126 Mr. A. Mallock. [Dec. 13, 



material, and the potassium chloride formed exercises its specific 

 influence on this reaction. 



The secondary action upon potassium iodide producing iodine is 

 practically an instantaneous one, unless the quantity of this substance 

 is below a certain minimum. Below this the velocity observed in the 

 mixture will be less than normal. The effect of increasing the amount 

 of this substance to much greater than the minimum is closely analo- 

 gous to that of a similar increase of any neutral salt. 



The velocity is an exponential function of the temperature, as was 

 observed in Messrs. Harcourt and Esson's investigations. As the 

 latter increases in arithmetical progression, the former increases in 

 geometrical progression. The rate is about doubled for a rise of 5 C. 

 The ratio in this progression is not, however, absolutely constant, but 

 varies a little with the temperature at which it is taken. Thus 

 between and 15 C. the rate is a little more than doubled for a 

 rise of 5 ; between 20 and 30 it is a little less than doubled. 



IV. " Determination of the Viscosity of Water." By A. MALLOCK. 

 Communicated by Lord RAYLEIGH, Sec. R.S. Received 

 November 30, 1888. 



The experiments here described, which were made during April and 

 May of the present year (1888), to determine ^he constant of viscosity 

 of water, may be of some interest on account of the newness of the 

 method employed, and also as being on rather a larger scale than 

 other experiments which have been made with the same object. 



Fig. 1 gives a section of the apparatus used. 



A and B are two coaxial cylinders ; of these A is mounted on the 

 vertical axis E, and can be made to rotate by a belt passing over the 

 wheel F. B is suspended by a long fine wire C, and the annular space 

 between A and B is filled with water or any other fluid to be experi- 

 mented on. 



A little way above the lower edge of B is fixed an air-tight dia- 

 phragm D, so that when the space between the two cylinders is filled 

 with liquid air is inclosed under D, and the liquid touches B only on 

 the cylindrical surface. 



The interior of B above D is filled with water which serves the 

 purposes of checking the torsional vibrations of B, of preventing any 

 rapid change of temperature of the liquid in the annulus, and of 

 holding the thermometer. 



The experiments were made by driving the cylinder A at a uniform 

 speed and recording the angle through which B is turned when it 

 comes to rest under the action of the fluid friction on its cylindrical 

 surface and the torsion of the suspending wire C. A was driven by 



