CHAPTER III. 

 THE REGULATION OF TEMPERATURE. 



THERMOMETER EFFECTS. 



Because of certain obvious analogies between a closed osmotic cell 

 and a sensitive thermometer, the name "thermometer effects" has been 

 given to a large group of exceedingly troublesome manifestations which 

 follow even slight fluctuations in bath temperature. The name is 

 appropriate only in a very restricted sense. The phenomena thus 

 classified are complex and often they are difficult to analyze satisfac- 

 torily. To understand them, one needs to keep constantly in mind 

 three fundamental facts: (1) That the capacity of the closed osmotic 

 cell is a nearly fixed quantity; (2) that every change in the volume of 

 its contents — due to rise or fall of temperature — is followed by a dis- 

 charge or intake of solvent through the membrane, both of which acts 

 also modify the volume and the osmotic pressure of the solutions ; and 

 (3) that the passage of the solvent through the membrane, in either 

 direction, is usually a much slower process than the changes in the 

 volume of the cell contents which result from fluctuations of tempera- 

 ture. The first and second of the enumerated facts are obviously true, 

 but the third, which is responsible in largest measure for the complex 

 and often perplexing results, can be learned only by experience. 



The four elementary fluctuations of temperature and their conse- 

 quences will be considered: 



(1) The temperature of the bath (previously constant) rises and 



becomes again constant at the higher level. 



(2) After rising, it falls again to the original level. 



(3) The temperature of the bath (previously constant) falls and 



remains constant at the lower level. 



(4) After falling, it rises again to the original level. 



The question to be answered is, what changes in cell pressure will the 

 observer at the telescope see in consequence of the temperature fluc- 

 tuations enumerated above? For convenience, all positive pressure 

 in the cell which is not osmotic will be called mechanical, and the sum of 

 the two will be spoken of as the total pressure. 



1. The conditions which are supposed to prevail are as follows : The 

 cell contains a solution of known concentration, the temperature is con- 

 stant, and the solution is exhibiting its true osmotic pressure only. 

 Subsequently the temperature rises and becomes constant again at the 

 higher level. This is the simplest of the four cases previously men- 

 tioned. The volume of the liquids in the cell — the mercury in the 

 manometer and the solution — and the tension of the gas in the manom- 



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