It is a permanent and universal law in vast numbers of unorganised bodies that their composition is definite and invariable, the same compound always consisting of the same elements united together in the same proportions. Two substances may indeed be mixed; but they will not combine to form a third substance different from both, unless their component particles unite in definite proportions; that is to say, one part by weight of one of the substances will unite with one part by weight of the other, or with two parts, or three, or four, &c., so as to form a new substance; but in any other proportions they will only be mechanically mixed. For example, one part by weight of hydrogen gas will combine with eight parts by weight of oxygen gas, and form water; or it will unite with sixteen parts by weight of oxygen, and form a substance called deutoxide of hydrogen; but, added to any other weight of oxygen, it will produce one or both of these compounds mingled with the portion of oxygen or hydrogen in excess. The law of definite proportion established by Dr. Dalton, on the principle that every compound body consists of a combination of the atoms of its constituent parts, is of universal application, and is in fact one of the most important discoveries in physical science, furnishing information previously unhoped for with regard to the most secret and minute operations of nature, in disclosing the relative weights of the ultimate atoms of matter. Thus an atom of oxygen uniting with an atom of hydrogen forms the compound water; but, as every drop of water however small consists of eight parts by weight of oxygen and one part by weight of hydrogen, it follows that an atom of oxygen is eight times heavier than an atom of hydrogen. In the same manner sulphuretted hydrogen gas consists of sixteen parts by weight of sulphur and one of hydrogen; therefore an atom of sulphur is sixteen times heavier than an atom of hydrogen. Also carbonic oxide is constituted of six parts by weight of carbon and eight of oxygen; and, as an atom of oxygen has eight times the weight of an atom of hydrogen, it follows that an atom of carbon is six times heavier than one of hydrogen. Since the same definite proportion holds in the composition of a vast number of substances that have been examined, it has been concluded that there are great differences in the weights of the ultimate particles of matter. Although Dalton’s law is fully established, yet instances have occurred from which it appears that the atomic theory deduced from it is not always maintained. M. Gay Lussac discovered that gases unite together by their bulk or volumes, in such simple and definite proportions as one to one, one to two, one to three, &c. For example, one volume or measure of oxygen unites with two volumes or measures of hydrogen in the formation of water.