THE DENSITY OF LIQUIDS 435 



The densities of the air at the times of the three weighings are 

 then found by interpellation in Wade and Merriman's table (2). 



Temp. Mass of z cc. of air at the following pressures in mm. 



The values found are: 0*001197, 0*001178, o*oon66 grams, 

 the weight of the bottle in vacuo = W 2 . 



W =W al (i+S/D-S/D'). 



= 31*7492 (1 +0*001197/2-50 — 0-001197/8*4). 

 = 3*7599 



the apparent weight of the bottle under the conditions of the 

 other two weighings can now be calculated. 



31-7599 = W ai (1 +0-001178/2-50 — 0-001178/8-4). 



W* =3 1 7494- 



and 31*7599 = W a5 (i +0*001166/2-50 — 0*001166/8-4). 



^<a = 3i'749S. 



The apparent weight of the water can now be obtained : 

 101*6971 — 31-7494= 69*9477; and that of the liquid 

 157-4290—31*7495 = 125*6795; while the approximate 

 value of the density is 125*68/69*95 = 1797; 

 These weights are now corrected to vacuum. 



W 2 = 69*9477 (* +0*001178/0*998 — 0*001178/8-4) = 70*0206. 

 W 3 = i2$ 6795(1 +0*001166/1-797 — 0*001166/8-4) = 125-7436. 



20°C 



and so S.G. — „' = 1*795809. 



and D=S.G.?^-- = 1-795809 x Z)^,^ 1-795809 x 0-998230 = 

 4 *■*• 



1*792630 



It should be noted that since there are seven significant 

 figures eight figure logs are necessary. 



The three main objections to the pyknometer method, 

 when extreme accuracy is required, are(i); (a) The diffi- 

 culty of keeping a large volume of unstirred water at a con- 

 stant and fixed temperature ; (b) The difficulty in removing 

 or correctly allowing for the effects of humidity in weighing 



