//.A MATIN. 251 
concentrated hydrochloric acid, at a temperature above 150° 0. Con- 
centrated sulphuric acid dissolves it, without any gas being evolved, 
giving rise to a dark red solution, from which water precipitates the 
substance known as " haematoporphyrin " (see p. 258), which, as it 
contains no iron, has been Bometimes spoken of ;is iron-free fuematin. 
This body is soluble in alkaline solutions, and both its acids and 
alkaline solutions exhibit very characteristic absorption-spectra. 
Alkaline solutions of ha-matin in thick layers, when examined by 
transmitted Liglit, appear red. whilst thin layers appear of an olive- 
green colour. Acid solutions, whatever the thickness of the stratum 
examined, always appear of ,-i brown colour. 
"When the spectrum of light transmitted through alkaline and acid 
solutions of haematin is examined by the photographic as well as by the 
direct method, it is seen that the last rays of the spectrum to be 
absorbed are the red rays up to B ; that the solutions are characterised 
by a defined absorption-band between C and D, which is shifted towards 
I) in the case of the alkaline, towards C in the case of the acid solutions ; 
that alkaline solutions, even when extremely diluted, effect a general 
absorption of the whole ultra-violet, violet, etc., rays ; that acid solutions, 
even when very highly diluted, whilst not exerting a general absorption 
of the ultra-violet, exhibit an absorption-band at the junction of the 
extreme violet and the ultra-violet, properly so called. 
The absorption-bands in the visible spectrum of both alkaline and 
acid solutions of haematin are shown in Plate II., Spectra 2, 4, and 6. 
The alkaline solutions exhibit one absorption-band between C and D, of 
which the more refrangible border adjoins I), whilst acid solutions exhibit 
an absorption-band also between C and L>, of which the less refrangible 
border adjoins C, though the position of the band is somewhat in- 
fluenced by the particular acid which has been employed. Attention 
is directed to the fact that the band between C and I) in the spectrum 
of methaemoglobin differs in position from the band in the spectrum 
of acid as well as from that of alkaline haematin. Whilst the absorption- 
band of the former is close to C and that of the latter close to 1), the 
band of methaemoglobin, in acid solutions, is separated by a marked 
interval both from C and D, though it is closer to the former than to 
the latter. 
Alkaline solutions of haematin in. the presence of certain foreign 
matters, when treated with reducing agents, exhibit a spectrum which 
is apparently identical with that which will be described under "Haemo- 
chromogen," and which was first described by Stokes as the spectrum of 
reduced hcematin. The band in the red disappears, and two characteristic 
bands appear in the green (Plate II., Spectrum 3). On now shaking 
the reduced liquid with air, the two bands first referred to disappear, 
and are replaced by the original haematin band. 
This experiment would appear to show that hsematin is hut oxidised hamio- 
chromogen, a conclusion which is false, and which is an illustration of the 
mistakes into which observers may be led who conclude as to the identity of 
two colouring matters from the identity of prominent absorption-bands in their 
spectra. 
A strong proof that oxidised ha?mochromogen is not identical with haematin 
is derived from my own observations on the absorption of the extreme violet and 
ultra-violet. Whilst haematin possesses even in solutions of great dilution the 
power of absorbing the whole of the ultra-violet, the violet and even the blue 
