Experience hits shewn that in the preceding process for organic analysis the quantity of hydrogen deduced from it 'is always slightly in excess, usually about 0.2 parts in 100, whilst, unless chromate of lead or chlomte of potaslt is employed, the carbon is sometimes as much deficient. A deficiency of carbon also occurs if the ash contain carbonates in any forra. Oc casionally sulphur and chlorine are among the constituents of organic bodies ; the methods of analysis must then be modified. For details upon these subjects the reader is referred to the treatise of Berzelius. ' We will suppose the labour of analysis thus brought to a successful issue. It is, however, evident that the information derived from this source alone is but scanty, as we can thereby form no idea either of the number of equiva lents of each element entering into the com position of an organic body, or of its relations to the substances concerned in its production or obtainable from it by its decomposition. Whenever it is possible,the equivalent or com bining proportion of the compound must be determined. This is effected by preparing- a compound of the body with some substance, whose equivalent is well known, and proceed ing to analyse the new product. If our or ganic substance be soluble in water, and capable of entering into combination with oxide of silver, this oxide is for many reasons preferred. Oxide of silver combines with very many organic bodies, and forms with them compounds insoluble or sparingly soluble in water. They may generally be formed by double decomposition, and washed ffoin all adhering- impurities ; fifteen or twenty grains of the silver compound is accurately vveighed in a counterpoised porcelain crucible. It is then carefully incinerated till pure silver alone re mains. On again weighing, the loss will give that of the body combined with the silver, and in addition that of one equivalent of oxygen expelled from the oxide of that metal at a red heat. The residual silver should dissolve without remainder in nitric acid. Now, since the equivalent number of silver on the hydrogen scale is 108, it is evident that by simple calculation we may determine the equi valent, number of the organic body that had combined with it.
An example will perhaps elucidate my mean• ing more distinctly.
Sometimes no compound with silver can- be obtained, and a salt of lead is then, if prac ticable, substituted for it. The residue, how ever, in this case does not consist entirely of rnetallic lead, neither is it all oxide of lead. It is atrefully weighed, treated with acetic acid in the crucible itself; the oxide of lead is thus dissolved and washed away. When the con tents of the crucible have been carefully dried, a second weighing gives the quantity of me tallic lead, whilst the loss furnishes that of the oxide. From the metal we calculate the quan
tity of oxide to which it is equivalent ; this added to the portion dissolved by acetic acid furnishes the whole quantity of oxide con tained in the compoundi—a calculation similar to that employed for the silver salt, then sup plies us with the means of determining the equivalent number of the body analysed. This method is not quite so accurate as the preceding; it involves more manipulation, and the com pounds of lead are apt to undergo slight loss by volatilization at a high temperature.
It would here be out of place to enter into detail into the methods of checking the cor rectness of an analysis in its various parts. Upon this point the reader is referred for in formation to Liebig's Introduction to Organic Analysis. The subject is an important one, and by no means sufficiently attended to by the majority of those who devote themselves to analytical researches of this description.
The number of authors who have written upon the methods of analysis is very great ; and their instructions are found more in de tached papers, scattered through the various scientific periodicals than in systematic treatises.
The works which may be consulted with espe cial advantage on proximate analysis are Berze lius's Lehrbuch Der Chemie, third German edition, translated by %Valet., 10 vols. 8vo. ; the fourth edition of Prout's Treatise on Diseases of the Stomach and Urinary Organs, and his papers in the Medico-Chirurgical and Philoso phical Transactions; G.O. Rees on the Analysis of Blood and Urine ; Lecanu, Ann. de Chimic, xlviii., and various papers on the blood ; Simon, Handbuch der augewandten Medizinischen Chemie, 2 vols. 8vo. 1840-42 ; one of the most recent and best treatises on animal che mistry, full of laborious and careful analyses, with copious and accurate directions for their performance. This work is now being trans lated into English.
For directions for analysing the inorganic constituents of organized compounds the reader is referred in particular to Rose's Analytica Chemistry, either the fourth German edition, o the English translation of the first edition b Griffin.
Ample instructions for the ultimate analysi of organic substances are furnished in Liebig' Organic Analysis, translated by Gregory, an forming one of the series of works publishe in Griffin's Scientific Miscellany, and in th fifth volume of Dumas' Traite de Chemi Appliquee aux Arts, as well as in the volume of Berzelius already referred to.
A valuable treatise ha.s recently been pub4 lished in German by Vogel, jun. on the ap plication of the microscope to the field of animal organic chemistry—" Anleitung zum Gebrauche des Mikroskops zur ZoocherniscLe Analyse und zur Mikroscopisch-chemische untersuchung uberhaupt." (IV..A. 31111er.)