796 



ORGANIC ANALYSIS. 



the original liquid, if it produce coagulation, 

 shews the presence of casein. We need not 

 seek for casein if the fluid shews an acid re- 

 action, asit is coagulated by free acids in general. 

 If mucus be present, some ambiguity may arise 

 from the action of acetic acid, as this re-agent 

 causes the coagulation of the mucus furnished 

 by the alimentary canal and its appendages. 

 When present, however, in appreciable quan- 

 tity, mucus always communicates to the fluid 

 a certain degree of ropiness which leads us to 

 suspect its presence. A confirmatory test for 

 casein under such circumstances consists in 

 adding a few grains of milk sugar and a little 

 washed rennet ; if the mixture be heated for an 

 hour or two to about 120, the casein coagu- 

 lates completely. 



<\ Fatty matters and cholesterin are revealed 

 by the microscope, and may be separated by 

 evaporating the fluid to dryness, digesting the 

 residue with ether and filtering; by sponta- 

 neous evaporation of the ethereal solution, 

 they are left behind with their usual physical 

 characters. 



d. The presence of sugar is best discovered 

 by mixing the suspected fluid with yeast and 

 placing it in an inverted tube over mercury for 

 twenty-four hours, at a temperature of from 

 70 to 80 F., making at the same time a 

 comparative experiment with an equal bulk of 

 the same yeast diluted to the same extent with 

 pure water. We cannot by yeast determine 

 with certainty the presence of sugar in a pro- 

 portion less than ^th of the liquid employed. 

 A much more delicate test, and one which fur- 

 nishes more immediate results, has lately been 

 proposed by Trornmer, founded upon the fact 

 that organic bodies, to which free alkali has 

 been added in excess, and especially solutions 

 of grape and milk sugar, dissolve freshly pre- 

 cipitated oxide of copper; when the saccharine 

 solutions are boiled they are decomposed, and 

 sub-oxide of copper is deposited of a charac- 

 teristic reddish brown colour. To apply this 

 test, add to the suspected liquid a few drops 

 of solution of caustic potass, so as to render 

 it distinctly alkaline, then a small quantity of 

 a dilute solution of sulphate of copper, agita- 

 ting to dissolve the precipitate ; a liquid of a 

 blue colour varying in intensity with the quan- 

 tity of copper held in solution is obtained. 

 Apply heat, and if sugar is present an ochre 

 yellow or red precipitate of sub-oxide takes 

 place, as soon as ebullition begins. I have 

 already mentioned that the presence of milk 

 sugar has the same effect as grape or dia- 

 betic sugar; other animal matters produce 

 a similar change. It is, however, very deli- 

 cate in its indications ; a negative result 

 therefore may be considered as decisive of the 

 absence of sugar. If a precipitate occur, the 

 presumption that we are examining a saccha- 

 rine liquid ought always to be confirmed by- 

 recourse to the unequivocal expedient of fer- 

 mentation. It is easy to concentrate the fluid 

 if the sugar is very small in quantity ; the only 

 case in which any ambiguity can arise. Sugar 

 of milk has never been found but in the secre- 

 tion from which it derives its name. 



e. Urea can only be discovered satisfactorily 

 by evaporating the suspected fluid to dryness- 

 and treating the residue with alcohol, again 

 evaporating this alcoholic solution to dryness, 

 redissolving in water,and adding nitric or oxalic 

 acid with the precautions to be mentioned 

 hereafter when treating specially of the deter- 

 mination of urea. When common salt is al- 

 lowed to crystallize from a moderately dilute 

 solution of urea, the form of the crystals is 

 modified, and instead of obtaining cubes or 

 octohedra the crystals developed assume a 

 more or less penniform appearance, often shew- 

 ing the figure of a Maltese cross with serrated 

 edges. These modifications have been proposed 

 as a test of the presence of urea : they are 

 not, however, certain indications, though useful 

 as affording a presumption of its presence. 

 When a fluid which contains urea is concen- 

 trated by evaporation, and iiitric acid is then 

 added, by spontaneous evaporation on a glass 

 phite we obtain lamellar crystals of nitrate of 

 urea in irregular rhomboidal plates with the 

 acute angles often truncated. 



,/'. To detect the existence of uric acid 

 and the urates, if albuminous principles are 

 present, the liquid is evaporated to dryness, 

 and the residue (lighted for some hours with 

 solution of caustic potash, till e'very thing 

 soluble has been taken up. It is diluted, 

 filtered, and supersaturated with hydro- 

 chloric acid ; a flocculent precipitate forms, 

 which is redissolved by excess of acid ; and 

 the uric acid separates ; this is collected 

 on a filter. We are enabled by this pro- 

 cess, where considerable quantities of uric 

 acid are present, as in the excrements of 

 birds of prey, to obtain results of considera- 

 ble accuracy ; but where the proportion of the 

 acid is very minute, it cannot be relied upon 

 for quantitative experiments ; the residue in 

 these cases must be subjected to microscopic 

 examination. If no azotised matters are pre- 

 sent, the mere addition of free hydrochloric 

 acid after a lapse of some hours usually causes 

 a separation of rhombic crystals of the acid ; 

 or by evaporating to dryness and treating the 

 residue with water acidulated with hydro- 

 chloric acid, the uric acid remains undissolved ; 

 the residue is proved to contain this compound, 

 either by the appearance of its crystals under 

 the microscope, or by evaporating a small por- 

 tion to dryness with nitric acid on a slip of 

 glass, when a red stain is left ; a drop of am- 

 monia added produces a fine crimson ; if this 

 be evaporated to dryness, a drop of solution 

 of caustic potash converts it into a beautiful 

 purple, which is destroyed by the application 

 of heat. 



The methods for testing the other acids will 

 be described when treating of their quantitative 

 determination. 



Having now obtained an idea of the nature 

 of the fluid we have to examine, we are ready 

 to proceed with its 



Quantitative analysis. 



A thin porcelain or platinum capsule is 

 carefully counterpoised, and filled with a given 

 weight of the fluid, usually 500 or 1000 grains. 



