The properties of carbonic acid are very remarkable; it is perfectly colorless and invisible; it is irrespirable, producing, when an attempt is made to breathe it, violent spasms of the glottis. If it be respired mixed with air, even in the proportion of one to ten, it gradually produces stupor and death, acting as a narcotic poison. Hence, when disengaged in large quantities, whether by natural operations or in process of manufacture, it accumulates in all cavities within its reach, and may cause fatal accidents to animals who enter unadvisedly. Carbonic acid does not support combustion; a taper plunged into a jar full of the gas is instantly extinguished. Carbonic acid is also a check on putrefaction, and arrests decay. SULPHUROUS ACID. Sulphurous acid exists at ordinary temperature and pressure in the gaseous form; it is one, however, of the most easily liquified gases. It is produced always when sulphur burns, either in air or in pure oxygen; sulphur not being capable of passing directly to a higher degree of oxydation. In the burning of sulphur, the volume of sulphurous acid gas formed is exactly equal to the amount of oxygen consumed. Sulphurous acid gas may also be simply prepared by heating three parts of flowers of sulphur with four of peroxide of manganese. The reaction is very simple: one part of the sulphur uniting with the metal, and another with the oxygen, form sulphuret of manganese and sulphurous acid. Another and quicker way to obtain this gas in small quantities is, to decompose a solution of hyposulphate of soda, by adding muriatic acid to it, so as to liberate the hyposulphurous acid, which immediately decomposes into sulphur and sulphurous acid. Sulphurous acid is absorbed by water. It is colorless and transparent, possessing an odor peculiarly irritating (the smell of burning sulphur), and cannot be breathed. It is not combustible, nor does it support combustion. Water dissolves about thirty-seven times its volume of sulphurous acid; the solution possesses the properties of the gas in a very high degree, and bleaches vegetable colors with great power; when kept for some time it gradually absorbs oxygen, and the sulphurous becomes changed into sulphuric acid. The sulphuric acid is one of the feeblest acids, and is expelled from its combinations by almost all but the carbonic acid. As has been demonstrated, all these gases are absorbed by water, and a saturated solution possesses the properties of the gases themselves. PHYSIOLOGY. GENERAL VIEW OF THE CIRCULATING APPARATUS OF MAN. The course and relative positions of the principal arteries and veins of the Systemic circulation are shown in this plate. The arteries commence from the great arterial trunk, called the aorta, and their branches are distributed to all parts of the system. The venous branches, which accompany the arteries, unite into two great veins, the superior and inferior vena cava, which convey the blood back to the heart.