June 1, 1901.] 
THE TROPICAL AGRICULTURIST. 
809 
weather favoured the pest, while rain deterred 
it But if we examine the second period, we Una 
that July, August, and September are distinctly 
wee months, and it is during tliese very months 
that the pest attains its maximum development. 
We and, therefore, th:-.6 the two periods of increase 
viz., December to March and June to August, are 
marked by dry weather in the first case and wet 
weather in the second. Similarly, the first period 
of inactivity (in April and May) occurs during 
heavy rains, and the second period (October anU 
and November) during comparatively dry weather. 
It is noticeable that both the maximum and mini- 
mum of attack occur during the wettest months. 
It should be observed that these deductions 
are made from records from a single estate in one 
district. To be of real value they should result 
from the comparison of a number of records 
from many estates in different districts. I would 
appeal to all those interested in the subject tor 
such particulars as will enable me to formulate 
more accurate deductions. There are doubtless 
many records of captures now available. I know 
that it is the custom to count the number ot 
insects bronghD in each day, and the figures are 
entered in tlie estate books. Even a statement 
of the number of coolies employed in the work, 
day by day, will be of coni^iderable value in eeti- 
maling the rise and fall of the attack. 
But the chief problem lequiriiig solution is, 
«'Whac becomes ..t the insect during the period 
or periods when it disappears li- ni the tea . 
Does it subsist upon some other plant? Does it 
lie dormant in some hole or cievice? Or doea 
the insect, towards the end tf its period ot 
activity, deposit specialized eggs that remain un- 
hatched for a much longer period than is ordi- 
narily the case ? 
These questions are of the greatest importance, 
and can only be answered by very careful and 
lon^-continued obseivation on the spot. I hope 
to undertake this investigation very shortly by 
repeated visits to an affected estate at the critical 
^^Untii these points have been satisfactorily 
elucidated I can suggest no other remedia 
measures than those wliich are now beu-g carried 
out on most affected estates, viz., (1) the carefu 
collection and destruction of the insects ; (2) 
plucking to the initial leaf, in badly attacked 
field=. to ensure the destruction of the eggs. 
E. ERNfcST GREEN. 
Gevernment Entomologist. 
Peradeniya, March, 1900. 
CORUNDUM GEMS. 
BY LEOPOLD CLARKMONT.* 
The mineral known as corandum (from the Indian 
" Korund" ) cOQsiats of almost pure crystallisecl oxiJe of 
aluminium (Al 2- O'S). It cvysUUises in the hexagonal 
or rhombohedral systems, aud is generally found in 
six-sidecl pyramids, scalenohedrons, and prisms, al- 
thout'h the coarse opaque variety also occurs massive 
and gvamilar— that is, devoid of any natural orystallme 
formation. , . . . , 
In hardness, corundum la only inferior .o the 
diamond, and it, therefore, occupies the position o£ 
No. 9 in the recognised scale of hardness. It occurs 
«This article appears in our calumns by permission of 
Messrs Claremont and Ward, lapidaries and experts in gems, 
83, (jondulb-screet, London, by whom the copyright is re- 
serretl. 
both opaque and translucent, and its specific gravity 
Taries between 3'90 and 4'16. The opaque variety of 
corundum, which is generftUy dull grey, reddish brown, 
or greenish, and sometimes colourless, is found in 
large masses in most of the Asiatic countries ; it also 
occurs plentifully in America and Australia. It is, on 
account of its extreme hardness, of value as an 
abrasive material, entering largely into the compos- 
ition of emery. It is, however, the translucent viriety 
of corundum with which we are chiefly concerned at 
present. Every imaginable shade of every colour pre- 
sents itself in this variety of the mineral, and the 
difference in the colour is due in a very great measure 
to the addition of minute quantities of metallic oxides 
to the alumina of which the mineral is composed. To 
every colour in which translucent corundum occurs a 
different name has become attached — e g., red corun- 
dum is called " ruby,'' blue corundum is called " sap- 
phire," the yellow " Oriental topaz," the green Oriental 
emerald, the purple "Oriental amethyst," &c., kc. 
Therefore, to the scientist, these gems are absolutely 
identically the same stone, only differing in an infiui- 
tesimally small degree in the nature of the colouring 
matter to which they owe their distinguishing quality. 
But to the prospector, the dealer in precious stones, and 
the wearer of jewels, this little distinction with re- 
gard to colour makes a great difference, for upon it 
depends the commercial value of the gems. It is, 
therefore, necessary for the present work that each 
precious stone, no matter what relationship it has to 
any other gem, should receive separate attention from 
my pen. 1 do not propose, however, to make a re- 
petition of all the physical and optical properties of 
these stones, except in the case of the ruby and sap- 
phire, 
RUBY. 
The ruby or red corundum is, undoubtedly, the 
most coveted of nature's treasures by the human 
race throughout the universe. In this most precious 
material it is possible to concentrate the greatest 
amount of wealth in the least possible bulk. 
A ruby equal in value to a " king's ransom " can be 
carried in the corner of one's waistcoat pocket without 
inconvenience. A fine ruby weighing, say, four carats, 
ia worth in the gem market to-day quite ten times as 
much as a fine white diamond of the same weight. As 
already explained, the ruby is one of the group of the 
precious stones, which consists of oxide of aluminium 
and, consequently, has many of its properties in 
commmwith all the other members of the same group. 
The specific gravity of ruby is 4. The hardness is 
generally recorded as 9, but, as a matter of fact, ruby 
is slightly softer than the other corundum gems, which 
occupy the position of being next in hardness to the 
diamond. 
The ruby acquires eleotrictity by friction and retains 
it for several hours, The lustre is vitreous, but the 
surfaces of the crystals are usually more or lees dull. 
The fracture is conchoidal or uneven, and the cleav- 
age basal, but imperfect. Some specimen? of semi- 
opaque and cloudy rallies display a shimmering six- 
pointed star when cut with a convex surface. These 
are called star rubies, and will be more fully described 
under the heading of " Star Stones." 
Ruby occurs in the hexagonal or rhorabohedral 
system, assuming the form of the hexagonal prism or 
pyramid. Water-worn and rolled fragments are, how- 
ever, of much more frequent occurrence, often show- 
ing, by the absence of any trace of crystalline for- 
mation, how severe are the many hardships endured 
during the unrecorded ages of their existence. The 
ruby is doubly refractive, but not to a very great ex- 
tent, and is always dichroio, showing the two squares 
of the dichroiacope of distiucsly different colours — 
namely, aurora red and carmine red — when viewed at 
right angles to the principal axis of the crystal. 
This is a tnost useful means of distinguishing luby 
from garnet and spinel, which show both aqnarea of 
the instrument of the same colour, no matter in what 
direction through the crystal they be viewed. 
