4S2 
THE DISCRIMINATION OF ORGANIC BODIES 
stituent, we must examine it in a pure spectrum. [The formation, of a pure spectrum 
was then explained, and such a spectrum was formed on a screen by the aid of the 
electric light. On holding a cell containing a salt of copper in front of the screen, and 
moving it from the red to the violet, it was shown to cast a shadow' in the red as if the 
fluid had been ink, while in the blue rays it might have been supposed to have been 
water. Chromate of potash similarly treated gave the reverse effect, being transparent 
in the red and opaque in the blue. Of course the' transition from transparency to opacity 
was not abrupt; and for intermediate colours the fluids caused a partial darkening. In¬ 
deed, to speak with mathematical rigour, the darkening is not absolute even when it ap¬ 
pears the greatest; but the light let through is so feeble that it eludes our senses. In 
this w r ay the behaviour of the substance may be examined with reference to the various 
kinds of light one after another; but in order to see at one glance its behaviour with re¬ 
spect to all kinds, it is merely requisite to hold the body so as to intercept the whole 
beam which forms the spectrum,—to place it, for instance, immediately in front of the 
slit.] 
To judge from the two examples just given, it might be supposed that the observation 
of the colour would give almost as much information as analysis by the prism. To show 
how far this is from being the case, two fluids very similar in colour, port-wine and a 
solution of blood, were next examined. The former merely caused a general absorption 
of the more refrangible rays; the latter exhibited two well-marked dark bands in the 
yellow and green. These bands, first noticed by Hoppe, are eminently characteristic of 
blood, and afford a good example of the facilities which optical examination affords for 
following a substance which possesses distinctive characters of this nature. On adding 
to a solution of blood a particular salt of copper (any ordinary copper salt, with the ad¬ 
dition of a tartrate to prevent precipitation, and then carbonate of soda), a fluid w r as 
obtained utterly unlike blood in colour, but showing the characteristic bands of blood, 
while at the same time a good deal of red was absorbed, as it would have been by the 
copper salt alone. On adding, on the other hand, acetic acid to a solution of blood, the 
colour was merely changed to a browner red, without any precipitate being produced. 
Nevertheless, in the spectrum of this fluid the bands of blood had w'holly vanished, while 
another set of bands less intense, but still very characteristic, made their appearance. 
This alone, however, does not decide whether the colouring-matter is decomposed or not 
by the acid; for as blood is an alkaline fluid, the change might be supposed to be merely 
analogous to the reddening of litmus. To decide the question, we must examine the 
spectrum when the fluid is again rendered alkaline, suppose by ammonia, which does not 
affect the absorption bands of blood. The direct addition of ammonia to the acid mix¬ 
ture causes a dense precipitate, which contains the colouring-matter, which may, how¬ 
ever, be separated by the use merely of acetic acid and ether, of w r hich the former was 
already used, and the latter does not affect the colouring-matter of blood. This solution 
gives the same characteristic spectrum as blood to which acetic acid has been added; but 
now' there is no difficulty in obtaining the colouring-matter in an ammoniacal solution. 
In the spectrum of this solution, the sharp absorption-bands of blood do not appear, but 
instead thereof there is a single band a little nearer to the red, and comparatively vague 
[this was shown on a screen]. This difference of spectra decides the question, and proves 
that hgematin (the colouring-matter prepared by acid, etc.) is, as Hoppe stated, a product 
of decomposition. 
The spectrum of blood may be turned to account still further in relation to the chemi¬ 
cal nature of that substance. The colouring-matter contains, as is well known, a large 
quantity of iron; and it might be supposed that the colour was due to some salt of iron, 
more especially as some salts of peroxide of iron, sulphocyanide for instance, have a blood- 
red colour. But there is found a strong general resemblance between salts of the same 
metallic oxide as regards the character of their absorption. Thus the salts of sesquioxide 
of uranium show a remarkable system of bands of absorption in the more refrangible 
part of the spectrum. The numberjand position of the bands differ a little from one 
salt to another; but there is the strongest family likeness between the different salts. 
Salts of sesquioxide of iron in a similar manner have a family likeness in the vagueness 
of the absorption, which creeps on from one part of the spectrum to another without 
presenting any rapid transitions from comparative transparency to opacity and the con¬ 
verse. [The spectrum of sulphocyanide of peroxide of iron was shown for the sake of 
contrasting with blood.] Hence the appearance of such a peculiar system bands of 
