The call for oxygen by the kidney 93 



all manner of false deductions. There is only one method by which 

 a measurement of the work can be made, viz. by the use of the 

 second law of thermodynamics and the laws of osmotic pressure. 

 Whenever a given solution (e.g. the blood), containing any number 

 of substances dissolved, is separated into two or more separate parts 

 of different concentrations (e.g. blood and urine), then work is done : 

 and this work is always positive and can never be negative. It 

 always requires the liberation of free energy outside to effect the 

 separation of a solution into two different solutions. 



In order to avoid confusion we must emphasise that actually and 

 commercially (so to speak), to carry out the separation of the several 

 bodies, far more work will probably be done than is theoretically 

 necessary. In the same way the theoretical minimum work in foot- 

 pounds, which it is necessary to do in order to carry bricks up a hill, 

 is given by the weight of the bricks in pounds multiplied by their 

 vertical rise in feet : in practice, however, it will inevitably be the 

 case that very much more work than this will actually be done in 

 carting the bricks up the hill, depending on the state of the road, the 

 wind, the friction of the wheels, and the training of the horse, or the 

 internal friction of the engine which carries them up. But, in spite 

 of that, the only general and valuable estimate of the work to be done 



is the product 



(weight of bricks) x (vertical rise in feet), 



for the actual amount of work expended depends entirely on the 

 method adopted, and the mechanism by which the work is done 

 and that of course we do not in general know. The secretion of 

 urine may be regarded as the separation of one fluid, the blood, into 

 two fluids of the same total volume, the concentrated blood and the 

 urine. The actual energy used in carrying out the process of secret- 

 ing a given sample of urine we cannot calculate, until we know the 

 inner mechanism by which secretion is performed. All we can do is 

 to calculate, from the "molecular concentrations" of the several salts 

 in the urine and the blood, a certain quantity W, which is the minimum 

 work which would have to be impressed on the blood in order to 

 separate it into concentrated blood and urine. W is always positive 

 for every conceivable change, as will be shown below, and is obtained 

 on the hypothesis of reversible changes being carried out in concen- 

 trating the blood by means of semi-permeable membranes : there are 

 many processes by which the separation can be carried out, but the 

 second law of thermodynamics asserts that whatever be the process, 



