Combustible Substances. 213 



weaker or stronger than proof. This method, 

 however, cannot be depended upon, as the bub- 

 bles of a weaker spirit more oily than ordinary 

 have the same appearance, and continue as long 

 as those of proof spirit ; and some dealers have 

 contrived to give their spirit a certain viscidity 

 which produces this appearance. 



Double rum is commonly examined by drop- 

 ping olive oil into it. If the oil sinks, the spirit 

 is considered as sufficiently strong; if it swims, 

 it is thought too weak. No doubt but the com- 

 paring of spirit with other liquors whose density 

 is nearly the same is the best test of their com- 

 parative strength, but it does not determine their 

 particular strength. 



In the custom-house they use a ball of metal 

 with a graduated stem* ; but they are obliged 

 to vary the instrument for the different spirits, 

 having particular weights to screw on for each ; 

 and as the temperature of the air alters much the 

 density of spirit (for spirit has been found to 

 expand seven times as much as quicksilver), they 

 have three or four weights for the different 

 seasons. Yet as these are adapted only to four 

 degrees of heat, as they have no directions on 

 what particular days they must be used, and as 

 what may be called cold in Jamaica may be very 

 warm weather here, this method must be very 

 inaccurate. 



Another method is to use a small glass bubble 



* See the lecture on Hydrostatics. 



