ree 
IN STUDYING QUESTIONS OF CHEMICAL MECHANICS. 391 
soon as the starch matter has been sufficiently disaggregated to 
remain, even temporarily, in suspension or in solution in water, 
whether pure or mixed with acids, it manifests a constant, very 
and iodine possessing these properties of mutability, and which was presented to 
me by M. Lassaigne himself. He had prepared it by pouring 3 decilitres ofecold 
water upon 0-4 gramme of triturated starch, and filtering the mixture in order 
to separate the portion which had become soluble, then adding 30 decilitres of 
an aqueous solution of iodine. This mixture, at first blue, lost its colour upon 
a suitable elevation of temperature, and again became blue on cooling; it pre- 
served this last tint constantly for several years, remaining at the same time 
perfectly limpid. Having lately submitted it to the same trials in a water-bath 
heated progressively, the flask being only half-open, the blue colour disappeared 
long before the bath approached the boiling-point, and it was not restored by 
cooling. But the cooled liquid immediately acquired a blue colour by a new 
addition of alcoholic tincture of iodine. And if this addition was made while 
it was hot, it acquired only a yellow tint, which gradually became blue as the 
temperature lowered. I took advantage of its decoloration to observe its optical 
power in a tube of 500 millimetres. It did not produce any appreciable devi- 
ation through this length, whence I conclude that the ponderable proportion 
of dissolved dextrine was less than +355; which shows at the same time the 
excessively small quantity of soluble matter which cold water is capable of re- 
moving from triturated starch, and the excessive sensibility of the chemical 
test which renders such small quantities evident from the colour it imparts to 
them. However, this colorization is no longer effected thus when the orga- 
nization of the granules of which the starch globules are composed has been 
completely destroyed. For syrup of dextrine, prepared by means of sulphuric 
acid, then separated from this acid by saturation, is no longer coloured blue by 
the tincture of iodine, whether hot or cold. It only takes a yellow tint more or 
less reddish, although it contains a considerable quantity of dextrine, that 
is to say, of starch completely disaggregated and reduced to the molecular state, 
which is manifested both by the great energy of its rotatory power and by 
the precipitation which alcohol causes in it. To observe the succession of 
tints which starch acquires from the iodine in the various phases of its disinte- 
gration, whether mechanical or chemical, I arrange a microscope so that the 
end of the tube which bears the object-glass may be turned upwards, and that 
the luminous pencils traverse it from above downwards, and are then brought 
horizontally towards the eye by total reflexion in an interior prism. ‘Then I 
place a polished and thin watch-glass upon the stage, which now occupies the 
uppermost position, into which I pour some water whick forms a meniscus, 
and into this water I add the grains of starch, whole or broken, which falling to 
the bottom become observable underneath through the glass. Two metallic 
arms, forming part of the stage, and extending laterally, bear at their extremi- 
ties two small spirit-lamps, which raise at will their temperature, and transmit 
it to the small glass containing the water, which may thus be brought gradu- 
ally to the boiling-point. By this means the expansion and progressive disag- 
gregation of the starch globules may be observed according as they are effected, 
They are seen to separate into flakes and into granules; and on adding a little 
solution of iodine, the progressive diversity of the tints which it gives to all 
these parts, according to their thickness and state of aggregation, is perceptible. 
We may, with the same apparatus, study the progressive action of alkalies and 
acids on the starch globules, on introducing these agents in quantities as small 
as we please. And without doubt it might also be employed to observe the 
progress ofa multitude of chemical reactions, whose phases of mutation it would 
be very interesting to investigate. These various additions to the microscope 
have been very ably effected by M. Ch. Chevallier. 
