ME. W. CEOOIvES OX THE ATOMIC WEIGHT OE THALLIUM. 
285 
That of 200 grains of glass . . =200-0970 grains. 
Weight of air displaced by water =0-976 grain. 
„ ,, glass =0-0970 „ 
A brass weight of 1000 grains in use in my laboratory displaces 0-1402 grain of air ; 
a 1000-grains platinum weight, 0-058271 grain of air. 
The weight of glass and water . 
,, air displaced by them 
W eight of glass and water 
= 1000-0000 grains. 
= 1-9736 „ 
1001-9736 
Glass and water =1001-9736 grains. 
Less air displaced by weight (brass) . = 0-1462 „ 
1001-8274 grains = the true value 
of the water and apparatus received from the assistant employing brass weights. 
Glass and water =1001-973600 grains. 
Less air displaced by weight (platinum) . = 0-058271 ,, 
1001-915329 grains = the true value 
of the water and glass received from the assistant employing platinum weights. 
(2) In a note to the translation of Desciianel’s 4 Natural Philosophy’* by Professor 
Evekett, it is said, “ In strictness the weight in grammes of a litre of air under the 
pressure of 760 millimetres of mercury is different in different localities, being propor- 
tional to the intensity of gravity — not because the force of gravity in the litre of air is 
different (for though this is true, it does not affect the numerical value of the weight 
when stated in grammes), but because the pressure of 760 millimetres of mercury varies 
as the intensity of gravity. So that more air is compressed into the space of a litre as 
gravity increases. The weight in grammes is another name for the mass. The force of 
gravity on a litre of air under the pressure of 760 millims. is proportional to the square 
of the intensity of gravity. This is an excellent example of the ambiguity of the word 
weight , which sometimes denotes a mass, sometimes a force ; and though the distinction 
is of no practical importance so long as we confine our attention to one locality, it 
cannot be neglected when different localities are compared.” In one locality we have 
to deal with differences of air-pressure alone. Assumed that we are weighing at 
Greenwich, where gravity is to that of Paris as 3457 to 3456, the weight of a litre of 
dry air at Paris being 1*293167 grin, at 0° C. at a barometric height of 760 millims., 
the weight of a litre of air at Greenwich at the same barometric and thermometric 
heights will be 1 "293561 grm. Knowing the weight of a litre of air at 760 millims. 
barometric height, at a lower height of the mercurial column the weight will be pro- 
portionately less, the temperature beings upposed constant; so that the weight of air 
displaced with the barometer at 7 40 millims. by the glass apparatus and water will be 
1-9216 grain, and at 715 millims. 1-8890 grain. The brass weight will displace 0-1424 
* Part I. p. 141. 
2 Q 
JIDCCCLXXIII. 
