390 Naiural-Philosophical Collections. 
which have been formed respecting the nature of light, for it is necessary that 
these general laws of the propagation of light could be deduced from these hypo- 
theses. This, in fact, is what results from the labours of M. Fresnel and M. 
Ampere’s memoir. They prove, that the laws in question results from the hy- 
pothesis, which consists in con sidering the phenomena of light as produced by 
the vibrations of an elastic fluid. It is found, that the direction of these vibra- 
tions is perpendicular to that of the luminous ray. This latter part of the hypo- 
thesis was long rejected by some persons, because they established their calcu- 
lation upon he supposition that the different parts of elastic fluids act upon each 
other only by being alternately compressed and expanded. 
M. Fresnel condidered these fluids under a diiterent aspect. He regarded 
them as composed of material points, acting upon each other at a distance. He 
shewed that there might be in them a communication of vibratory motions, even 
when the fluid might only be susceptible of condensations and dilatations as 
small as might be BOLE MTEC so that on considering them as null, the direction 
of the vibrations ought, at this limit, to remain perpendicular to that of the lumi- 
nous rays. Now, this view appears to be confirmed by ulterior researches. MM. 
Ampére concluded from this, that before the hypothesis in question can with 
propriety be considered as uncertain, it is necessary to deduce the general laws 
yielded by experiment from a different physical notion, which no person has yet 
done. 
On the Crystallization of Barley-Sugar; by Tuomas GRrawAM, A.M,, 
F.R.S.E.—The change in appearance, arising from crystallization, which sticks 
of barley-sug ar undergo in keeping, is always instanced as a case of chrystalliza. 
tion occurring in a solid body, without solution, and independently of external 
agents. ‘The barley-sugar certainly does not then become a hydrate; and pre- 
bably at the completion of the change is exactly cf the same weight as before it 
began. But from an observation I have made, it would appear that the pre- 
sence of a little moisture is n eee for the change, and probably that every por- 
tion of barley-sugar which suffers this change has been successively loosened and 
held in solution by that sm ail portion of water, which begins to act on the outer 
surface of the stick and iravels inwards. 
Two fresh sticks ef oe sugar, ary and transparent, were introduced at the 
Same time into separate phials: one of them with a stick of caustic. potash, and 
the other by itself, corked up, and laid in a drawer. ‘The barley-sugar, in com- 
pany with the caustic potash, which would preserve it perfectly dry, did not un- 
dergo the slightest alteration in six months, but remained as transparent as at 
first. ‘The Log sugar in the other phial was scarcely altered during the first 
four months; but during the last two months, which were colder and damper, it 
became Geacne on the surface, and the crystallization thereafter was propagated 
inwards to a considerable depth. 
_ The effect of a small quantity of moisture in enabling solid amorpheus matter 
to crystallize was observed very distinctly in the case of another substance. A 
quaitity of sulphate of soda was rendered anhydrous by heat, and became a 
heavy powder. Placed in a confined atmosphere, kept purposely humid, the 
powder slaked like lime, swelling to several times its original bulk. It regains 
in two weeks its usual combined water (twelve atoms), and was then dry, and 
not in the slightest degree crystalline. Two days afterwards the powder was 
found a massof crystals of the usual form of sulphate of soda, so dry as not 
to adhere to the blade of a knife; and it was not till after weighing that 
I satisfied myself of the presence of uncombined moisture among the crystals 
hearly to the extent of an additional atom. Here a small quantity of water 
‘allowed the powdery particles to right themselves, and adopt a crystalline ar 
rangement which they were incapable of assuming without it. 
On Chrome Orange Colour,—It is Sew th at, although no other colour 
-has been so much run upon for a couple of years in cotton yarns, no account of 
the mode of raising this beautiful tint, so far as I can learn, has hitherto been 
published ; yet the precess is universally known, and follawed by dyers. 
