liNTERESTING PAPER ON THE THEORIES OF LIGHT. 
45 
For Ihe purposes of agriculture it is some- 
times calcined, which deprives it of its water of 
crystallization, which in the hydrous gypsum 
amounts to 2 atoms, d'his preparation is at- 
tended to in France, where the expense of the 
process is less than in other countries. In 
England, Germany , &c. it is generally em- 
ployed in the crude state. The effect which 
calcination produces, is to render the gypsum 
more rapid in its operation, though the benefi- 
cial effects are less durable. In France it is 
burned in a kind of limekiln by means of coal, 
after being reduced to powder. 
It can be obtained in this state in Gard, for 
oneshilling the llOlbs. avoird., and it costs 
double the expense in Alais. Extensive na- 
tural deposits occur in England in the neigh- 
bourhood of the Humber, from whence it is 
brouglitto Glasgow and Manchester for the 
useofthe bleachers, who now employ it in 
considerable quantities. Its purity may be 
negatively tested by vinegar, which, if it causes 
no effervescence, .shews that there is no carbo- 
nate of lime present. If it swells up when 
water is thrown on it, and then assumes con- 
sistence, it is a sign that it has been properly 
calcined. The best plaster will absorb the 
greatest quantity of water. It is chiefly on 
artificial meadows that we observe the best 
effects from its application, more especially 
on clover, lucern, sainfoin, and in general on 
the leguminous tribe of plants possessing large 
and thick leaves. It has a powerful effect also 
upon natural meadows which contain much 
clover, vetches, and other analogous plants; 
but upon the grasses the effect of gypsum is 
trifling. It acts, according to M. Thibaud, 
by extracting the moisture from the air, and 
stimulating the vital action of plants. 
It sometimes doubles the product of clover, 
lucern, and sainfoin. In France it is sowed 
like corn with the hand, about March or April 
when the plants are a few inches above the 
soil, so as to allow the gypsum to fall on the 
leaves. It should be done previous to rain, 
but not during the fall of rain, or the exis- 
tence of wind, or during frost. 
The quantity of gypsum applied to the land 
must vary of course with the nature of the soil. 
In the course of fifteen or twenty days the 
good effects resulting from its use are visible, 
if circumstances have been favourable. The 
benefits of one application last for two or three 
years, so that it is unnecessary to spread it 
every year. In Gard and Heraults ainfoin is 
principally cultivated for pasture, and seems 
to thrive well in dry soils, especially in stony 
calcareous situations. About Alais, for the 
cultivation of this plant in artificial meadows, 
the ground is first ploughed in November, then 
again in December, and the seed is sown in 
tlie beginning of April. 
In Provence and in the southern parts of 
Languedoc, where the effects of frost are 
less dreaded, it is sown in autumn. The 
sainfoin thus cultivated in inferior soil affords 
one or two crops in the year, and lasts for four 
or six years ; then it is ploughed up and corn is 
substituted for it. It is worthy of remark, 
that lands which previously could not pro- 
duce corn, has, by the use of gypsum in the 
manner described, been able to raise good 
crops in the midland parts of France, The 
agriculturists of Alais may procure gypsum 
from Anduze, Salle, Rochebelle, and Blana- 
ves. 'I’o Drome it may be carried fiom Gard 
and Ardeche. At Heraultit may be otbained 
at Cruzy, Quarante, Calzouls, Ilerepain, 
Beziers, Clermont, Loubes, and Lodeve. It 
is extensively employed in Canada with the 
most happy results. It was tried in York- 
shire by Lord Dundas without any benefit, 
but the soil upon which it was spread was as- 
certained to contain a quantity of gypsum. 
It might be employed, there can belittle 
doubt, with great advantage in the border 
counties, where the trifolium pratense has in 
many places failed. 'J’his plant necessarily 
from itsstrong and luxuriant nature, obviously 
must require a considerable quantity oftiie 
manure. If it be deficient in quantity the 
plants may vegetate, but must speedily perish 
from the effect of the first frost on their deli- 
cate structures. 
ROYAL INSTITUTION.— COMPARI- 
SON OF THE NEWTONIAN AND 
UNDULATORY THEORIES OF 
LIGHT. 
30t/t January, 
Dr. Ritchie commenced his lecture on the 
two theories of light which have been advo- 
cated by different philosophers for many 
years, with a few observations with regard to 
the difficulty of acquiring knowledge of the 
subject by direct experiment, in consequence 
of the almost spiritual nature of the substance 
upon which it is necessary to operate. 
Newton whose opinion was long in vogue, 
having had his attention directed to the mo- 
tions of bodies, considered light as a substance 
consisting of revolving spherical particles is- 
suing from luminous bodies moving in straight 
lines, and producing reflection or refraction 
according as the extremities of the spheres, 
which came in contact with a denser medi- 
um, were sharp or obtuse. This theory re- 
quired certain postulates which appear, 
however, to be entirely gratuitous. By the 
undulatory theory, which is often called the 
theory of Huyngens, which was suggested to 
his mind in cosequence of his attention being- 
directed to the motions of the pendulum, al- 
though it was known before his time, light is 
considered to consist of the undulations of an 
ethereal fluid filling all space, and existing 
between the particles of bodies. If such a 
fluid does exist, we might expect that it would 
act in retarding the motions of the heavenly 
bodies. It is obvious, however, that the plants 
can suffer no retardation, because, inconse- 
quence of their revolutions, the ether will also 
acquire motion and be carried along with 
them, but in reference to the comets, which 
are extremely light bodies, we find a decided 
retardatibn, which after making all allow- 
ances, can only be accounted for on the sup- 
position of the existence of an etherial medi- 
um. This has been clearly proved by Sir 
John Herschel, in his article on light in the 
Encyclopaedia Metropolitana. Dr. Ritchie 
stated that he had only become a convert to 
the undulatory theory of light about two years 
ago, in consequence ofHerschel’s arguments, 
and an attentive comparison of the two theo- 
