ENERGY CHANGES INVOLVED IN SECRETION 163 



viz. 30 grm. in a 2 per cent, solution, measuring accordingly 

 1500 c.c. The molecular weight of urea is 60, and it is not dis- 

 sociated, so that there is no correction for dissociation, also the 

 usual figure of 0*04 per cent, may be taken for the concentration 

 in the plasma. 



The expression for the work done is 



M 



If we express this amount of work as heat energy in small 

 calories the value of the constant R becomes T98, if T be taken 

 at 40 C., the value of T in absolute scale becomes 273 + 40 = 313, 

 and hence the value of RT is 620 at this temperature ; l Q is 30 

 grm., and the value of M, the molecular weight in grm., is 60, 



so that ^f becomes 0*5 ; the value of the ratio of is the same 

 M p 



as that of the two concentrations of the urea in secretion and 



2 

 plasma respectively = ^ ^^ = 50, and for Iog e 50, we can substitute 



Iog 10 50, on dividing by the Briggs modulus for transference from 

 Napierian to common logarithms, the value of log 1Q 50 is very 

 closely 1-7, and the value of the modulus is 0-434 ; so that we 

 finally get on making all these substitutions in the above equation, 

 for the value of the work done expressed in small calories : 



W = 620 x 0-5 x 1-7 -f 0-434 = 1214 cal. 



This amount of energy may be expressed as mechanical work 

 by remembering that the small calory is approximately equivalent 



1 The value of 5-8 rational calories or 580 small calories, given in a previous 

 chapter, was the usual value based on a temperature of 15 C., the value 620 

 small calories used above is that which the expression ET has at a temperature 

 of 40 C., the approximate temperature of secretion of the urine. The value of 



PV 



1-98 for R is obtained by using the formula PV = RT, or R = -^-, and then sub- 



stituting the values for P, V, and T for a grm. molecule at any given values of 

 pressure, volume, and temperature corresponding to one another. Thus a grm. 

 molecule at C. has a volume of 22,330 c.c., a pressure of 76 c.m. of mercury 

 = 76x13-4x981 dynes, and T is 273 on absolute scale. Also 1 small calory 

 = 42xl0 6 dynes, and on substituting these values in the above equation we 

 obtain for the value of R in small calories : 



22330 x 76 x 13'4 x 981 _ 



273x42xl0 6 >98 ' 



