This general absence of crystalline form in animal principles, and the consequent difficulty of ascertaining that they are free from all mois ture, which does not chemically enter into their constitution, have,• by rendering us uncertain of the purity of the substances analysed, mainly contributed to the slow and uncertain proglress of this department of chemistry, and have given rise to the numerous contradintnrv statpmpnts with which it abounds. By multiplied researches and experiments we are, how ever, at leng,th arriving at results on the accuracy of which tolerable confidence may be placed. • The determination of the four elements, carbon, hy drogen, oxygen, and nitro gen, as they constitute the bulk of most organic sub stances, is that part of the process which now claims our attention. It is to Gay Lussac and Thenard that we are indebted for the funda mental principle that re gulates our operations. The process proposed by them has subsequently been modified and improved by many chemists, especially by Berzelius, Prout, and Liebig, and in the hands of the lat ter eminent philosophrit_has acquired a de gree of facility and accuracy hitherto unap proached in any other department of analytical research.
Our object being to determine the relative proportion in which each of the ultimate ele ments exists, it becomes necessary to the success of any analytical process that we should pro cure them in the form of definite compounds that can easily be collected ; and it has been found most convenient, by supplying the sub stance to be analysed with a sufficient quantity of oxygen, to convert the carbon into carbonic acid, which may be absorbed by potassa and weighed, and the hydrogen into water, which may likewise, by passing over a substance that has a powerful attraction for it, such as chloride of calcium or sulphuric acid, be collected and weighed, whilst the nitrogen escapes as gas, which is collected over mercury and measured.
In cases where nitrogen is present, it has re cently been proposed to heat the substance to be analysed along with hydrate of soda or potash; all the nitrogen is thus converted into ammonia, in which form, like carbonic acid and water, it admits of being weighed. By calculation it is easy to find the weight of the carbon, hydrogen, and nitrogen respectively contained in the car bonic acid, water, and ammonia collected. Car bonic acid contains three-elevenths of its weight of carbon ; water, one-ninth of hydrogen, and ammonia fourteen-seventeenths of nitrogen. When by incineration of a portion of the mass the proportion of saline matter has been deter mined, the quantity of oxygen the substance contains may be known by deducting the united weight of the carbon, hydrogen, nitrogen, and salts from the total weight of the body ana lysed ; the deficiency (supposing sulphur and phosphorus not to have been present) is oxygen.
Scrupulous attention to the purity of the matter submitted to analysis is of course of primary importance, a very slight admixture with other compounds being sufficient to vitiate the conclusions deducible from our experi ments. liaving ascertained the purity of our substance, the next care is to ensure its corn plete desiccation. For this purpose the fol lowing plan, recommended by Liebig, will be found the most efficient (fig. 429). A small quantity of the material to be dried is placed in an inverted syphon-tube (c), the bend of which is plunged into a vessel (D), contain ing water gradually heated to the boiling point. When plain water is used, the tem perature of course will not rise above 212°; but by substituting for it different saline solu tions we may at pleasure obtain any degree of heat between 212' and 300°, according to the nature of the compound to be analysed. A current of dry air is made to pass over the substance by connecting, one limb of the sy phon with a tube containing chloride of calcium (A), and the other with a vessel (E) closed at top, excepting the aperture by which it is con nected with the syphon-tube, and filled with water, vdtich is allowed to run out at the bottom with a speed regulated by a stop-cock V), the place of the liquid being supplied by air, which has passed over the chloride of cal cium and then through the syphon-tube. Vo latile liquids that are unchanged by distillation should be allowed to stand two or three days upon fragments of fused chloride of calcium; the liquid should then be decanted and dis tilled in a small retort; in other cases, as in the examination of fats or fixed oils, it may be more convenient to dry the material in a watch glass placed in an ordinary water-bath or the hot water oven previously described. The further progress of the analysis will vary according to the form and composition of the substance to be examined.
we shall describe the methods of analysing 1. A solid, which does not contain nitrogen.
2. A fluid, which does not contain nitrogen.
3. A substance, which does contain nitrogen.
1. Analysis of a solid not containing nitrogen.
The combustible which answers best in these experiments is charcoal ; it is the least expen sive, and very manageable, but dusty. Spirits of wine or pyroxylic spirit, no doubt, are cleaner, but their expense is a great objec tion. Gas has been tned by myself and others in a variety of ways, but though some modi fications of burner answer tolerably well, it is not on the whole to be recommended.