Aug. r, 1H94.] 
THE TROPICAL AGRICULTURIST. 
85 
Decay of Cement. 
The decay of cements exposed to the action of 
sea water is attributed to the replacement of lime 
by magnesia. Professor Hartley would condemn 
cements used for harbour works which contain 
more than 1 per cent of magnesia. According 
to Lechartier, cements containing a considerable 
proportion of magnesia undergo a change in 
volume and disintegrate. 
These views, referring the decay of cement to 
the action of magnesia present either in the 
cement or in the sea water, are recent, and as 
they are no doubt based upon careful observations 
are probably correct, though they appear to be 
opposed to the older views which regarded the 
magnesia in well-made cements, as adding greatly 
to the hardness and stability of the cement. It 
is easy to understand that the stronger affinity 
possessed by magnesia, as compared with lime, 
for silica, may gradually lead to the first of 
these metallic bases replacing the second in 
the case of cements exposed to sea water ; but 
it is evident that this action of sea water would 
rather be a reason for originally replacing the 
lime in the cement as far as practicable with 
magnesia ; but if Lechartier's view is also correct, 
it is evident that the presence of magnesia in 
the cement itself is a source of weakness. 
The older view regarding the part played by 
magnesia in mortars and cement is well put in an 
article on "Cements" in "Chemistry: Theore- 
tical, Practical and Analytical,'' from which, as 
the subject is one of great importance, I quote 
the follow ing :-- 
" Magnesia. — When minerals chiefly composed 
of silicate of magnesium are brought into contact 
with lime they have no tendency to unite with 
it, and, therefore, form no cements. If, however, 
the magnesia is present iii excess as in dolomite, 
thru it contributes t<> tin' formation of a powerful 
mortar. This behaviour of magnesia is owing 
to the great affinity w hich existsbetween it and 
Silica. In the lirsl case, where lime and natural 
silicate of magnesium are. brought into con talc t; 
there is no combination, because the silicic acid is 
united more energetically with the magnesium 
than it could be with the calcium, and, therefore, 
no chemical change is produced which might give 
rise to a silicate of calcium when the powdered 
mass is mixed with water. But in the case where 
there is an excess of magnesia, not combined in 
the natural state with the silicic acid, then, on 
slaking, the magnesia and silica rapidly unite, 
and a very firm compound results. Indeed, so 
powerful is the affinity of silica for magnesia, 
that the dolomite limestones always afford a 
better and more binding compound than, if the 
metal were wholly calcium. Dolomites containing 
only carbonates of calcium and magnesium, after 
being burned, will yield a mortar that will set 
under water ; and if silicic acid be present in 
the proportion of 6 to 10 per cent, or more, a 
very superior hydraulic mortar results from the 
formation of a double silicate of magnesium and 
calcium. Hence, when magnesia is one of the 
components of the hydraulic lime, and is not 
combined with the silica in the natural state, 
the setting of the mortar is afterwards to be 
attributed in a great measure to this body. The 
degree of compactness which mortar assumes is, 
however, to some extent dependent upon the 
molecular state of the silica, and also upon the 
amount of the bases combined with it. 
PLUMBAGO. 
Plumbago is a mineral mainly composed of 
graphitic carbon. In some places, this form of 
carbon is undoubtedly due to the alteration of 
coal formations by heat. It occurs in various 
different forms. Dana enumerates (a) foliated, (b) 
columnar and sometimes radiated, (c) scaly 
massive nnd slaty, (d) granular massive, (c) earthy 
ahiorpbq . without metallic lustre, except in 
the streak ) in radiated concretions. 
Plumbago occurs abundantly in Ceylon, particu- 
larly in the Western, Southern and North-Western 
Provinces, and is the most important mineral 
export of the Island. Its chief uses are for the 
manufacture of crucibles for metallurgy, as a 
lubricant, a polishing material for metallic surfaces, 
and for the manufacture of lead pencils, although 
iD is commonly asserted that Ceylon plumbago is 
unsuitable for the last purpose. It is at all events 
used in conjunction with the graphite of other 
countries for this purpose. 
In analysing plumbago for commercial purposed, 
it is important to determine the total percentage 
of graphite and of sulphur, the latter being re- 
garded as an objectionable impurity. The follow- 
ing shews the amount of carbon and of sulphur 
in a number of samples of Ceylon plumbago dust. 
Lump plumbago has not hitherto been analysed 
locally for commercial purposes. 
Analyses of Ceylon Plumbago dust. 
1. 
2. 
3. 
4. 
Carbon per cent. 
Sulphur ,, 
99-3 
trace. 
99 
trace. 
98-05 
•02 
98- 
•06 
( ai l ion per cent. 
Sulphur ,, 
5. 
97 'S 
•09 
6. 
95-3 
•59 
7. 
95-9 
•83 
8. 
91-35 
•75 
9. 
10. 
11. 
12. 
13. 
Carbon 
). c. 
!iir;> 
SS-G 
88 G 
88-75 
87-4 
Sulphur 
•02 
•19 
•38 
•72 
"75 
The preceding analyses were made by the 
process of J. Macintosh, the object of which is 
to yet quit of the impurities, and to leave the 
