ELECTRICAL MEASUREMENT. 251 



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 sulphuric 

 acid is not strong enough, and frequently, when an electrometer has 

 failed, by putting in stronger acid, the defect has been perfectly 

 remedied. 



The PRESIDENT, in rising to move a vote of thanks to Sir William 

 Thomson for his very profound observations on a subject which is, per- 

 haps, one of the most difficult in physical science, said : he would only 

 allow myself to make one criticism, namely, that Sir William Thomson 

 had dealt more with the labours of others than with his own. We all 

 know that he has worked more than any other living man to bring 

 theory and practice into one focus regarding this subject of applied 

 electricity. There are many apparatus with which his name is con- 

 nected, and will always be connected, such as electrometers for the 

 measurement of currents, and apparatus for working long submarine 

 lines. These he has not dwelt upon, except in a very cursory manner. 

 Regarding the electrometer, I heard Sir William Thomson give a 

 lecture at the Royal Institution, some years ago, when he brought some 

 Scottish electricity to London in this very instrument. These electro- 

 meters have had not only a theoretical importance, but are of practical 

 uses to the electrician, especially one variety of them, which is used 

 largely by those who apply electricity to practical work, Without 

 occupying your time longer, after listening to so lucid an explanation, 

 I beg to call upon you to pass a hearty vote of thanks to Sir William 

 Thomson. 



The vote of thanks having been passed, 



The PRESIDENT said : I will now call on M. Tresca to give us a 

 discourse upon his very remarkable experiments on the flow of solids. 

 This is a subject which is peculiarly the work of M. Tresca, of whom 

 we have all of us more or less heard, but we have not all of us had the 

 author himself to give us a lucid explanation of them, as I am sure he 

 will do now. 



