104 LABORATORY WORK IN PHYSIOLOGICAL CHEMISTRY. 



when it is remembered that solutions of urea and sodium 

 chloride with the same specific gravity would have widely 

 differing J, since urea is non-dissociable, while sodium chlo- 

 ride in the strength in which it exists in the urine disso- 

 ciates to a considerable degree. 



The freezing-point determination is made by the use of 

 the Beckmann apparatus (see demonstration). The manip- 

 ulation is simple, rapid, and requires little experience. 



Another property of the urine which is dependent upon 

 the presence of electrolytes in the solution is termed the 

 electrical condiictivity. Electrolytes are substances which 

 become capable of conveying electricity in virtue of the fact 

 that they become dissociated in solution into (electrically) 

 active, positive or negative parts or ions. Therefore the 

 electrical conductivity is in direct proportion to the degree 

 of dissociation or ionization of the urine. This property is 

 unaffected by variations in the content of urea in the 

 urine. Such determinations are accomplished by the use 

 of a rather complicated apparatus based upon the theory 

 of the Wheatstone bridge. (See demonstration.) The im- 

 perfect knowledge which exists in regard to the conditions 

 obtaining in the various fluids of the body does not permit 

 of exact interpretations of the results acquired by these 

 methods. 



It must suffice, therefore, to merely mention that various 

 factors are derived from a combination of the estimation of 

 the J and that of urinary substances, such as NaCl, total 

 nitrogen, etc., and that such factors are constantly increas- 

 ing in value as data serving to present a clearer picture of the 

 conditions under which a given sample of urine may have 

 been excreted. 



