Hardman — Zinc for Magnesium in Minerals. 533 
hand succeeded in obtaining bars of granular selenium through which 
the current of one Leclanche cell causes a very considerable deflection 
of the needle. This form is, we find, in its electric resistance almost 
unaffected by light. Between these two forms of granular selenium — 
the apparently non-conducting and the comparatively highly con- 
ducting — there is another of intermediate resistance. This modification 
is highly sensitive to light ; its conductivity when in the form of flat 
bars increasing in direct sunlight 75 to 100 per cent., and in artificial 
light in ratios ranging from 10 to 50 per cent. 
One form of granular selenium, as Hittorf first showed, has its 
resistance considerably diminished by heat : indeed, he says that 
could it be heated red hot its conductivity would not be inferior to that 
of the metals. Our own experiments confirm the diminution of con- 
ductivity by heat, but we have found, in at least one modification 
which we have produced, a body which so far conforms to the me- 
tallic type as to have its resistance strikingly increased by heat. 
We have made bars of selenium which when placed in the circuit 
of a battery and galvanometer, have shown a deflection of 48°, while 
upon completing the circuit of a nitric acid battery, the current of 
which heated a spiral of patinum wire surrounding the bar of selenium, 
the needle gradually fell to 15°, as the temperature of the bar became 
greater. 
Tor the present we refrain from comment upon these results ; and 
although we have been engaged during many months in this investi- 
gation, we defer details of our experiments, and especially of the 
conditions under which the different electrically resisting and light- 
sensitive forms of seleniam are obtained, until with larger experience 
we may hope to bring before the Academy results tending more closely 
to the solution of the questions we have proposed to ourselves. 
XLYIII. — On a supposed Substitution of Zinc for Magnesium in 
Minerals. By Edward T. Hardman, F. C. S., &c. (of the Geological 
Survey of Ireland.) 
[Read January 26, 1874.] 
The investigation undertaken by me— viz.. An Inquiry into the Effect 
of the Contact of Igneous Dykes and Masses with other Kock Masses, 
and the true nature of the Alteration so often observed under such cir- 
cumstances — has been necessarily a tedious one, and one which will 
require a careful analysis and comparison of very man};^ rock-specimens 
ere a conclusion of any value can be arrived at on the subject. I shall 
therefore reserve, for the present, the account of my work on the ori- 
ginal matter of inquiry. 
In the course of my research, however, a discovery of much interest 
and some importance was made ; and I wish, therefore, in this Report 
