246 Mr. G. J. Parks on Heat Evolved 



or 



found to be f of a division of the eyepiece micrometer, that 

 is "00025 cm. Suppose now there are n such average grains to 

 a gramme of powder, then, since the specific gravity of the 

 powder is 2*2, we have 



nx*"('00025) 3 x 2*2 = 1 



on the assumption that the grains are spherical, and the area of 

 surface is equal to 



n.7r(-00025) 2 = 6x ~ 2 x -A_ = 10900 sq. cm. 



That is, one gramme of the precipitated silica exposed a surface 

 of about 10900 sq. cm. Another variety of silica used in 

 these experiments was examined under the microscope, and 

 the average diameter of the grains was found to be 1*2 

 divisions or '00010 cm., so that the area of surface per 

 gramme was 



6X: 0004 >< F2 =B820sq - cm - 



Experiments were also made with some fine grey sand of 

 specific gravity 2'6 ; the average diameter of the grains was 

 •010 cm., and the area of the surface exposed by one gramme 

 of the sand was therefore 



6X 2 T 6 X •01 =231 sq * Cm '' 



on the assumption that the grains were spherical. 



The probable error in estimating the surface of a powder 

 by this method is considerable, because the particles are 

 irregular in shape and size, especially those of the sand. 

 Experiments were therefore made with "cotton silicate" or 

 a glass wool," a kind of glass of specific gravity 2*7, in the 

 form of fine filaments which, when examined under the 

 microscope, are seen to be almost perfect cylinders, of fairly 

 uniform size. The length of each cylindrical filament was 

 very great compared with its diameter, so that in calcu- 

 lating the area of its surface only the curved surface was 

 considered. The average diameter was found to be '00175 cm.; 

 hence the area of surface exposed by one gramme of the 

 silicate was 



2-7x 4 00175 =847sq - Cm - 



The accompanying table (I.) shows the results of some of 

 the experiments. Other experiments were made, but the 

 results are not shown because they are considered to be 



