Intelligence and Miscellaneous Articles. 319 
News’ of the 20th of October last, because in my anxiety to admit 
of any one testing the accuracy of my results, I had described how 
the process might be conducted with what is generally sold as pure 
acid, but which, if properly tested, is rarely free from arsenic. 
During these last experiments, it occurred to me to distil the sands 
with a second and a third dose of acid, and in most cases I have 
found the yield of arsenic and antimony to be much greater, say from 
two to five times, in the second distillate than in the first; and in 
some I have found the third distillate to give more than the first, 
but in others less. 
These results induce me to say that, before a sand could be pro- 
nounced not to contain any arsenic or antimony, it should be distilled 
to dryness with at least three distinct doses of acid, each distillate 
being tested carefully in the manner described in my former com- 
munication. 
I am, Gentlemen, 
Your obedient Servant, 
Ducatp CAMPBELL. 
7 Quality Court, Chancery Lane, 
March 25th, 1861. 
NOTE ON A MODIFICATION OF THE APPARATUS EMPLOYED FOR 
ONE OF AMPERE’S FUNDAMENTAL EXPERIMENTS IN ELECTRO- 
DYNAMICS. BY PROFESSOR TAIT. 
My attention was recalled by Principal Forbes’s note* (read to the 
Royal Society of Edinburgh on January 7), to his request that I should 
at leisure try to repeat Ampére’s experiment for the mutual repulsion 
of two parts of the same straight conductor, by means of an apparatus 
which he had procured for the Natural Philosophy Collection in the 
University. Some days later I tried the experiment, but found that, 
on account of the narrowness of the troughs of mercury, it was im- 
possible to prevent the capillary forces from driving the floating wire 
to the sides of the vessel. I therefore constructed an apparatus in 
which the troughs were two inches wide, the arms of the float being 
also at that distance apart. Making the experiment according to 
Ampére’s method with this arrangement, I found one small Grove’s 
cell sufficient to produce a steady motion of the float from the poles 
of the pile; in fact, the only difficulty in repeating the experiment 
lies in obtaining a perfectly clean mercurial surface. 
Two objections have been raised against Ampére’s interpretation 
of this experiment, one of which is intimately connected with the 
subject of Principal Forbes’s note. This is, the difficulty of ascer- 
taining exactly what takes place where a voltaic current passes from 
one conducting body to another of different material. It is known 
* Phil. Mag. for February 1861. 
