432 SCIENCE PROGRESS 



perature ; the apparatus is then quickly cooled and the 

 pyknometer dried and weighed. To make allowance for the 

 changes with temperatures in the volume of the apparatus, it 

 is necessary to know the weight of the pyknometer empty and 

 also its weight full of water at two temperatures. 



If the pyknometer contains v ccs. water (wt, w grms.) at 

 temp, t, and v 1 ccs. water (wt, w 1 grms.) at temp, t 1 , and v e ccs. 

 of liquid (wt, w e ) whose density is required at temp. (boiling 

 point of this liquid), (i.e. A d ) } then : 



v 1 — v = ^av(t 1 — t) 

 v e — v= T ) av(0 — t) 



v,-v = {vi-v) (0-t) h i- foi-P) 



= k(0-t) 





V e V+ k(d-t) 



(ii) Wade and Merriman's modification of Gay-Lussac's 

 pyknometer (2) consists of a small, flat-bottomed flask with a 

 capillary neck expanding at the top to a small cup which is 

 closed by a rubber stopper ; there is also a special filling 

 apparatus. With a 25 cc. pyknometer having a neck of o'j 

 mm. internal diameter, an error in setting of o'i mm. corre- 

 sponds to an error of about one in the sixth place. The india- 

 rubber stopper is used with the idea of preventing the small 

 vapour leak which occurs with ground-glass joints ; but 

 rubber absorbs some vapours, e.g. chloroform, so readily, and 

 is altogether so difficult a substance to weigh exactly that the 

 authors prefer to use a well ground-in glass stopper instead. 

 The kind of losses to be expected by this leak are shown by 

 the following figures below : 



Bousfield pyknometer S.G. Bottle with 



70 c.c. cap 50 cc. 



Weight after 45 min. in balance-case . 114*3722 grams. 66*6592 grams. 

 Weight after 90 min. in balance-case . 114*3643 „ 66*6588 „ 



Loss in 45 min 0*0079 „ -0004 „ 



Temp, of balance-case = 25°C. Ether 

 was the liquid used. 



In this method the stopper can always be inserted to 

 exactly the same extent in the bottle. It can also be filled 

 easily without letting the liquid come in contact with moist 



