250 SECTION MECHANICS. 



instrument below. The more convenient arrangement of the pumice 

 is now made, but that is the only alteration. The mechanical arrange- 

 ment of the disc is only changed in the portable electrometer as it 

 now exists, and two specimens have been sent out to the Arctic 

 expedition. Just one word of practical advice, with respect to the 

 electrometers. I have been continually asked how to keep them in 

 order, and have frequently heard complaints that these will not hold ; 

 that they do not retain the charge. In each of these electrometers 

 there is a porous jar, the heterostatic system being adopted in each of 

 them. It is necessary the insulating should be perfect, and then it all 

 depends afterwards on the cleanness of the surface of the glass. If you 

 will allow me to use the phrase of Lord Palmerston, with regard to 

 water, when he said that, " Dirt is matter in its wrong place," and to 

 consider that water, or any moisture on the face of the glass which 

 ought to be perfectly dry is 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 

 incantations to get perfect cleanness of the surface of the glass, but I 

 doubt much whether I have 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 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 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 every- 

 thing except water, then dry it, and you will 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 



