1 70 
Observations, <$~c. 
[March, 
parts broken down, by grinding or pounding, until reduced to an impalpable pow- 
der, or to have them separated by repeated washings. The blue clay, on the con- 
trary, as obtained from the Medway at low water, by digging from one to two feet 
below the surface of the mud, is in a sufficiently minute state of division, without 
requiring either of those troublesome processes. 
31. The most suitable clays for artificial cements would, therefore, be those which 
contain the least of animal or vegetable matter, and which, consequently, do not 
sensibly alter their colour or chemical properties, by continued exposure to air, 
and which are at the same time in a state of minute division. I am not aware 
that any clay of the above description, quite fine enough for such purposes, is to 
be found in this neighbourhood. 
32. None but plastic clays, or those which are capable of burning into hard 
sound bricks, or earthenware, are fit ingredients for water cements. Those which 
burn into unsound slag or scoria, ought to be rejected. 
33. In ascertaining, by experiment, whether any mixture will answer the purpose 
of an artificial cement, make it up, with a moderate quantity of water, into balls 
or cubes of about inches diameter, or side, if you propose to burn it in a com- 
mon fireplace ; or into balls of about 1 inch in diameter, if you propose to use a 
crucible. The ingi-edients cannot be pounded too fine, or mixed too well. Burn 
them in the manner, and to the degree dii'ected in Article 3, and then try the 
strength of your cement according to Article 5. Before you put the mixture into 
the fire, whether protected by a crucible or not, let it be dried gradually, and heated 
a little. 
34. A curious result developed itself in the course of our expei'iments, at Chat- 
ham. The carbonate of magnesia is, in itself, an excellent water cement. When 
calcined, and made up into a moist ball with water, it does not heat perceptibly) 
and it sets very slowly ; but if allowed to stand for about 12 hours, it may then 
be put under water, and will, by degrees, become eqx^al to the natural cements in 
33. Magnesian lime stone, mixed with plastic clay alone, no matter of what 
kind, produces a good water cement; for where magnesia is present in any 
artificial cement mixture, the oxide of iron may be dispensed with. Hence mag- 
nesian lime stone and pipe clay make an excellent cement, although chalk and 
pipe clay do not always form a good one ; and the magnesian lime stone will 
always succeed with the blue or changeable alluvial clavs. althmiob rhalk and the 
hardness. 
