THE DISCRIMINATION OF ORGANIC BODIES 
434. 
pretty transparent with regard to another part. Carthamine, murexide, platino-cyanide 
of magnesium may he mentioned as examples. Such substances reflect copiously, like a 
metal, those rays with respect to which they are intensely opaque, but more feebly, like 
a vitreous substance, those rays for which they are tolerably transparent, Hence, when 
white light is incident upon them the regularly-reflected light is coloured, often vividly, 
those colours preponderating which the substance is capable of absorbing with intense 
avidity. But perhaps the most remarkable example known of the connection between 
intense absorption and copious reflection occurs in the case of crystals of permanganate 
of potash. These crystals have a metallic appearance, and reflect a greenish light. They 
are too dark to allow the transmitted light to be examined; and even when they are 
pulverized, the fine purple powder they yield is too dark for convenient analysis of the 
transmitted light. But the splendid purple solution which they yield may be diluted at 
pleasure, and the analysis of the light transmitted by it presents no difficulty. The 
solution absorbs principally the green part of the spectrum; and when it is not too strong, 
or used in too great thickness, five bands of absorption, indicating minima of transpa¬ 
rency, make their appearance (these were shown on a screen). Now, when the green 
light reflected from the crystals is analysed by a prism, there are observed bright bands, 
indicating maxima of reflecting power, corresponding in position to the dark bands in 
the light transmitted by the solution. The fifth bright band, indeed, can hardly, if at 
all, be made out, but the corresponding dark band is both less strong than the others and 
occurs in a fainter part of the spectrum. When the light is reflected at a suitable angle, 
and is analysed both by a Nicol’s prism, placed with its principal section in the plane of 
incidence, and by an ordinary prism, the whole spectrum is reduced to the bands just 
mentioned. The Nicol’s prism would, under these circumstances, extinguish the light 
reflected from a vitreous substance, and transmit a large part of the light reflected from a 
metal. Hence we see that as the refrangibility of the light gradually increases, the sub¬ 
stance changes repeatedly, as regards the character of its reflecting power, from vitreous 
to metallic and back again, as the solution (and therefore it may be presumed the sub¬ 
stance itself) changes from moderately to intensely opaque, and conversely. 
These considerations leave little doubt as to the chemical state of the copper present 
in a certain glass which was exhibited. This glass was coloured only in a very thin 
stratum on one face. By transmission it cut off a great deal of light, and was bluish. 
By reflection, especially when the colourless face was next the eye, it showed a reddish 
light visible in ell directions, and having the appearance of coming from a fine precipi¬ 
tate, though it was not resolved by the microscope, at least with the power tried. It 
evidently came from a failure in an attempt to make one of the ordinary red glasses 
coloured by suboxide of copper, and the only question was as to the state in which the 
copper was present. It could not be oxide, for the quantity was too small to account for 
the blueness, and in fact the glass became sensibly colourless in the outer flame of a 
blowpipe. Analysis of the transmitted light by the prism showed a small band of ab¬ 
sorption in the place of the band seen in those copper-red glasses which are not too deep, 
and therefore a small portion of copper was present in the state of suboxide, i. e. a silicate 
of that base. The rest was doubtless present as metallic copper, arising from over-reduc¬ 
tion in the manufacture, and accordingly the blue colour, which would have been purer 
if the suboxide had been away, indicates the true colour of copper by transmitted light, 
•quite in conformity with what we have seen in the case of gold. Hence, in both metals 
alike, the absorbing and the reflecting powers are, on the whole, greater for the less than 
for the more refrangible colours, the law of variation with refrangibility being of course 
somewhat different in the two eases. 
Time would not permit of more than a very brief reference to the second property to 
which the speaker had referred as useful in tracing substances in impure solutions—that 
of fluorescence. The phenomenon of fluorescence consists in this, that certain substances, 
when placed in rays of one refrangibility, emit during the time of exposure compound 
light of lower refrangibility. When a pure fluorescent substance (as distinguished from 
a mixture) is examined in a pure spectrum, it is found that on passing from the extreme 
red to the violet and beyond, the fluorescence commences at a certain point of the spec¬ 
trum, varying from one substance to another, and continues from thence onwards, more 
or less strongly in one part or another according to the particular substance. The colour 
of the fluorescent light is found to be nearly constant throughout the spectrum. Hence, 
when in a solution presented to us, and examined in a pure spectrum, we notice tha 
