Page:Proceedings of the Royal Society of London Vol 2.djvu/40

Rh when forcibly compressed between two plates of glass, it also exhibited beautiful colours, that were complementary to each other in the two images of a candle seen through it, by means of a prism of Iceland spar; and when the pressure was removed, these complementary tints disappeared.

Inasmuch as these colours might be supposed owing to the thinness to which the plate of jelly was reduced by pressure, Dr. Brewster cut the cake to the same thinness which it had possessed while under compression, but without any production of colours till pressure was again applied.

In the author's concluding experiment, he formed one twentieth of an inch thick of the same jelly, by melting it between two plates of glass. When merely consolidated by cooling, this had no power of depolarization; but by pressure it instantly restored the evanescent image, and exhibited, as in the former cases, the complementary colours, showing, says the author, that pressure communicates a modification of structure correspondent to that of crystallized minerals. 



The author's ultimate object is to ascertain the manner in which certain poisons act in destroying life; but for this purpose he found it necessary previously to determine how far the powers of the nervous and sanguiferous systems depend on each other; and though it be generally allowed that the powers of the nervous system cannot continue long after the cessation of the circulation of the blood, the converse is not so generally admitted; since there are persons who maintain that the nervous power may be wholly destroyed without impairing the vigour of the heart.

The present inquiry relates solely to this part of the subject, how far the power of the heart is influenced by the state of the nervous system; and the author designs, at some future time, to investigate experimentally, by what steps certain poisons destroy the powers of both.

M. Le Gallois maintains, that though the destruction of the brain does not impair the action of the heart, it is immediately and extremely debilitated by destruction of the cervical part of the spinal marrow. Dr. Philip, however, did not find this to be the case in his experiments, of which the first ten, performed on rabbits, relate almost exclusively to the effect of destroying the spinal marrow.

The animals were in general first rendered insensible by a blow on the occiput, after which the circulation was found to depend wholly on the continuance of respiration by artificial means, and not to be in any degree altered by subsequent removal or destruction of the spinal marrow, which was effectually done by means of a hot wire.