654 



SCIENCE. 



[N. S. Vol. XXII. No. 569. 



sities, which we all feel as such, can be 

 converted into far-reaching sharply formu- 

 lated equations, as was done by Carnot and 

 Clausius. These principles were first ap- 

 plied in chemistry by Horstmann. Then, 

 by successive application to chemical prob- 

 lems by Massieu, Gibbs, Helmholtz and 

 others, was won a system of relations touch- 

 ing the problem of affinity, to which I can 

 give only brief attention: 



1. Affinity may be defined as the maxi- 

 mum quantity of work that a chemical 

 change can produce. Equilibrium ensues 

 when this quantity is zero. 



2. The mass law can be obtained in a 

 well-founded and somewhat modified form, 

 restricted to dilute gases and solutions. 



' 3. The Thomsen-Berthelot principle as- 

 sumes a modified form in the rule that a 

 fall of temperature induces the formation 

 of that w^hich develops heat. It is, for 

 instance, in accordance with this rule that 

 at ordinary temperatures water is stable 

 in comparison with detonating gas, and 

 that at high temperatures this relation is 

 reversed, as it was found by Deville to be. 

 4. Lastly, we have the phase rule, indi- 

 cating, for example, in what cases chemical 

 phenomena will be comparable with melt- 

 ing and freezing, and in what cases they 

 will be comparable with evaporation and 

 condensation. 



Most curious of all, we can treat prob- 

 lems of affinity in an absolutely trust- 

 worthy way, so that our calculations fur- 

 nish a check upon experiment, without ad- 

 mitting anything concerning the nature of 

 affinity or of the matter wherein the affinity 

 is supposed to reside. 



J. H. van't Hoff. 



THE PROBLEM OF RENAL FUNCTION} 

 In my first lecture I touched upon a 



series of physiological problems that have 

 ^ Being the second of the Herter Lectures de- 



livel^d at the Johns Hopkins Medical School. 



been elucidated in a pharmacological way. 

 I treated these problems in a merely cur- 

 sory manner and did not enter upon details 

 of the various investigations. Permit me 

 to-day to discuss more fully a problem 

 which has for a long time claimed my 

 attention and which has for many years 

 been a topic of research in my laboratory, 

 namely, the problem of renal function. 



As is well known, there are two leading 

 and opposing theories on the nature of 

 urinary secretion. According to one of 

 these theories, which was developed most 

 fully by Heidenhain, we have to deal with 

 a true secretory process by which water 

 and perhaps the salts pass through the 

 glomerulus, whereas the specific constitu- 

 ents of the urine are liberated from the 

 tubules so that the sum of both secretions 

 is represented by the outflowing urine. 

 According to the other hypothesis, which 

 was first proposed by Ludwig and subse- 

 quently modified (in a biological sense) 

 by his successors, there goes on in the kid- 

 ney, side by side with the glomerular 

 activity, dependent essentially on the mech- 

 anical conditions of the circulation, and 

 independently also on the secretion of cer- 

 tain urinary constituents, a process of re- 

 sorption in the urinary tubules. Through 

 this resorption the slightly concentrated 

 secretion of the glomerulus, corresponding 

 to the water of the blood, undergoes con- 

 centration to a point characteristic of the 

 urine. 



The output of urine is chiefly conditioned 

 on the largely physical excretory process, 

 which, on account of its dependence on the 

 blood flow, and the blood pressure in the 

 kidneys, one is justified in regarding as a 

 kind of filtration or transudation. On the 

 other hand, the resorption of water through 

 the tubules is not directly dependent on the 

 circulation of the blood. That is, it is in 

 nowise proportional to the abundance of 



