Absorbed on contact of Liquid with Solid. 245 



and then could be immediately removed. A piece of glass 

 tubing was attached to the brass tube so that the screw worked 

 along the axis of the glass tube, which served as a handle of 

 non-conducting material. The water equivalent of this instru- 

 ment was 2'35 grammes, and that of each thermometer was 

 1'50 grammes. 



In making an experiment, the bulb containing the powder 

 was placed under the surface of the water in the calorimeter, 

 and allowed to remain there for some hours, generally about 

 twenty-four hours. The temperature was then observed 

 every five minutes, and if several consecutive readings were 

 the same, the bulb was broken, and the temperature again 

 observed until it was constant. The rise of temperature was 

 generally complete in three minutes or less, the liquid being 

 gently stirred together with the powder. 



The mass of water. M, was always large compared with the 

 mass of the powder, p, and hence the error, if any, involved 

 in taking the specific heat of the water as 1, must have been 

 exceedingly small. The temperatures at which the experi- 

 ments were made did not differ very much, and the variation 

 due to these small differences of temperature was neglected ; 

 from theoretical considerations it follows that the variation in 

 the heat evolved is not more than *3 per cent, per degree 

 centigrade, and later experiments tend to confirm this. 



The specific heat of the glass bulb and its contents was 

 taken as '19 : any error in this assumption could not have 

 affected the results, since the mass of the bulb and its contents 

 was always small compared with the mass of water. 



The average diameter of the grains of powder was obtained 

 by measuring many hundreds of grains by means of a 

 microscope supplied with stage micrometer and eyepiece 

 micrometer. The microscope was so adjusted that thirty 

 divisions of the eyepiece micrometer exactly corresponded 

 to one tenth of a millimetre on the stage micrometer, so that 

 one division of the eyepiece micrometer represented J_ cm. 



It was found that in the dry state the smaller grains of 

 powder were often joined together, forming larger grains? 

 and it was not easy to distinguish a lump consisting of several 

 small grains from a complete grain ; hence any measure- 

 ments of powder in the dry state are likely to make the 

 powder appear much coarser than it really is. When the 

 powder was put in a drop of water on a glass slide under 

 the microscope and stirred with a small brash, the larger 

 pieces of silica were >een to break up into smaller grains of 

 fairly uniform size. The average diameter of the grains was 



