IN STUDYING QUESTIONS OF CHEMICAL MECHANICS. 389 
of a sensible plate of gypsum, they are seen to consist of a heap 
of ovoidal globules, composed of solid, transparent strata, inti- 
mately superposed, but distinguished by their peculiar densities 
or by their mode of apposition, and constituted by meridional 
sections around a point of the superficies of the globule called the 
umbilicus, in the same manner as the sides of a pear or the zones 
of a melon are arranged around the peduncle through which they 
derive their nutriment. And, further, this structure is locally 
fixed as in the above fruits; for the effects which it produces on 
polarized light subsist without modification in each fragment of 
a broken globule*. As long as these preserve their natural state 
* To make these observations, I employ an achromatic microscope, contain- 
ing asmall Nicol’s prism fixed behind the object-glass on the side of the eye-glass. 
I insert some particles of starch in a thin layer of water contained between two 
glasses, one of which is plane, and the other slightly concave internally, but 
plane without. When this system has been placed on the stage and brought 
into focus, I place above it a second Nicol’s prism, which is inserted in a ring 
centred on the axis of the incident pencil, and I turn this prism on itself, until, 
by the perpendicularity of its principal section to that of the other prism be- 
hind the object-glass, the field of vision becomes completely black in all the 
parts where the course of the luminous rays is not interrupted by the starch 
globules. Then, each of the luminous pencils which traverses these globules 
undergoes a deplacement of its plane of polarization, which shows their form, 
and gives them the appearance indicated in the text. Ifthe starch is fresh, we 
generally perceive in each globule a black cross, the arms of which intersect 
in its umbilicus, and appear straight or curved, according as the axis of the 
globule is presented straight or obliquely to the eye. These appearances are 
especially regular when the starch has been previously washed with cold water 
slightly acidulated with one- or two-thousandth parts of hydrochloric acid, 
which cleans the surface of the globules of any organic matters which often 
adhere to them, and which may be produced, either by ruptures of the cellular 
membranes enclosing them, or by their mutual friction in preparing them. But 
when the starch is old, the globules are generally deformed and rugose, which 
produces the appearance of unequal sectors having nevertheless always the 
umbilicus for their summit. These phenomena of polarization are manifested 
in the same manner and with similar characters in all the starches, the smallest 
as well as the largest. But they are more energetic in proportion as the vo- 
lume of the globules is more considerable ; and in a great number their inten- 
sity is such as to produce colours analogous to those of plates of alum, except- 
ing that the outlines of the coloured portions are curved, because they follow 
the inflexions of the tangent plane at each division of the surface of the glo- 
bules. They subsist in each portion of the globule accidentally broken, as if it 
were entire; and when two or more globules are naturally connected together, 
which often happens in the large kinds, each individual still produces them 
around its own axis. These characters of direction relative to meridional planes 
could evidently not result from the existence of an interior liquid: they can 
only belong to a system regularly aggregated by organization. The lamellar 
character of this kind of action is manifested likewise by the modifications it pro- 
duces in the proper colours of thin crystallized laminz interposed in the passage 
of the rays, posteriorly to the first prism. ‘To make these effects most manifest, a 
thin lamina of gypsum should be selected, whose thickness is such, that its extra- 
ordinary tint is Newton’s violaceous purple-gray of flax, which answers in his 
