BORIC ACID AND BORAX. 27 



SPECIAL DIFFICULTIES CONNECTED WITH THE WORK. 

 COLLECTION OF EXCRETA. 



Aside from the usual difficulties connected with analytical practice, 

 which must alwa} r s be taken into consideration, there are some special 

 points in connection with a work of this kind which must be men ; 

 tioned. These difficulties are connected chiefly with the collection and 

 analysis of the excreta. The principal object in the analysis of the 

 excreta, as is evident, is to, establish the relation between certain 

 ingested elements and those which appear in the excreta. Certain 

 forms of food are more or less completely changed in passing through 

 the body, and are oxidized and manifested as heat and energy. The 

 fats and carbohydrates are types of foods of this kind. Certain other 

 elements in foods, while they undergo marked changes of combination 

 during digestion, assimilation, and excretion, appear in the excreta in 

 practically the same quantity in which they are found in the food. 

 Among these substances may be particularly mentioned nitrogen, 

 sulphur, and phosphorus. 



In a state of equilibrium, where the body is exercising all of its 

 functions in a normal manner, and where there is neither increase nor 

 decrease in body weight, the quantities of nitrogen, sulphur, and 

 phosphorus which are excreted should be the same as those which are 

 ingested in the food. This should not be construed to imply that the 

 actual elements eaten on one day appear in the excreta of the next 

 day. This is far from being the case. It may require many days, 

 weeks, or even months, for a given particle of nitrogen, sulphur, or 

 phosphorus ingested in the food to reappear in the excreta. It is 

 sufficient, however, for the purpose of establishing the balance between 

 these ingested substances and those which are recovered in the excreta 

 to assume that the quantities forced out of the body each day in a 

 normal state are equivalent in all respects to those which are intro- 

 duced. As an illustration, the case of a tube long enough to hold a 

 hundred marbles may be cited. If an additional marble be forced in 

 at one end of the tube, a marble of equal magnitude will be forced out 

 at the other, and thus the balance will be maintained in the tube. So 

 in a state of equilibrium each molecule or atom of nitrogen, phos- 

 phorus, or sulphur entering the body will be represented by a similar 

 molecule or atom of these, respective substances forced out of the 

 body. 



Were it practicable in experiments such as these to collect abso- 

 lutely every particle of emergent nitrogen, for instance, the balance 

 between the entering and departing nitrogen should be complete. In 

 these experiments, however, no attempt was made to collect any of 

 the nitrogen except that removed from the body in the urine and 

 feces. This, of course, represents nearly all of the nitrogen excreted, 



