ORGANIC ANALYSIS. 



819 



best made by passing a piece of glass tube 

 through a cork fitting accurately into the mouth 

 of the combustion tube and connecting the 

 brass apparatus to this small glass tube by a 

 caoutchouc connector ; the limb (6) is fastened 

 by a similar joint to a glass tube (e) bent at 

 right angles near one end, with a straight por- 

 tion upwards of thirty inches long, the other 

 extremity of which is turned up at an acute an- 

 gle for the convenience of safely delivering the 

 gas ; the tube is placed in a vertical direction 

 with its lower upturned extremity dipping into a 

 small mercurial trough ; the stop-cock tube (r) 

 is connected with an exhausting syringe and 

 a vacuum produced ; the apparatus is left for 

 half an hour to ascertain that all the joints are 

 tight : if the mercury after this lapse of time 

 still stands at the same level, the experiment is 

 proceeded with ; a moderate heat is applied by 

 a spirit lamp to the end of the retort contain- 

 ing the carbonate ; by this means carbonic acid 

 is set free and displaces the last portions of air; 

 the exhaustion and disengagement of gas are 

 repeated alternately three or four times, taking 

 care to leave sufficient carbonate undecorn- 

 posed to renew this expulsive process at the 

 termination of the experiment. The stop-cock 

 (c) is now closed, the air-pump is removed, 

 and a graduated jar containing some solution 

 of potash is inverted in the mercury over the 

 recurved extremity of the long glass tube. 

 The copper turnings are then brought to red- 

 ness in the usual way by charcoal, and the 

 experiment conducted with the customary pre- 

 cautions, the decomposition being caused to 

 take place less rapidly than usual ; when the 

 part of the retort containing the matter for 

 analysis is red-hot through its entire extent, 

 heat is gradually applied to the carbonate at 

 the end, and the last portions of gas from the 

 combustion in the apparatus are driven into 

 the receiver by the disengaged carbonic acid. 



As the products of combustion are only 

 water, carbonic acid, and nitrogen, the two 

 former are retained by the solution of potash, 

 whilst the latter alone presents itself for mea- 

 surement. I need hardly say that the height 

 of the barometer and thermometer must be 

 carefully noticed, when the apparatus by stand- 

 ing for an hour or two has reached the tem- 

 perature of the atmosphere ; as the gas will be 

 saturated with moisture, its volume must be 

 corrected by the known methods for the three 

 points of temperature, pressure, and moisture; 

 then, since 100 cubic inches of nitrogen at 

 standard temperature and pressure weigh 

 30.15 grs., the weight of the nitrogen that a 

 given quantity of the matter analysed contains 

 is easily determined. In this process, as in 

 every case where the proportion of nitrogen 

 alone forms the object of our experiment, after 

 the weight of the material for analysis has been 

 once accurately determined, it is evident there is 

 nothing to fear from the absorption of moisture. 

 Occasionally the quantity of nitrogen, where 

 large, is advantageously determined by making 

 the combustion just as though we were going 

 to ascertain the proportion of carbon and hy- 

 drogen; but, instead of condensing the car- 



bonic acid and weighing it, the whole of the 

 gases produced are collected over mercury. 

 A bent gas-delivering tube is substituted for 

 the usual drying apparatus. (Fig. 431.) In 

 this case it is best to begin at the closed ex- 

 tremity of the tube, and having expelled the 

 atmospheric air by a portion of gas generated 

 from the substance, to collect the rest of the 

 gaseous products in a graduated jar; by agi- 

 tating the gas with solution of potash the pro- 

 portion of nitrogen to carbon is at once deter- 

 mined, as equal volumes of carbonic acid and 

 nitrogen represent single equivalents of carbon 

 and nitrogen. It is not necessary in this case 

 to determine accurately the quantity of mate- 

 rial acted upon. 



Experience has 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 chlorate of potash 

 is employed, the carbon is sometimes as much 

 deficient. A deficiency of carbon also occurs 

 if the ash contain carbonates in any form. 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 from all 

 adhering impurities ; fifteen or twenty grains of 

 the silver compound is accurately weighed 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. 



3 o 2 



