New Forms of Stereometers. 313 



method, the volume being altered by the introduction of 

 brass cylinders of known volume : — 



Calculated Vol. Found. % Error. 



279-13 2787 --15 



230-47 231-2 +-32 



27779 279-1 -^-47 



301*45 301 '9 +*I5 



313-27 313-0 --09 



The method is only adapted to the determination of 

 specific gravities when the volume of the substance bears a 

 large proportion to the whole capacity of the apparatus, as 

 otherwise the experimental error is enormously multiplied. 

 Taking the last number of the above series for example, 

 we have : — 



Vol. of apparatus, 325-1 ; Cylinder introduced, 11.83 



Vol. of Cylinder. 



Vol. of CO 2 (calctd.) Found. 



313-27 313-0 



Calctd. Found. 



11*83 I2-I 



Error % = -09 Error % = 2-28 



Neither this method, nor the ordinary stereometric one 

 can be used for a substance such as glass wool. A long series 

 of experiments made by us with this substance shows that its 

 power of condensing gases on its surface is an insuperable 

 obstacle in the way of the determination of its specific 

 gravity by these methods. 



The conclusions to be drawn from our experience of 

 these instruments are : 



(i) Their accuracy is inferior to that of the hydrostatic 

 method, which should therefore always be employed 

 when possible. 



(2) Porous substances cannot be employed. 



(3) The methods of pressure and dilatation should always 



be both applied. 



(4) The air in the stereometer should either be dry or 



saturated with moisture. 



