ELECTRICAL MEASUREMENT. 453 



any moisture on the inner surface of the glass 

 which ought to be perfectly dry is " matter 

 in its wrong place," and is, therefore, dirt, you 

 will understand what I mean. If there is no dirt 

 on the glass it is certain to insulate well. But then 

 how to get the glass perfectly clean ? In the first 

 place, wash it well with soap and water. If you 

 like you may try nitric acid, and then pure water, 

 or you may wash it with alcohol, and then with 

 pure water. I have gone through almost in- 

 cantations to get perfect cleanness of the surface of 

 the glass, but I doubt much whether I ever got 

 any result which I could not have got with soap 

 and water, and then running pure water over the 

 surface of the glass after it is done. Wash it 

 well, somehow or other. You may use acids, or 

 alcohol, if you like ; but I think you will generally 

 find that soap and water, and enough of clean 

 water to end with, will answer as well as anything. 

 Then shake it well, and get it well dry, but do 

 not use a duster, however clean, to dry it. Shake 

 the moisture off, and take a little piece of blotting 

 paper, and suck up very carefully any little portion 



