310 LECTURE xxvr. 



In whatevei' manner we compare the specific gravities of bodies with that 

 of water, it is necessary, for very accurate experiments, either that the water 

 be employed at the temperature of the air when moderately warm, or that a 

 proper correction should be made for its change of bulk at different tempera- 

 tures. Platina, the densest known substance, is S3 times as heavy as dis- 

 tilled water, gold 194-, mercury 134-, lead 1]^, silver 11, copper 9, iron and 

 steel 7-|., stony substances usually about 24-, rectified spirits 1^, naphtha, the 

 lightest liquid-i^, cork about^-, conjmon air .g4^, steam -^sW, and pure hydrogen 

 gas , ,, 000 . From this comparison, the weight of a cubic foot of any of these 

 substances may be easily determined; since a cubic foot of water weighs 

 nearly 1000 ounces avoirdupois, or more nearly 998; thus a cubic foot of 

 gold would weigh about 195 000 ounces, and be worth above 60 000 pounds 

 sterling; a cubic foot of iron weighs 7750 ounces, and a cubic foot of com- 

 mon stone about 2500. 



The method of measuring the bulk of solid bodies by immersing them in a 

 fluid was applied, by its inventor Archimedes, to the detection of a fraud in 

 the composition of a mixed metal: and at present the principal use of hydro- 

 meters is for ascertaining, by the specific gravity of a compound of alcohol and 

 water, the proportional quantities of its ingredients. But in all experiments of 

 this kind, it is necessary to be aware, that a considerable change of the joint 

 bulk of two substances is often produced by their mixture: and that in ge- 

 neral their dimensions are considerably contracted. Thus, 1 8 gallons of 

 water, and 18 of alcohol, instead of 36 gallons, make only 35, consequently 

 the specific gravity of the compound is one 35th greater than the mean of the 

 specific gravities of the ingredients. And in some cases the whole dimensions 

 of a single substance may even be contracted by the addition of another sub- 

 stance: thus iron, by the addition of one eighth of its bulk of platina, becomes 

 contracted one fortieth of that bulk. 



The use of the spirit level depends on the tendency of all fluids to preserve 

 a horizontal surface, and the freedom, with which the particles qf fluids move 

 on each other, renders it an instrument capable of the greatest delicacy. A 

 tube, which is very slightly curved, being nearly filled with alcohol or ether, 

 and then perfectly closed, the bubble will always rise to the highest part of 

 the tube, and will never be stationary at the point which is marked as its 



