868 
PEOFESSOE W. A. MILLEE ON THE PHOTOOEAPHIC 
caoutchouc against the ground surfaces of the plate-glass. No cement was employed, 
and the trough was taken to pieces and cleansed between each experiment — a stratum 
of liquid 0'75 inch thick being used in each case. 
10. In the preparation of the various compounds for examination, much care was taken 
to employ the materials in a state of purity. In one or two instances, however, it has 
happened that an acid which usually forms highly diactinic salts has exhibited an 
anomalous and excessive absorptive power, although in combination with a base which 
in other instances furnishes strongly diactinic salts. Here some impurity, in quantity so 
small as to escape the tests in ordinary use, but sensitive to the action of light, has 
probably been present, and has impaired the diactinic capacity of the substance. Cases in 
which such impurity is suspected are indicated in the Table by the mark (I) subjoined to 
them. I intended to have prepared fresh portions of each of these substances with a view 
to their re-examination; but by the time I had arrived at this stage of my experiments 
I learned from my L iend Professor Stokes that he had been engaged in a similar inquiry, 
but had been turning to account for this purpose the property of fluorescence ; and as I 
found that my results, where both of us had employed the same substance, were in close 
accordance with those which he had obtained, I determined to postpone the further exami- 
nation of these bodies till after the details of Professor Stokes’s experiments have appeared. 
I have also refrained from extending my observations to the compounds of organic che- 
mistry, many of which I had otherwise proposed to submit to a similar investigation. 
It may here be observed that the solution of a salt in water always to a certain extent 
impairs the diactinic quality of the liquid, however limpid the solution may be, producing 
an effect which may be compared to opalescence or turbidity in a liquid employed in the 
transmission of luminous rays. 
11. I have not been able to trace any special connexion between the chemical com- 
plexity of a substance and its diactinic power. Carbon in its pure form as diamond we 
regard as an element. In thin slices it transmits portions of the chemical rays of nearly 
all degrees of refrangibility, though none of the specimens which I examined exhibited 
any approach to the actinic limpidity of quartz. Phosphorus, on the other hand, though 
transparent to light in its melted condition, and equally regarded as elementary, appeal's 
to be nearly adiactinic, or impermeable to the chemical rays. In many cases the peculim 
diactinic or adiactinic action of an element is traceable in its simpler chemical com- 
pounds. Thus the simpler combinations of sulphur, such as sulphuretted hydrogen, 
sulphurous acid gas, bisulphide of carbon, and chloride of sulphur, are all powerful actinic 
absorbents, while in the more complicated form of sulphuric acid and the sulphates of 
certain bases the compounds are highly diactinic. On the other hand, the silicates are 
much less diactinic than silica in the form of quartz, or the bases which enter into the 
formation of the silicates : probably this may arise, as Professor Stokes suggests, from 
the difficulty of obtaining silicates, either natural or artificial, after fusion, perfectly free 
from ii’on. 
12. No solid or liquid substance that I have as yet tried surpasses rock-crystal in 
