purposes, especially those which contain large 
particles of siléx. 
Puzzolanas are either natural or artificial. The natural is 
found in situations which have been acted upon by subterraneous 
ficat. ‘They all consist of silex, alumine, oxide of iron, and a 
little lime, the properties of which vary greatly. Silex is always 
tlie predominating ingredient 5 the lime and iron are sometimes, 
though rarely, wanting. The scoria of forges and furnaces, bro- 
ken pottery, and pulverized brick or tile, are artificial substances 
analogous to puzzolanas. 
There is one class of puzzolanas which dissolve readily in sul 
phurie 
