454 POPULAR LECTURES AND ADDRESSES. 



of water which may remain by cohesion, but do 

 not rub it with anything that can leave shreds 

 or fibres ; that is dirt. The finest cambric will 

 leave on the glass what will answer Lord 

 Palmerston's definition. When you have got the 

 glass clean of everything except water, then dry 

 it, and you will be sure to find it answer. The 

 way to dry it, and to keep it dry, is to have the 

 sulphuric acid in the proper receptacle. Each of 

 these instruments has a receptacle for sulphuric 

 acid, which must be freed from volatile vapours 

 by a proper process ; boiling with sulphate of 

 ammonia suffices. The sulphuric acid need not 

 be chemically pure, but it must be purified from 

 volatile vapours, and it must be very strong. I 

 believe, oftener than from any other cause, these 

 instruments fail to hold well because the sul- 

 phuric acid is not strong enough, and frequently, 

 when an electrometer has failed, by putting in 

 stronger acid the defect has been perfectly 

 remedied. 



