Mr Davy, we must now turn our attention to Mr Dalton, one of the most ingenious and industrious philosophers of the present day. By the introduction of a very simple but most important theory, he has contributed very essentially to the improvement of chemical analysis, and to the accuracy of our notions respecting the constitution of compound bodies. According to him, when substances combine together, they unite either to each other atom to atom, or one atom of the one combines with a determinate number of atoms of the other. Water, for example, is composed by the union of one atom of oxygen with one atom of hydrogen. Now water is a compound of about 87.5 parts by weight of oxygen, and 12.5 of hydrogen, or of 7 parts of oxygen, and 1 of hydrogen. Hence we know that the relative weights of an atom of each of these bodies are to each other as 7 to 1. Therefore, if we denote the weight of an atom of hydrogen by 1, we must denote that of an atom of oxygen by 7. In like manner, ammonia is composed of an atom of hydrogen, combined with an atom of azote. Now it consists of 81.5 parts by weight of azote, 18.5 parts of hydrogen, or nearly of 4.5 azote, and I hydrogen. Hence, if the weight of an atom of hydrogen be represented by 1, that of an atom of azote will be 4.5. In like manner, from the analysis of carbonic acid, an atom of charcoal will be found to weigh about 5, an atom of sulphur weighs about 18, and an atom of phosphorus about 9. Now, knowing the relative weights of the different atoms, and the num ber of atoms which combine, it is easy to determine the composition of any compound, and likewise the relative weight of a particle of it. The following table exhibits a few examples of these determinations, to render them more familiar to the reader :— 2. Water is composed of 1 oxygen and 1 hydrogen. A particle weighs & 1 hydrogen. 1 carbon. 1 hydrogen. Carbureted hydrogen 1 carbon 2 hydrogen. 2 & acid, give us the relative proportions of the whole class of salts. soda lime - By the same mode of proceeding, we The same method furnishes us with the relative weight of a particle of the earths and alkalies. For the salts must be considered as composed of a particle of acid united to a particle of base. Thus, sulphate of barytes is a compound of one particle of sulphuric acid and one particle of barytes. Now, by the most accurate analysis, it is composed of 33 parts of sulphuric acid, and 67 barytes. Hence, if the weight of a particle of sulphuric acid be denoted by 34, as in the preceding table, that of a particle of barytes must be denoted by 69. In the same way we may obtain the weight of a particle of the earths and alkalies, as in the following table : 1 strontian 34 46 strontian 19 lime magnesia 19 46 19 23 20 Carbonate of potash The supersalts, or those which contain an excess of acid, are formed by the union of two particles of acid with one of base. Thus the super-sulphate of potash is compo sed of 34 x 2=68 acid, +42 potash. On the other hand, the subsalts, or those which contain an excess of base, are composed of two particles of base united to one particle of acid. Thus borax is composed of two particles of soda and one particle of boracic acid. Calcareous spar, and all the other earthy combinations of carbonic acid, as they are commonly exhibited, are in reality subsalts, being combinations of two particles of earth with one of carbonic acid. An additional dose of carbonic acid may be made to combine with these earthy bases, but it easily makes its escape again on account of its elasticity. The nitrates constitute exceptions to the general rule. They are composed of two particles of acid and one particle of base united together. They are all of course supersalts, though they do not redden vegetable blues. The triple salts are formed by the union of a particle of each of the salts which enters as a constituent. Thus tartrate of potash and soda, or Rochelle salt, is composed of a particle of tartrate of potash united to a particle of tartrate of soda. In the same way the metalline salts admit of analysis. They are usually composed of a particle of oxide united to a particle of acid. But to enter into particular details would oblige us to extend this article too far. Such is a short sketch of Mr Dalton's most curious and important theory. It applies to all the compounds which have been analysed, especially to the salts, with such uncommon precision, that it is impossible for the most sceptical chemist, who takes the trouble to examine the subject with sufficient care, to refuse his assent to it. For further details respecting this theory, we refer the reader to Mr Dalton's New System of Chemical Philosophy, two volumes of which have been published; or to the third volume of Dr Thomson's System of Chemistry, 3d or 4th editions, where the subject is stated at considerable length. 13. Some additions have been re cently made to our knowledge of animal substances, and of some of the animal functions, which deserve to be noticed. Mr William Brande has analysed a considerable number of urinary calculi, which are deposited in the Hunterian Museum in London, and has corrected some errors of preceding experimenters, and contributed some additional facts of his own. Preceding experimenters had detected a considerable number of sub. stances in urinary calculi; the principal of which are uric acid, phos. phate of lime, phosphate of magnesia and ammonia, oxalate of lime. Fourcroy and Vauquelin had announced also urate of ammonia as a pretty common constituent; but Mr Brande has shewn that they were misled, partly by the urea, and partly by other alkaline salts, with which the uric acid was mixed. Urate of ammonia, it would appear from his experiments, is never found in urinary calculi. The calculi formed in the kidney are usually composed of uric acid; though, when they lodge in that organ, they are sometimes coated over with the phosphates. Calculi composed of oxalate of lime are much rarer than any of the other species. Mr Brande has shewn that the medicines usually prescribed do not act as solvents, but often produce an additional deposite of calculous matter; the alkalies diminish the proportion of uric acid in urine; but they tend to increase the deposite of the phosphates. Acids, on the other hand, promote the depositions of uric acid. 14. The phenomena of respiration have attracted the particular attention of chemists, ever since the discoveries in pneumatic chemistry enabled them to ascertain the changes produced upon the air by drawing it into the lungs. Priestley, Goodwin, Menzies, Lavoisier, and Davy distinguished themselves particularly in these inquiries. It was ascertained that a portion of the oxygen of the air drawn in disappeared, and that a quantity of carbonic acid gas was found in its place; but doubts were entertained whether the bulk of the air inhaled was diminished, whether the azote which it contained was diminished, and what was the proportion of the carbonic acid formed to the oxygen withdrawn. A very accurate set of experiments has been made by Messrs Allen and Pepys to elucidate these particulars. The apparatus used was simple and ingenious, and every precaution was taken to ensure accuracy. The following may be considered as results established by these experiments. The bulk of the air is not altered by respiration; the azote is not altered, but remains invariably the same. The carbonic acid formed is just equal in bulk to the oxygen 'which has disappeared. The air drawn into the lungs comes out loaded with about nine per cent. of carbonic acid gas. The quantity of this gas given out by a middle-sized man is 302 cubic inches in eleven minutes, which amounts to rather more than eleven ounces troy of carbon in 24 hours. When oxygen gas is breathed, a portion of it is replaced by azotic gas. This singular result cannot be accounted for in the present state of our knowledge. It is possible that it may have made its way into the oxygen by some unknown means through the body. When a mixture of hydrogen gas and oxygen gas in the same proportion as common air is breathed, the animal exhibits no symptoms of uneasiness, but always falls asleep. It would appear, therefore, that hydrogen produces a soporific effect when it comes in contact with the lungs. The experiment was made upon a Guinea pig. Such are the most material discoveries in chemistry that have been made during the period to which our history extends. We deem it unnecessary to collect the insulated facts which have been added to different departments of the science, because it would be impossible to render them intelligible, without entering into much greater details than are consistent with their importance. II. The investigation of the ana tomy and physiology of plants was begun about 150 years ago by Grew and Malpighi; and since their time, many eminent philosophers have devoted their attention to this delightful study. Dr Hailes and Mr DuHamel particularly distinguished themselves by their discoveries. The most successful cultivator of this study at present in Britain is Mr Knight. His discoveries have been numerous and important, and he has prosecuted the subject for more than a dozen of years with much zeal and assiduity. One of his dissertations published within the period of our history claims our notice. It is well known that the sap of plants is absorbed by the roots, and that it moves upwards from these organs to the leaves, where it is digested and converted into the peculiar juices of plants. In trees, it had been ascertained that the sap moves through the alburnum to the leaves; and large vessels, called tracheas by Grew and Malpighi, and alburnous vessels by Mr Knight, were conceived to be the vessels through which the sap passed. But a set of experiments, recently made by Mr Knight, have rendered this opinion very doubtful, if they have not altogether overturned it. These vessels are always full of air, except during the season when trees bleed. Though they be completely cut through, the sap still continues to find its way to the extremi. ty of the branch, and the branch continues to live. Hence Mr Knight concludes, that the sap moves not through the alburnous vessels, but through the cellular substance of the alburnum. The alburnous vessels he considers partly as reservoirs for the sap, and partly as intended to increase the strength and lightness of the plant, on the same principle as the bones of animals are hollow. The strongest objection to this opinion is, the force with which the sap issues out at the bleeding season; a force, as Dr Hailes found, able to overcome a column of mercury 34 inches high. Now it is difficult to conceive how it could move with such force through the cellular substance of the alburnum, unless that substance were precisely similar to a vessel, and had the property of contracting. III. The Linnæan society still continue their useful labours. The ninth volume of their Transactions, published during the period to which our history extends, contains a great number of valuable dissertations, both in the departments of botany and zoology. The botanical papers, as usual, relate, with a few exceptions, to foreign plants, hence they cannot well be abridged. We shall notice some of the most striking novelties in the volume. The mosses, within these few years, have undergone a great change in their botanical arrangement. A great number of new genera have been invented, and the old genera, especially the brium, hypnum, and mnium, have been subdivided. These subdivisions must be admitted to be great improvements. They simplify the characters, and, by diminishing the number of species belonging to each genus, diminish the labour of finding the botanical name of every particular moss. Dr Smith has added a new genus to the number, to which he has given the name of Hookeria, in honour of Mr Hooker of Norwich. The characters of this new genus are the following: The capsule is ovate, and finely reticulated, from a scaly, lateral perechetium. The peristomium has sixteen teeth, both internally and externally, and internally it is membrana. ceous. The calyptra is reticulated and entire. There are ten species described, all of them foreign, except one, formerly called hypnum lucens, which is a native of Britain; the rest are chiefly from New Zealand and New Holland. The genus lichen constitutes one of the most difficult and interesting among the cryptogameous plants. It is exceedingly numerous; no fewer than 360 species, natives of Britain, having been already described. Dr Acharius has lately published a most interesting work upon these plants. He has subdivided them into a variety of new genera, which greatly facilitates their investigation, and has very much improved the method of describing these plants. His new division deserves to be adopted. Mr Dawson Turner, who has distinguished himself so much in the description of cryptogameous plants, has published an account of eight new British lichens, accompanied by very accurate engravings. They are all natives of the south of England, and were all first observed by Mr Burrer. Some of them are new species. One of the most difficult genera of the syngenesia class of plants is the hieracium. Different species of it, ranked among British plants, have never been seen by any living author growing native in our island. The hieracium dubium, and auricula may be mentioned as examples. Dr Smith has published a historical detail of all the facts known concerning these two species, which are arranged among British plants on the sole authority of Hudson. The hieracium murorum of Linnæus is the variety B of the Flora Britannica. What has hitherto been described as the hieracium sylvaticum was placed among the |