ANALYSIS. 



211 



found in animal substances. The object of organic 

 analysis is to determine the nature and relative 

 proportions of these constituents. A knowledge 

 of the composition of vegetable and animal pro- 

 ducts is absolutely essential to the progress of 

 every investigation ; it forms indeed the only true 

 basis of a scientific research into their nature. In- 

 numerable as are the organic compounds, and few 

 as their ultimate elements have been above stated 

 to be, yet correct analyses have shown but few 

 compounds that possess exactly the same elements 

 in the same proportions. The method followed in 

 earlier times, for analysing organic compounds, was 

 extremely imperfect, and does not resemble in the 

 least that now followed. The present plan has 

 reached a very high degree of perfection, and al- 

 most entirely by the instrumentality of chemists 

 still alive Gay Lussac, Berzelius, and Liebig. 



The simplest niode of analysing organic com- 

 pounds would appear, at first sight, to be that of 

 obtaining the elements separate and pure ; but 

 such a plan is impossible, or at least, has never yet 

 been practised. Nor is it at all necessary, for suf- 

 ficient accuracy is attained when we separate the 

 elements into known compounds. Nitrogen alone 

 is obtained in the simple uncombined form, and in- 

 deed is always so estimated; but carbon, oxygen 

 and hydrogen, are never, in organic analysis, obtained 

 pure, but are estimated in the case of carbon, as 

 carbonic acid, and in that of the other two ele- 

 ments as water. It will be obvious, that from the 

 composition of carbonic acid being already esta- 

 blished, the quantity of carbon can be correctly 

 determined when the amount of carbonic acid is 

 given ; and in like manner, from the known compo- 

 sition of water, the proportions of hydrogen and 

 oxygen may be calculated with the greatest ac- 

 curacy. 



The means, then, employed to arrive at a perfect 

 knowledge of the composition of an organic com- 

 pound, is by converting a known weight of it, into 

 carbonic acid and water. When the substance con- 

 tains nitrogen likewise, this body is collected gen- 

 erally in a separate process in the simple form. 

 Perfectly to effect this conversion, and conveniently 

 to estimate the quantities of the products, is the 

 intention of the modes of organic analysis as now 

 practised; this end is attained by mixing the body 

 to be analysed with another highly oxygenated 

 substance, which will readily part with its oxygen 

 at a high temperature ; for this purpose chlorate of 

 potash, oxide of copper, and chromate of lead have 

 been employed. The most usual, and for general ana- 

 lysis, the best, is the black oxide of copper, obtained 

 by exposing the nitrate of copper to redness. This 

 substance has the valuable property of resisting any 

 amount of heat by itself, and yet of yielding up 

 oxygen readily to organic substances at a moderate 

 heat. It becomes reduced thereby to metallic cop- 

 per. The carbonic acid gas and water which re- 

 sult from this oxidation, escape from the heated 

 mixture of substance and oxide of copper, and arc 

 collected in apparatus shortly to be described, for 

 ascertaining their quantity. The water is con- 

 densed by means of a substance to which it is 

 known to have an immense affinity, namely, chloride 

 of calcium, and the carbonic acid is absorbed by 

 caustic potash. The increase of weight on the 

 part of the chloride of calcium, affords the data for 

 calculating the proportion of hydrogen in the com- 

 pound ; for water contains exactly A of its weight 

 of hydrogen. There are two ways of estimating 



the quantity uf carbonic acid; by collecting the 

 whole gases given off during the" operation over 

 mercury, and then condensing the carbonic acid 

 by means of caustic potash, and judging of its 

 quantity from the amount of condensation. The 

 process takes place, of course, in graduated vessels. 

 But the newer, and the better way of estimating 

 the carbonic acid, is by forcing the gases evolved 

 to bubble through a solution of caustic potash, and 

 the amount of carbonic acid is determined by the 

 increase in weight which this solution has attained. 

 Carbonic acid contains 27'64. p. c. of carbon. By 

 these means, then, the content of the substance 

 in carbon and hydrogen has been determined, and 

 if there be no other constituent, it is obvious that 

 the two quantities thus obtained, must together 

 give a sum equal to the quantity consumed in the 

 analysis. Many oils and other bodies contain besides 

 carbon and hydrogen no other constituent. If the 

 substance analysed contained besides those only 

 oxygen, the quantity of this element is obtained by 

 subtracting the sum afforded, by adding together 

 the two quantities of carbon and hydrogen, from 

 the whole weight of substance used. Thus, if the 

 weight used be 10 grs. and it be found, by experi- 

 ment and calculation, that the carbon and hydrogen 

 together, amount to 8 grs., the oxygen is then '2 

 grs. in ten, or 20 p. c. If the substance contain 

 nitrogen, its quantity must be added to that of the 

 carbon and hydrogen, and then the sum subtracted 

 from the whole weight used to ascertain the amount 

 of oxygen. 



The particular mode of procedure, in analysing 

 a substance that does not contain nitrogen, is briefly 

 this : a quantity, varying from three to twelve 

 grains of the substance in tine powder, and perfectly 

 dry, is well mixed with recently ignited oxide of 

 copper. The mixture is introduced into a tube 

 about 12 inches long, and an inch wide. This 

 tube must be of good potash glass, which does not 

 fuse readily. It is sealed at one end, at the other 

 it is stopped with a sound dry cork, into which 

 passes a narrow tube, for conducting the evolved 

 gases and vapours, through a bulb filled with pieces 

 of chloride of calcium, to absorb the water. A 

 small tube of caoutchouc connects this bulb with 

 an apparatus destined to collect the carbonic acid. 

 This apparatus consists of five bulbs, into which is 

 introduced the liquid caustic potash. The con- 

 struction and disposition of those parts will be easily 

 understood. 



The furnace is of malleable iron, and has slits 

 at the bottom for allowing a supply of air; the 

 combustible used is charcoal. The chloride of 

 calcium and caustic potash apparatus, having been 

 weighed before and after the combustion, the in- 

 crease in weight of the different parts, afford the 

 data for calculating the composition of the sub- 

 stance submitted to analysis. The above process 

 is used for determining the carbon, hydrogen, and 

 oxygen of all organic substances, as well of those 

 which contain nitrogen, as of those which do not. 

 But when nitrogen is present, another process is 

 superadded to the above, for the purpose of ob- 

 taining the nitrogen in a state of purity. There 

 are two modes of doing this ; they resemble each 

 other in the substance being mixed and burned 

 with oxide of copper, as has been before described, 

 but differ in the modes pursued for determining 

 the nitrogen. One consists in obtaining the gas 

 pure by exhausting the apparatus of air, and caus- 

 ing the mixed carbonic acid and nitrogen gases to 

 o2 



