In addition to the above solvents, others are employed for special operations, as circumstances seem to indicate. Thus the alcohol or ether may be used in an acid or alkaline condition, as described for water ; and benzoic, chloroform, bisulphide of carbon, and similar liquids, may also be pressed into service under certain circumstances. As before stated, much must depend upon the knowledge, judgment, and °beer ration of tho experimenter.
The treatment of the several liquid extracts obtained as above described is the next point for consideration ; but hero the field of experiment becomes so greatly enlarged that anything like a detailed description of it would 'lead us far beyond the limits of the present article. Suffice it to my, that fractional distillation, fractional crystal ligation, and fractional precipitation form the bases of operation adopted in the separation and isolation of organic proximate prin ciples.
The examination of the residue after an organic material has been successively subjected to tho action of the above liquids, is not a matter of very great difficulty, inasmuch as it can only consist of such substances as lignin and cellulose. Nor is it difficult to extract one proximate principle only from organic material, experience of the properties of similar principles here guiding to the most proper method.
The physical and chemical properties that should characterise an organic proximate principle have already been alluded to at the com mencement of this article ; it now, therefore, only remains to consider Ultimate Organic Analysis.
From what has been stated, it is evident that this operation mainly consists in the determination of the quantities of carbon, hydrogen, oxygen, and nitrogen contained in any organic substance. Chlorine, bromine, iodine, sulphur, and phosphorus are comparatively very rarely met with ; they are estimated in the manner described under each of those elements, or by some slight modification of the methods there given.
The firstpoint to bo attended to in an organic analysis, or com bustion, as st is practically termed, is the removal of hygroscopic water—the removal of that moisture which most bodies absorb by mere contact with the atmosphere. This may be done in several ways ; a very good one consists in placing the body over a surface of oil of vitriol under the receiver of an air-pump and exhausting, when complete desiccation speedily takes place. This process has the advantage of
not requiring the application of heat, sometimes a point of great importance. Substances that boar a temperature of 212° Fahr. are dried either in a water oven, or are placed in a curved tube of the annexed form, which is introduced into boiling water and dry air drawn through it. Sometimes mere exposure to dried air at common temperatures is sufficient to desiccate a substance.
The next step in the process is the accurate weighing of the material. For this purpose the latter is usually inclosed in a small tube, and a balance employed that will indicate of a grain.
The following are the principles involved in the actual analysis : First, of compounds containing carbon, oxygen, and hydrogen • for nitrogen must be estimated by a separate operation. It is well known that when coal or wood, or anything containing the three elements alluded to, is burned, the carbon takes up oxygen from the air and off under the form of carbonic acid, while the hydrogen corn t the quantity of oxygen requisite to form water and passes off as steam. Further, it is also well known that these combinations take place in fixed and unvarying proportions ; that six parts of carbon always combine with sixteen of oxygen to form carbonic acid; and that one part of hydrogen always combines with eight parts of oxygen to form water. On these facts, then, is founded the process about to be described; for, obviously, if a weighed quantity of the wood, coal, &c., were taken, and the whole of the carbonic acid and water formed could be collected and weighed, we should be in possession of all the data required for finding the amounts of carbon, and oxygen originally present. The total amount of carbonic acid divided by 22 and then multiplied by 6, would give the quantity of carbon ; tho total amount of water divided by 9 would show the ' quantity of gen • and the difference between the combined weights of the carbon and hydrogen, snd the weight of the original substance, would give the quantity of oxygen.