24 GLASS APPARATUS IN COMMON USE 



Cover-slips. 



1. Wash the cover-slips thoroughly in running water. 



2. Boil the cover-slips in 10 per cent, solution of 

 chromic acid, as for new cover-slips. 



3. Wash thoroughly in running water. 



4. Pick out those cover-slips which show much ad- 

 herent dirty matter, and rub them between thumb 

 and forefinger under the water tap. The dirt usually 

 rubs off easily, as it has become friable from contact 

 with the chromic acid. 



5. Return all the cover-slips to the beaker, fill in 

 fresh chromic acid solution, and treat as new cover- 

 slips. 



NOTE. Test-tubes, plates, capsules, etc., which, from long use, 

 have become scratched and hazy, or which cannot be cleaned in 

 any other way, may be dealt with by immersing them in an 

 enamelled iron bath, containing water acidulated to i per cent, 

 with hydrofluoric acid, for ten minutes, rinsing thoroughly in 

 water, drying, and polishing. 



PLUGGING TEST-TUBES AND FLASKS. 



Before sterilisation all test-tubes and flasks must 

 be carefully plugged with cotton-wool, and for this 

 purpose best absorbent cotton-wool (preferably that 

 put up in cylindrical one-pound packets and inter- 

 leaved with tissue paper known as surgeons' wool) 

 should be employed. 



1. For a test-tube or a small flask, tear a strip of 

 cotton- wool some 10 cm. long by 2 cm. wide from the 

 roll. 



2. Turn in the ends neatly and roll the strip of wool 

 lightly between the thumb and fingers of both hands 

 to form a long cylinder. 



3. Double this at the centre and introduce the now 

 rounded end into the open mouth of the tube or flask. 



4. Now, whilst supporting the wool between the 

 thumb and fingers of the right hand, rotate the test- 



