SCIENTIFIC SUMMAKY. 
319 
These numbers closely accord with those required by the formula Fe 2 0 3 
3S0 s 12H 2 0, and ihleite appears to be a distinct mineral species ; it differs 
from the eoquimbite, analysed by Gustav Rose, in containing 12 in place of 
9 equivalents of water. — JSeues Jahrhuch fur Miner alogie , 1877, 251. 
PHYSICS. 
The Electric Conductivity of Water. — It seems at first sight curious that 
in the case of so common a substance as water there should be any doubt 
among physicists as to the power which the liquid possesses of conducting 
electricity. Not that the subject has by any means been neglected, as the 
labours of Magnus, Pouillet, Becquerel, Oberbeck, Rossetti, and Quincke 
abundantly testify. But the results obtained by these several investigators 
differ so widely among themselves that it is not easy to know which are to 
be accepted. The figures brought out by Pouillet, for example, are sixty 
times greater than those deduced by Magnus. Professor Kohlrausch has, 
therefore, thought it high time that the subject was settled; and after a 
most carefully conducted investigation, he has published the results of his 
inquiry in Poggendorf s u Annalen ” ( Ergdnzungsb . iii. 1877, p. 1). Many 
substances have their electric conductivity affected to a remarkable extent 
by the presence of impurities, even when these are present in only minute 
proportion. Matthiessen showed that copper, for instance, has its con- 
ductivity lowered 40 per cent, by presence of a mere trace of arsenic. In 
like manner Kohlrausch has found it necessary to pay scrupulous attention 
to the purity of the water which he examined, and indeed the discrepancies 
among the results of previous inquirers may probably be explained by in- 
attention to this point. Having prepared the liquid in as pure a state as 
the resources of the laboratory can permit, Kohlrausch tested the con- 
ductivity by examining a shell of this water enclosed between two hemi- 
spherical vessels of platinum nearly fitting one into the other, and serving 
as electrodes. On passing a current of electricity through this arrange- 
ment, it was found that the pure water offered remarkable resistance ; in 
fact its conductivity was only of that assigned to it by Pouillet. Rain, 
which is, of course, the purest natural form of water, conducted electricity 
twenty-five times better than the artificially-purified liquid which served 
as a standard. 
Influence of Light on the Electric Resistance of Metals. — Some two or 
three years ago we were startled by the announcement that the electric con- 
ductivity of selenium is capable of being affected by light. It has, however, 
been since determined that not only selenium but also the allied element 
tellurium has its electric resistance diminished after exposure to luminous 
rays. Desirous of determining whether other bodies are similarly affected, 
Dr. Bornstein has carried out some interesting researches in the Physical 
Institute at Heidelberg. His results are published in a paper which has 
been translated by Mr. R. E. Day, in the “Philosophical Magazine ” (June, 
1877, p. 481). Gold, silver, and platinum are the only metals which Dr. 
Bornstein has yet examined, but as he finds that all these are sensitive to 
